chapter 46 biology book

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46Arthropods

Concept Outline

46.1 The evolution of jointed appendages has madearthropods very successful.

Jointed Appendages and an Exoskeleton. Arthropodsprobably evolved from annelids, and with their jointedappendages and an exoskeleton, have successfully invadedpractically every habitat on earth.Classification of Arthropods. Arthropods have beentraditionally divided into three groups based onmorphological characters. However, recent researchsuggests a restructuring of arthropod classification isneeded.General Characteristics of Arthropods. Arthropodshave segmented bodies, a chitinous exoskeleton, and oftenhave compound eyes. They have open circulatory systems.In some groups, a series of tubes carry oxygen to theorgans, and unique tubules eliminate waste.

46.2 The chelicerates all have fangs or pincers.

Class Arachnida: The Arachnids. Spiders and scorpionsare predators, while most mites are herbivores.Class Merostomata: Horseshoe Crabs. Among themost ancient of living animals, horseshoe crabs are thoughtto have evolved from trilobites.Class Pycnogonida: The Sea Spiders. The spiders thatare common in marine habitats differ greatly fromterrestrial spiders.

46.3 Crustaceans have branched appendages.

Crustaceans. Crustaceans are unique among livingarthropods because virtually all of their appendages arebranched.

46.4 Insects are the most diverse of all animal groups.

Classes Chilopoda and Diplopoda: The Centipedes andMillipedes. Centipedes and millipedes are highlysegmented, with legs on each segment.Class Insecta: The Insects. Insects are the largest groupof organisms on earth. They are the only invertebrateanimals that have wings and can fly.Insect Life Histories. Insects undergo simple orcomplete metamorphosis.

The evolution of segmentation among annelids markedthe first major innovation in body structure among

coelomates. An even more profound innovation was the de-velopment of jointed appendages in arthropods, a phylumthat almost certainly evolved from an annelid ancestor.Arthropod bodies are segmented like those of annelids, butthe individual segments often exist only during early devel-opment and fuse into functional groups as adults. Inarthropods like the wasp above (figure 46.1), jointed ap-pendages include legs, antennae, and a complex array ofmouthparts. The functional flexibility provided by such abroad array of appendages has made arthropods the mostsuccessful of animal groups.

FIGURE 46.1An arthropod. One of the major arthropod groups is representedhere by Polistes, the common paper wasp (class Insecta).

component of plants, and sharessimilar properties of toughness andflexibility. Together, the chitin andprotein provide an external coveringthat is both very strong and capableof flexing in response to the contrac-tion of muscles attached to it. Inmost crustaceans, the exoskeleton ismade even tougher, although lessflexible, with deposits of calciumsalts. However, there is a limitation.The exoskeleton must be muchthicker to bear the pull of the mus-cles in large insects than in smallones. That is why you don’t see bee-tles as big as birds, or crabs the sizeof a cow—the exoskeleton would beso thick the animal couldn’t move itsgreat weight. Because this size limi-tation is inherent in the body designof arthropods, there are no largearthropods—few are larger thanyour thumb.

The Arthropods

Arthropods, especially the largestclass—insects—are by far the mostsuccessful of all animals. Well over1,000,000 species—about two-thirdsof all the named species on earth—are members of this phylum (figure46.2). One scientist recently esti-mated, based on the number and di-versity of insects in tropical forests,that there might be as many as 30million species in this one classalone. About 200 million insects arealive at any one time for eachhuman! Insects and other arthro-pods (figure 46.3) abound in everyhabitat on the planet, but they espe-cially dominate the land, along withflowering plants and vertebrates.

The majority of arthropod speciesconsist of small animals, mostly abouta millimeter in length. Members of

the phylum range in adult size from about 80 micrometerslong (some parasitic mites) to 3.6 meters across (a giganticcrab found in the sea off Japan).

Arthropods, especially insects, are of enormous eco-nomic importance and affect all aspects of human life.They compete with humans for food of every kind, play akey role in the pollination of certain crops, and cause bil-

914 Part XII Animal Diversity

Jointed Appendagesand an ExoskeletonWith the evolution of the first an-nelids, many of the major innova-tions of animal structure had alreadyappeared: the division of tissues intothree primary types (endoderm,mesoderm, and ectoderm), bilateralsymmetry, a coelom, and segmenta-tion. With arthropods, two more in-novations arose—the development ofjointed appendages and an exoskeleton.Jointed appendages and an exoskele-ton have allowed arthropods (phy-lum Arthropoda) to become the mostdiverse phylum.

Jointed Appendages

The name “arthropod” comes fromtwo Greek words, arthros, “jointed,”and podes, “feet.” All arthropods havejointed appendages. The numbers ofthese appendages are reduced in themore advanced members of the phy-lum. Individual appendages may bemodified into antennae, mouthpartsof various kinds, or legs. Some ap-pendages, such as the wings of certaininsects, are not homologous to theother appendages; insect wingsevolved separately.

To gain some idea of the impor-tance of jointed appendages, imagineyourself without them—no hips,knees, ankles, shoulders, elbows,wrists, or knuckles. Without jointedappendages, you could not walk orgrasp any object. Arthropods usejointed appendages such as legs forwalking, antennae to sense their envi-ronment, and mouthparts for feeding.

Exoskeleton

The arthropod body plan has a sec-ond major innovation: a rigid external skeleton, or ex-oskeleton, made of chitin and protein. In any animal, theskeleton functions to provide places for muscle attach-ment. In arthropods, the muscles attach to the interiorsurface of their hard exoskeleton, which also protects theanimal from predators and impedes water loss. Chitin ischemically similar to cellulose, the dominant structural

46.1 The evolution of jointed appendages has made arthropods very successful.

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MollusksChordates

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Spiders

Crustaceans

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FIGURE 46.2Arthropods are a successful group.About two-thirds of all named species arearthropods. About 80% of all arthropodsare insects, and about half of the namedspecies of insects are beetles.

lions of dollars of damage to crops, before and after har-vest. They are by far the most important herbivores in allterrestrial ecosystems and are a valuable food source aswell. Virtually every kind of plant is eaten by one ormore species of insect. Diseases spread by insects causeenormous financial damage each year and strike every

kind of domesticated animal and plant, as well as humanbeings.

Arthropods are segmented protostomes with jointedappendages. Arthropods are the most successful of allanimal groups.

Chapter 46 Arthropods 915

The jointed appendages of insectsare all connected to the central bodyregion, the thorax. There are three pairsof legs attached there and, most often,two pairs of wings (some insects likeflies have retained only one wing pair).The wings are sheets of chitin and protein.

Insects eliminate wastes by collectingcirculatory fluid osmotically in Malpighiantubules that extend from the gut into theblood and then reabsorbing the fluid, butnot the wastes.

Insects breathe through small tubescalled tracheae that pass throughout thebody and are connected to the outsideby special openings called spiracles.

Insects havecomplex sensoryorgans located onthe head, includinga single pair ofantennae andcompound eyescomposed of manyindependent visualunits.

Arthropods have beenthe most successfulof all animals. Two-thirdsof all named specieson earth are arthropods.

Antenna

Eye

Head

Thorax Air sacMalpighiantubules

Abdomen

Rectum

Poisonsac

Sting

MidgutSpiracles

Mouthparts

PHYLUM ARTHROPODA: Jointed appendages and exoskeleton

FIGURE 46.3The evolution of jointed appendages and an exoskeleton. Insects and other arthropods (phylum Arthropoda) have a coelom,segmented bodies, and jointed appendages. The three body regions of an insect (head, thorax, and abdomen) are each actually composed ofa number of segments that fuse during development. All arthropods have a strong exoskeleton made of chitin. One class, the insects, hasevolved wings that permit them to fly rapidly through the air.

Classification of the ArthropodsArthropods are among the oldest of animals, first appearingin the Precambrian over 600 million years ago. Ranging insize from enormous to microscopic, all arthropods share acommon heritage of segmented bodies and jointed ap-pendages, a powerful combination for generating novelevolutionary forms. Arthropods are the most diverse of allthe animal phyla, with more species than all other animalphyla combined, most of them insects.

Origin of the Arthropods

Taxonomists have long held that there is a close relation-ship between the annelids and the arthropods, the twogreat segmented phyla. Velvet worms (phylum Ony-chophora), known from the Burgess Shale (where upside-down fossils were called Hallucigenia) and many other earlyCambrian deposits, have many features in common withboth annelids and arthropods. Some recent molecular stud-ies have supported the close relationship between annelidsand arthropods, others have not.

Traditional Classification

Members of the phylum Arthropoda have been tradition-ally divided into three subphyla, based largely on morpho-logical characters.

1. Trilobites (the extinct trilobites). Trilobites, com-mon in the seas 250 million years ago, were the firstanimals whose eyes were capable of a high degree ofresolution.

2. Chelicerates (spiders, horseshoe crabs, sea spiders).These arthropods lack jaws. The foremost ap-pendages of their bodies are mouthparts called che-licerae (figure 46.4a) that function in feeding, usuallypincers or fangs .

3. Mandibulates (crustaceans, insects, centipedes, milli-pedes). These arthropods have biting jaws, calledmandibles (figure 46.4b). In mandibulates, the mostanterior appendages are one or more pairs of sensoryantennae, and the next appendages are the mandibles.Among the mandibulates, insects have traditionallybeen set apart from the crustaceans, grouped insteadwith the myriapods (centipedes and millipedes) in ataxon called Tracheata. This phylogeny, still widelyemployed, dates back to benchmark work by thegreat comparative biologist Robert Snodgrass in the1930s. He pointed out that insects, centipedes, andmillipedes are united by several seemingly powerfulattributes:

A tracheal respiratory system. Trachea are small,branched air ducts that transmit oxygen fromopenings in the exoskeleton to every cell of thebody.

Use of Malpighian tubules for excretion. Malpighiantubules are slender projections from the digestivetract which collect and filter body fluids, emptyingwastes into the hindgut. Uniramous (single-branched) legs. All crustacean ap-pendages are basically biramous, or “two-branched” (figure 46.5), although some of theseappendages have become single-branched by re-duction in the course of their evolution. Insects, bycontrast, have uniramous, or single-branched,mandibles and other appendages.

Doubts about the Traditional Approach

Recent research is casting doubt on the wisdom of thistaxonomic decision. The problem is that the key morpho-logical traits used to define the Tracheata are not as pow-erful taxonomically as had been assumed. Taxonomistshave traditionally assumed a character like branching ap-


Eyes

Chelicera

Pedipalp

Antenna

Mandible

(a) Chelicerate (b) Mandibulate

FIGURE 46.4Chelicerates and mandibulates. In the chelicerates, such as aspider (a), the chelicerae are the foremost appendages of the body.In contrast, the foremost appendages in the mandibulates, such asan ant (b), are the antennae, followed by the mandibles.

Exopodite

Endopodite

Crayfish maxilliped(biramous)

Insect appendage(uniramous)

FIGURE 46.5Mandibulate appendages. A biramous leg in a crustacean(crayfish) and a uniramous leg in an insect.

pendages to be a fundamental one, conserved over thecourse of evolution, and thus suitable for making taxo-nomic distinctions.

However, modern molecular biology now tells us thatthis is not a valid assumption. The branching of arthro-pod legs, for example, turns out to be controlled by a sin-gle gene. The pattern of appendages among arthropods isorchestrated by a family of genes called homeotic (Hox)genes, described in detail in chapter 17. A single one ofthese Hox genes, called Distal-less, has recently beenshown to initiate development of unbranched limbs in in-sects and branched limbs in crustaceans. The sameDistal-less gene is found in many animal phyla, includingvertebrates.

A Revolutionary New Phylogeny

In recent years a mass of accumulating morphological andmolecular data has led many taxonomists to suggest newarthropod taxonomies. The most revolutionary of these,championed by Richard Brusca of Columbia University,considers crustaceans to be the basic arthropod group, andinsects a close sister group (figure 46.6).

Morphological Evidence. The most recent morphologi-cal study of arthropod phylogeny, reported in 1998, wasbased on 100 conserved anatomical features of the centralnervous system. It concluded insects were more closely re-lated to crustaceans than to any other arthropod group.They share a unique pattern of segmental neurons, andmany other features.

Molecular Evidence. Molecular phylogenies based on18S rRNA sequences, the 18S rDNA gene, the elongationfactor EF-1a, and the RNA polymerase II gene, all placeinsects as a sister group to crustaceans, not myriapods, andarising from within the crustaceans. In conflict with 150years of morphology-based thinking, these conclusions arecertain to engender lively discussion.

Arthropods have traditionally been classified intoarachnids and other chelicerates that lack jaws and havefang mouthparts, and mandibulates (crustaceans andtracheates) with biting jaws. A revised arthropodtaxonomy considers Tracheata to be the productsconvergent evolution, with insects and crustaceanssister groups.


Ancestralarthropod

Ancestralarthropod

Trilobites (extinct) Trilobites (extinct)

Eurypterids (extinct)

Horseshoe crabs Horseshoe crabs

Arachnids Arachnids

Sea spiders Sea spiders

Chelicerates Chelicerates

Crustaceans

Crustaceans

Mandibulates

Traditional Phylogeny Revised Phylogeny

Insects

InsectsTracheata

Centipedes

Centipedes

Millipedes

Millipedes

Myriapoda

Moderncrustaceans

Eurypterids (extinct)

A crustacean?

FIGURE 46.6A proposed revision of arthropod phylogeny. Accumulating evidence supports the hypothesis that insects and modern crustaceans aresister groups, having evolved from the same ancient crustacean ancestor in the Precambrian. This implies that insects may be viewed as“flying crustaceans,” and that the traditional Tracheata taxon, which places centipedes, millipedes, and insects together, is in fact apolyphyletic group.

General Characteristics ofArthropodsArthropod bodies are segmented like annelids, a phylum towhich at least some arthropods are clearly related. Mem-bers of some classes of arthropods have many body seg-ments. In others, the segments have become fused togetherinto functional groups, or tagmata (singular, tagma), suchas the head and thorax of an insect (figure 46.7). This fus-ing process, known as tagmatization, is of central impor-tance in the evolution of arthropods. In most arthropods,the original segments can be distinguished during larval de-velopment. All arthropods have a distinct head, sometimesfused with the thorax to form a tagma called thecephalothorax.

Exoskeleton

The bodies of all arthropods are covered by an exoskeleton,or cuticle, that contains chitin. This tough outer covering,against which the muscles work, is secreted by the epider-mis and fused with it. The exoskeleton remains fairly flexi-ble at specific points, allowing the exoskeleton to bend andappendages to move. The exoskeleton protects arthropodsfrom water loss and helps to protect them from predators,parasites, and injury.

Molting. Arthropods periodically undergo ecdysis, ormolting, the shedding of the outer cuticular layer. Whenthey outgrow their exoskeleton, they form a new one un-derneath. This process is controlled by hormones. Whenthe new exoskeleton is complete, it becomes separated fromthe old one by fluid. This fluid dissolves the chitin and pro-tein and, if it is present, calcium carbonate, from the oldexoskeleton. The fluid increases in volume until, finally,the original exoskeleton cracks open, usually along the

back, and is shed. The arthropod emerges, clothed in anew, pale, and still somewhat soft exoskeleton. The arthro-pod then “puffs itself up,” ultimately expanding to full size.The blood circulation to all parts of the body aids them inthis expansion, and many insects and spiders take in air toassist them. The expanded exoskeleton subsequently hard-ens. While the exoskeleton is soft, the animal is especiallyvulnerable. At this stage, arthropods often hide understones, leaves, or branches.

Compound Eye

Another important structure in many arthropods is thecompound eye (figure 46.8a). Compound eyes are com-posed of many independent visual units, often thousandsof them, called ommatidia. Each ommatidium is coveredwith a lens and linked to a complex of eight retinular cells


Cephalothorax(fused head andthorax)

Abdomen

Abdomen

(a) Scorpion (b) Honeybee

HeadThorax

FIGURE 46.7Arthropod evolution from many to few body segments. The(a) scorpion and the (b) honeybee are arthropods with differentnumbers of body segments.

Ommatidium

Optic nerve

Nervefiber

Corneal lens

Crystalline core

Rhabdom

Retinular cells

Pigmentcells

Cross sectionof

ommatidium

FIGURE 46.8The compound eye. (a) The compound eyes found in insects are complex structures. (b) Three ocelli are visible between the compoundeyes of the robberfly (order Diptera).

(a) (b)

and a light-sensitive central core, or rhabdom. Com-pound eyes among insects are of two main types: apposi-tion eyes and superposition eyes. Apposition eyes arefound in bees and butterflies and other insects that are ac-tive during the day. Each ommatidium acts in isolation,surrounded by a curtain of pigment cells that blocks thepassage of light from one to another. Superposition eyes,such as those found in moths and other insects that areactive at night, are designed to maximize the amount oflight that enters each ommatidium. At night, the pigmentin the pigment cells is concentrated at the top of the cellsso that the low levels of light can be received by many dif-ferent ommatidia. During daylight, the pigment in thepigment cells is evenly dispersed throughout the cells, al-lowing the eye to function much like an apposition eye.The pigment in the pigment cells gives the arthropod eyeits color, but it is not the critical pigment needed for vi-sion. The visual pigment is located in an area called therhabdom found in the center of the ommatidium. The in-dividual images from each ommatidium are combined inthe arthropod’s brain to form its image of the externalworld.

Simple eyes, or ocelli, with single lenses are found inthe other arthropod groups and sometimes occur togetherwith compound eyes, as is often the case in insects (figure46.8b). Ocelli function in distinguishing light from dark-ness. The ocelli of some flying insects, namely locusts anddragonflies, function as horizon detectors and help the in-sect visually stabilize its course in flight.

Circulatory System

In the course of arthropod evolution, the coelom has be-come greatly reduced, consisting only of cavities thathouse the reproductive organs and some glands. Arthro-pods completely lack cilia, both on the external surfaces of

the body and on the internal organs. Like annelids,arthropods have a tubular gut that extends from themouth to the anus. In the next paragraphs we will discussthe circulatory, respiratory, excretory, and nervous sys-tems of the arthropods (figure 46.9).

The circulatory system of arthropods is open; theirblood flows through cavities between the internal organsand not through closed vessels. The principal componentof an insect’s circulatory system is a longitudinal vesselcalled the heart. This vessel runs near the dorsal surface ofthe thorax and abdomen. When it contracts, blood flowsinto the head region of the insect.

When an insect’s heart relaxes, blood returns to itthrough a series of valves. These valves are located in theposterior region of the heart and allow the blood to flowinward only. Thus, blood from the head and other anteriorportions of the insect gradually flows through the spacesbetween the tissues toward the posterior end and then backthrough the one-way valves into the heart. Blood flowsmost rapidly when the insect is running, flying, or other-wise active. At such times, the blood efficiently delivers nu-trients to the tissues and removes wastes from them.

Nervous System

The central feature of the arthropod nervous system is adouble chain of segmented ganglia running along the ani-mal’s ventral surface. At the anterior end of the animal arethree fused pairs of dorsal ganglia, which constitute thebrain. However, much of the control of an arthropod’s ac-tivities is relegated to ventral ganglia. Therefore, the ani-mal can carry out many functions, including eating, move-ment, and copulation, even if the brain has been removed.The brain of arthropods seems to be a control point, or in-hibitor, for various actions, rather than a stimulator, as it isin vertebrates.


RectumMalpighiantubules

Heart

Ovary

Tympanalorgan

Compound eye

Ocelli

HeadThorax

Abdomen

Antennae

Spiracles

MouthNerve ganglia

Brain

Aorta

Crop Stomach

Gastricceca

(a) (b)

FIGURE 46.9A grasshopper (order Orthoptera). This grasshopper illustrates the major structural features of the insects, the most numerous group ofarthropods. (a) External anatomy. (b) Internal anatomy.

Respiratory System

Insects and other members of subphylum Uniramia, whichare fundamentally terrestrial, depend on their respiratoryrather than their circulatory system to carry oxygen to theirtissues. In vertebrates, blood moves within a closed circula-tory system to all parts of the body, carrying the oxygenwith it. This is a much more efficient arrangement than ex-ists in arthropods, in which all parts of the body need to benear a respiratory passage to obtain oxygen. As a result, thesize of the arthropod body is much more limited than thatof the vertebrates. Along with the brittleness of their chitinexoskeletons, this feature of arthropod design places severelimitations on size.

Unlike most animals, the arthropods have no singlemajor respiratory organ. The respiratory system of mostterrestrial arthropods consists of small, branched, cuticle-lined air ducts called tracheae (figure 46.10). These tra-cheae, which ultimately branch into very small tracheoles,are a series of tubes that transmit oxygen throughout thebody. Tracheoles are in direct contact with individual cells,and oxygen diffuses directly across the cell membranes. Airpasses into the tracheae by way of specialized openings inthe exoskeleton called spiracles, which, in most insects,can be opened and closed by valves. The ability to preventwater loss by closing the spiracles was a key adaptation thatfacilitated the invasion of the land by arthropods. In manyinsects, especially larger ones, muscle contraction helps toincrease the flow of gases in and out of the tracheae. Inother terrestrial arthropods, the flow of gases is essentiallya passive process.

Many spiders and some other chelicerates have a uniquerespiratory system that involves book lungs, a series ofleaflike plates within a chamber. Air is drawn in and ex-pelled out of this chamber by muscular contraction. Booklungs may exist alongside tracheae, or they may function

instead of tracheae. One small class of marine chelicerates,the horseshoe crabs, have book gills, which are analogousto book lungs but function in water. Tracheae, book lungs,and book gills are all structures found only in arthropodsand in the phylum Onychophora, which have tracheae.Crustaceans lack such structures and have gills.

Excretory System

Though there are various kinds of excretory systems in dif-ferent groups of arthropods, we will focus here on theunique excretory system consisting of Malpighian tubulesthat evolved in terrestrial uniramians. Malpighian tubules areslender projections from the digestive tract that are attachedat the junction of the midgut and hindgut (see figure 46.3).Fluid passes through the walls of the Malpighian tubules toand from the blood in which the tubules are bathed. As thisfluid passes through the tubules toward the hindgut, nitroge-nous wastes are precipitated as concentrated uric acid orguanine. These substances are then emptied into the hindgutand eliminated. Most of the water and salts in the fluid arereabsorbed by the hindgut and rectum and returned to thearthropod’s body. Malpighian tubules are an efficient mech-anism for water conservation and were another key adapta-tion facilitating invasion of the land by arthropods.

All arthropods have a rigid chitin and proteinexoskeleton that provides places for muscle attachment,protects the animal from predators and injury, and,most important, impedes water loss. Many arthropodshave compound eyes. Arthropods have an opencirculatory system. Many arthropods eliminatemetabolic wastes by a unique system of Malpighiantubules. Most terrestrial insects have a network of tubescalled tracheae that transmit oxygen from the outside tothe organs.


Spiracles

Spiracle

Tracheoles

Trachea

FIGURE 46.10Tracheae and tracheoles. Tracheae and tracheoles are connected to the exterior by specialized openings called spiracles and carry oxygento all parts of a terrestrial insect’s body. (a) The tracheal system of a grasshopper. (b) A portion of the tracheal system of a cockroach.

(a) (b)

Class Arachnida: The ArachnidsChelicerates (subphylum Chelicerata) are a distinct evolu-tionary line of arthropods in which the most anterior ap-pendages have been modified into chelicerae, which oftenfunction as fangs or pincers. By far the largest of the threeclasses of chelicerates is the largely terrestrial Arachnida,with some 57,000 named species; it includes spiders, ticks,mites, scorpions, and daddy longlegs. Arachnids have a pairof chelicerae, a pair of pedipalps, and four pairs of walkinglegs. The chelicerae are the foremost appendages; theyconsist of a stout basal portion and a movable fang oftenconnected to a poison gland.

The next pair of appendages, pedipalps, resemble legsbut have one less segment and are not used for locomotion.In male spiders, they are specialized copulatory organs. Inscorpions, the pedipalps are large pincers.

Most arachnids are carnivorous. The main exception ismites, which are largely herbivorous. Most arachnids caningest only preliquified food, which they often digest ex-ternally by secreting enzymes into their prey. They canthen suck up the digested material with their muscular,pumping pharynx. Arachnids are primarily, but not exclu-sively, terrestrial. Some 4000 known species of mites andone species of spider live in fresh water, and a few miteslive in the sea. Arachnids breathe by means of tracheae,book lungs, or both.

Order Opiliones: The Daddy Longlegs

A familiar group of arachnids consists of the daddy long-legs, or harvestmen (order Opiliones). Members of thisorder are easily recognized by their oval, compact bodiesand extremely long, slender legs (figure 46.11). Theyrespire by means of a primary pair of tracheae and are un-usual among the arachnids in that they engage in directcopulation. The males have a penis, and the females anovipositor, or egg-laying organ which deposits their eggsin cracks and crevices. Most daddy longlegs are predatorsof insects and other arachnids, but some live on plant juicesand many scavenge dead animal matter. The order includesabout 5000 species.

Order Scorpiones: The Scorpions

Scorpions (order Scorpiones) are arachnids whose pedi-palps are modified into pincers. Scorpions use these pincersto handle and tear apart their food (figure 46.12). The ven-omous stings of scorpions are used mainly to stun theirprey and less commonly in self-defense. The stinging appa-ratus is located in the terminal segment of the abdomen. Ascorpion holds its abdomen folded forward over its body

when it is moving about. The elongated, jointed abdomensof scorpions are distinctive; in most chelicerates, the ab-dominal segments are more or less fused together and ap-pear as a single unit.

Scorpions are probably the most ancient group of terres-trial arthropods; they are known from the Silurian Period,some 425 million years ago. Adults of this order of arach-nids range in size from 1 to 18 centimeters. There are some1200 species of scorpions, all terrestrial, which occurthroughout the world. They are most common in tropical,subtropical, and desert regions. The young are born alive,with 1 to 95 in a given brood.



FIGURE 46.11A harvestman, or daddy longlegs.

FIGURE 46.12The scorpion, Uroctonus mordax. This photograph shows thecharacteristic pincers and segmented abdomen, ending in astinging apparatus, raised over the animal’s back. The white massis comprised of the scorpion’s young.

Order Araneae: The Spiders

There are about 35,000 namedspecies of spiders (order Araneae).These animals play a major role invirtually all terrestrial ecosystems.They are particularly important aspredators of insects and other smallanimals. Spiders hunt their prey orcatch it in silk webs of remarkable di-versity. The silk is formed from afluid protein that is forced out ofspinnerets on the posterior portion ofthe spider’s abdomen. The webs andhabits of spiders are often distinctive.Some spiders can spin gossamer floatsthat allow them to drift away in thebreeze to a new site.

Many kinds of spiders, like the familiar wolf spiders andtarantulas, do not spin webs but instead hunt their prey ac-tively. Others, called trap-door spiders, construct silk-linedburrows with lids, seizing their prey as it passes by. Onespecies of spider, Argyroneta aquatica, lives in fresh water,spending most of its time below the surface. Its body is sur-rounded by a bubble of air, while its legs, which are usedboth for underwater walking and for swimming, are not.Several other kinds of spiders walk about freely on the sur-face of water.

Spiders have poison glands leading through their che-licerae, which are pointed and used to bite and paralyzeprey. Some members of this order, such as the black widowand brown recluse (figure 46.13), have bites that are poiso-nous to humans and other large mammals.

Order Acari: Mites and Ticks

The order Acari, the mites and ticks, is the largest in termsof number of species and the most diverse of the arachnids.Although only about 30,000 species of mites and ticks havebeen named, scientists that study the group estimate thatthere may be a million or more members of this order inexistence.

Most mites are small, less than 1 millimeter long, butadults of different species range from 100 nanometers to 2centimeters. In most mites, the cephalothorax and ab-domen are fused into an unsegmented ovoid body. Respira-tion occurs either by means of tracheae or directly throughthe exoskeleton. Many mites pass through several distinctstages during their life cycle. In most, an inactive eight-legged prelarva gives rise to an active six-legged larva,which in turn produces a succession of three eight-leggedstages and, finally, the adult males and females.

Mites and ticks are diverse in structure and habitat.They are found in virtually every terrestrial, freshwater,and marine habitat known and feed on fungi, plants, andanimals. They act as predators and as internal and externalparasites of both invertebrates and vertebrates.

Many mites produce irritating bites and diseases in hu-mans. Mites live in the hair follicles and wax glands of yourforehead and nose, but usually cause no symptoms.

Ticks are blood-feeding ectoparasites, parasites thatoccur on the surface of their host (figure 46.14). They arelarger than most other mites and cause discomfort by suck-ing the blood of humans and other animals. Ticks can carrymany diseases, including some caused by viruses, bacteria,and protozoa. The spotted fevers (Rocky Mountain spottedfever is a familiar example) are caused by bacteria carriedby ticks. Lyme disease is apparently caused by spirochaetestransmitted by ticks. Red-water fever, or Texas fever, is animportant tick-borne protozoan disease of cattle, horses,sheep, and dogs.

Scorpions, spiders, and mites are all arachnids, thelargest class of chelicerates.


FIGURE 46.13Two common poisonous spiders. (a) The black widow spider, Latrodectus mactans.(b) The brown recluse spider, Loxosceles reclusa. Both species are common throughouttemperate and subtropical North America, but bites are rare in humans.

FIGURE 46.14Ticks (order Acari). Ticks (the large one is engorged) on thehide of a tapir in Peru. Many ticks spread diseases in humans andother vertebrates.

(a) (b)

Class Merostomata: HorseshoeCrabsA second class of chelicerates is the horseshoe crabs (classMerostomata). There are three genera of horseshoe crabs.One, Limulus (figure 46.15), is common along the EastCoast of North America. The other two genera live in theAsian tropics. Horseshoe crabs are an ancient group, withfossils virtually identical to Limulus dating back 220 millionyears to the Triassic Period. Other members of the class,the now-extinct eurypterans, are known from 400 millionyears ago. Horseshoe crabs may have been derived fromtrilobites, a relationship suggested bythe appearance of their larvae. Individ-uals of Limulus grow up to 60 centime-ters long. They mature in 9 to 12 yearsand have a life span of 14 to 19 years.Limulus individuals live in deep water,but they migrate to shallow coastal wa-ters every spring, emerging from thesea to mate on moonlit nights whenthe tide is high.

Horseshoe crabs feed at night, pri-marily on mollusks and annelids. Theyswim on their backs by moving theirabdominal plates. They can also walkon their four pairs of legs, protectedalong with chelicerae and pedipalps bytheir shell (figure 46.16).

Horseshoe crabs are a very ancientgroup.

Class Pycnogonida: The Sea SpidersThe third class of chelicerates is the sea spiders (class Pyc-nogonida). Sea spiders are common in coastal waters, withmore than 1000 species in the class. These animals are notoften observed because many are small, only about 1 to 3centimeters long, and rather inconspicuous. They arefound in oceans throughout the world but are most abun-dant in the far north and far south. Adult sea spiders aremostly external parasites or predators of other animals likesea anemones (figure 46.17).

Sea spiders have a sucking proboscis in a mouth located atits end. Their abdomen is much reduced,and their body appears to consist almostentirely of the cephalothorax, with nowell-defined head. Sea spiders usuallyhave four, or less commonly five or six,pairs of legs. Male sea spiders carry theeggs on their legs until they hatch, thusproviding a measure of parental care. Seaspiders completely lack excretory andrespiratory systems. They appear to carryout these functions by direct diffusion,with waste products flowing outwardthrough the cells and oxygen flowing in-ward through them. Sea spiders are notclosely related to either of the other twoclasses of chelicerates.

Sea spiders are very common inmarine habitats. They are notclosely related to terrestrial spiders.


FIGURE 46.15Limulus. Horseshoe crabs, emerging fromthe sea to mate at the edge of DelawareBay, New Jersey, in early May.

Operculum Walking legs

Book gills

Pedipalp

Chelicera

Mouth

Carapace

Cephalothorax

Abdomen

Telson

FIGURE 46.16Diagram of a horseshoe crab, Limulus, from below. This diagram illustrates theprincipal features of this archaic animal.

FIGURE 46.17A marine pycnogonid. The sea spider,Pycnogonum littorale (yellow animal),crawling over a sea anemone.

CrustaceansThe crustaceans (subphylum Crustacea) are a large groupof primarily aquatic organisms, consisting of some 35,000species of crabs, shrimps, lobsters, crayfish, barnacles,water fleas, pillbugs, and related groups (table 46.1). Mostcrustaceans have two pairs of antennae, three types ofchewing appendages, and various numbers of pairs of legs.All crustacean appendages, with the possible exception ofthe first pair of antennae, are basically biramous. In somecrustaceans, appendages appear to have only a singlebranch; in those cases, one of the branches has been lostduring the course of evolutionary specialization. Thenauplius larva stage through which all crustaceans pass(figure 46.18) provides evidence that all members of thisdiverse group are descended from a common ancestor.The nauplius hatches with three pairs of appendages andmetamorphoses through several stages before reachingmaturity. In many groups, this nauplius stage is passed inthe egg, and development of the hatchling to the adultform is direct.

Crustaceans differ from insects but resemble cen-tipedes and millipedes in that they have appendages ontheir abdomen as well as on their thorax. They are theonly arthropods with two pairs of antennae. Theirmandibles likely originated from a pair of limbs that tookon a chewing function during the course of evolution, aprocess that apparently occurred independently in thecommon ancestor of the terrestrial mandibulates. Manycrustaceans have compound eyes. In addition, they havedelicate tactile hairs that project from the cuticle all overthe body. Larger crustaceans have feathery gills near thebases of their legs. In smaller members of this class, gasexchange takes place directly through the thinner areas ofthe cuticle or the entire body. Most crustaceans have sep-arate sexes. Many different kinds of specialized copulationoccur among the crustaceans, and the members of someorders carry their eggs with them, either singly or in eggpouches, until they hatch.

Decapod Crustaceans

Large, primarily marine crustaceans such as shrimps, lob-sters, and crabs, along with their freshwater relatives, thecrayfish, are collectively called decapod crustaceans (figure46.19). The term decapod means “ten-footed.” In these ani-mals, the exoskeleton is usually reinforced with calciumcarbonate. Most of their body segments are fused into acephalothorax covered by a dorsal shield, or carapace,which arises from the head. The crushing pincers commonin many decapod crustaceans are used in obtaining food,for example, by crushing mollusk shells.

In lobsters and crayfish, appendages called swimmeretsoccur in lines along the ventral surface of the abdomen andare used in reproduction and swimming. In addition, flat-tened appendages known as uropods form a kind of com-pound “paddle” at the end of the abdomen. These animalsmay also have a telson, or tail spine. By snapping its ab-domen, the animal propels itself through the water rapidlyand forcefully. Crabs differ from lobsters and crayfish inproportion; their carapace is much larger and broader andthe abdomen is tucked under it.


Table 46.1 Tradional Classification of the PhylumArthropoda

Subphylum Characteristics Members

Chelicerata Mouthparts are chelicerae The chelicerates: spiders, horseshoe crabs, mites

Crustacea Mouthparts are mandibles; The crustaceans: biramous appendages lobsters, crabs,

shrimp, isopods, barnacles

Uniramia Mouthparts are mandibles; Chilopods uniramous appendages (centipedes),

diplopods (millipedes), and

insects


FIGURE 46.18Although crustaceans are diverse, they have fundamentallysimilar larvae. The nauplius larva of a crustacean is an importantunifying feature found in all members of this group.

Terrestrial and Freshwater Crustaceans

Although most crustaceans are marine, many occur in freshwater and a few have become terrestrial. These include pill-bugs and sowbugs, the terrestrial members of a large orderof crustaceans known as the isopods (order Isopoda). Abouthalf of the estimated 4500 species of this order are terres-trial and live primarily in places that are moist, at least sea-sonally. Sand fleas or beach fleas (order Amphipoda) areother familiar crustaceans, many of which are semiterres-trial (intertidal) species.

Along with the larvae of larger species, minute crus-taceans are abundant in the plankton. Especially significantare the tiny copepods (order Copepoda; figure 46.20),which are among the most abundant multicellular organ-isms on earth.

Sessile Crustaceans

Barnacles (order Cirripedia; figure 46.21) are a group ofcrustaceans that are sessile as adults. Barnacles have free-swimming larvae, which ultimately attach their heads to apiling, rock, or other submerged object and then stir foodinto their mouth with their feathery legs. Calcareous platesprotect the barnacle’s body, and these plates are usually at-tached directly and solidly to the substrate. Although mostcrustaceans have separate sexes, barnacles are hermaphro-ditic, but they generally cross-fertilize.

Crustaceans include marine, freshwater, and terrestrialforms. All possess a nauplius larval stage and branchedappendages.


Cheliped

EyeCephalothorax Abdomen

Swimmerets

Telson

Uropod

Antenna

Antennule

Walking legs

FIGURE 46.19Decapod crustacean. A lobster, Homarus americanus. The principal features are labeled.

FIGURE 46.20Freshwater crustacean. A copepod with attached eggs, a memberof an abundant group of marine and freshwater crustaceans (orderCopepoda), most of which are a few millimeters long. Copepodsare important components of the plankton.

FIGURE 46.21Gooseneck barnacles, Lepas anatifera, feeding.These are stalked barnacles; many others lack a stalk.

Millipedes, centipedes, and insects, three dis-tinct classes, are uniramian mandibulates. Theyrespire by means of tracheae and excrete theirwaste products through Malpighian tubules.These groups were certainly derived from annelids, probably ones similar to theoligochaetes, which they resemble in their embryology.

Classes Chilopoda and Diplopoda: TheCentipedes and MillipedesThe centipedes (class Chilopoda) and millipedes(class Diplopoda) both have bodies that consistof a head region followed by numerous seg-ments, all more or less similar and nearly allbearing paired appendages. Although the namecentipede would imply an animal with a hundredlegs and the name millipede one with a thousand, adult cen-tipedes usually have 30 or more legs, adult millipedes 60 ormore. Centipedes have one pair of legs on each body seg-ment (figure 46.22), millipedes two (figure 46.23). Eachsegment of a millipede is a tagma that originated duringthe group’s evolution when two ancestral segments fused.This explains why millipedes have twice as many legs persegment as centipedes.

In both centipedes and millipedes, fertilization is in-ternal and takes place by direct transfer of sperm. Thesexes are separate, and all species lay eggs. Young milli-pedes usually hatch with three pairs of legs; they experi-ence a number of growth stages, adding segments andlegs as they mature, but do not change in general appearance.

Centipedes, of which some 2500 species are known,are all carnivorous and feed mainly on insects. The ap-pendages of the first trunk segment are modi-fied into a pair of poison fangs. The poison isoften quite toxic to human beings, and manycentipede bites are extremely painful, some-times even dangerous.

In contrast, most millipedes are herbivores,feeding mainly on decaying vegetation. A fewmillipedes are carnivorous. Many millipedes canroll their bodies into a flat coil or sphere be-cause the dorsal area of each of their body seg-ments is much longer than the ventral one.More than 10,000 species of millipedes havebeen named, but this is estimated to be no morethan one-sixth of the actual number of speciesthat exists. In each segment of their body, mostmillipedes have a pair of complex glands thatproduces a bad-smelling fluid. This fluid is ex-

uded for defensive purposes through openings along thesides of the body. The chemistry of the secretions of dif-ferent millipedes has become a subject of considerable in-terest because of the diversity of the compounds involvedand their effectiveness in protecting millipedes from at-tack. Some produce cyanide gas from segments near theirhead end. Millipedes live primarily in damp, protectedplaces, such as under leaf litter, in rotting logs, under barkor stones, or in the soil.

Centipedes are segmented hunters with one pair of legson each segment. Millipedes are segmented herbivoreswith two pairs of legs on each segment.



FIGURE 46.22A centipede. Centipedes, like this member of the genus Scolopendra, are activepredators.

FIGURE 46.23A millipede. Millipedes, such as this Sigmoria individual, are herbivores.

Class Insecta: The InsectsThe insects, class Insecta, are by far the largest group of or-ganisms on earth, whether measured in terms of numbersof species or numbers of individuals. Insects live in everyconceivable habitat on land and in fresh water, and a fewhave even invaded the sea. More than half of all the namedanimal species are insects, and the actual proportion isdoubtless much higher because millions of additional formsawait detection, classification, and naming. Approximately90,000 described species occur in the United States andCanada, and the actual number of species in this area prob-ably approaches 125,000. A hectare of lowland tropical for-est is estimated to be inhabited by as many as 41,000species of insects, and many suburban gardens may have1500 or more species. It has been estimated that approxi-mately a billion billion (1018) individual insects are alive atany one time. A glimpse at the enormous diversity of in-sects is presented in figure 46.24 and later in table 46.2.


(a)

(b)

(d) (e) (f)

(c)

FIGURE 46.24Insect diversity. (a) Luna moth, Actias luna. Luna moths and their relatives are among the most spectacular insects (order Lepidoptera).(b) Soldier fly, Ptecticus trivittatus (order Diptera). (c) Boll weevil, Anthonomus grandis. Weevils are one of the largest groups of beetles(order Coleoptera). (d) A thorn-shaped leafhopper, Umbonica crassicornis (order Hemiptera). (e) Copulating grasshoppers (orderOrthoptera). (f) Termite, Macrotermes bellicosus (order Isoptera). The large, sausage-shaped individual is a queen, specialized for layingeggs; most of the smaller individuals around the queen are nonreproductive workers, but the larger individual at the lower left is areproductive male.

External Features

Insects are primarily a terrestrial group, and most, if notall, of the aquatic insects probably had terrestrial ances-tors. Most insects are relatively small, ranging in size from0.1 millimeter to about 30 centimeters in length orwingspan. Insects have three body sections, the head, tho-rax, and abdomen; three pairs of legs, all attached to thethorax; and one pair of antennae. In addition, they mayhave one or two pairs of wings. Insect mouthparts all havethe same basic structure but are modified in differentgroups in relation to their feeding habits (figure 46.25).Most insects have compound eyes, and many have ocellias well.

The insect thorax consists of three segments, each witha pair of legs. Legs are completely absent in the larvae ofcertain groups, for example, in most members of theorder Diptera (flies) (figure 46.26). If two pairs of wingsare present, they attach to the middle and posterior seg-ments of the thorax. If only one pair of wings is present, itusually attaches to the middle segment. The thorax is al-most entirely filled with muscles that operate the legs andwings.

The wings of insects arise as saclike outgrowths of thebody wall. In adult insects, the wings are solid except forthe veins. Insect wings are not homologous to the other ap-pendages. Basically, insects have two pairs of wings, but insome groups, like flies, the second set has been reduced to apair of balancing knobs called halteres during the course ofevolution. Most insects can fold their wings over their ab-domen when they are at rest; but a few, such as the dragon-flies and damselflies (order Odonata), keep their wingserect or outstretched at all times.

Insect forewings may be tough and hard, as in beetles.If they are, they form a cover for the hindwings and usu-ally open during flight. The tough forewings also serve aprotective function in the order Orthoptera, which in-cludes grasshoppers and crickets. The wings of insects aremade of sheets of chitin and protein; their strengtheningveins are tubules of chitin and protein. Moths and butter-flies have wings that are covered with detachable scalesthat provide most of their bright colors (figure 46.27). Insome wingless insects, such as the springtails or silverfish,wings never evolved. Other wingless groups, such as fleasand lice, are derived from ancestral groups of insects thathad wings.

Internal Organization

The internal features of insects resemble those of theother arthropods in many ways. The digestive tract is atube, usually somewhat coiled. It is often about the samelength as the body. However, in the order Hemiptera,which consists of the leafhoppers, cicadas, and relatedgroups, and in many flies (order Diptera), the digestivetube may be greatly coiled and several times longer thanthe body. Such long digestive tracts are generally found in


(a) (b) (c)

FIGURE 46.25Modified mouthparts in three kinds of insects. Mouthparts aremodified for (a) piercing in the mosquito, Culex, (b) sucking nectarfrom flowers in the alfalfa butterfly, Colias, and (c) sopping upliquids in the housefly, Musca domestica.

FIGURE 46.26Larvae of a mosquito, Culex pipiens. The aquatic larvae ofmosquitoes are quite active. They breathe through tubes from thesurface of the water, as shown here. Covering the water with athin film of oil causes them to drown.

insects that have sucking mouthpartsand feed on juices rather than onprotein-rich solid foods because theyoffer a greater opportunity to absorbfluids and their dissolved nutrients.The digestive enzymes of the insectalso are more dilute and thus less effec-tive in a highly liquid medium than in amore solid one. Longer digestive tractsgive these enzymes more time to workwhile food is passing through.

The anterior and posterior regionsof an insect’s digestive tract are linedwith cuticle. Digestion takes place pri-marily in the stomach, or midgut; andexcretion takes place throughMalpighian tubules. Digestive enzymesare mainly secreted from the cells thatline the midgut, although some arecontributed by the salivary glands nearthe mouth.

The tracheae of insects extendthroughout the body and permeate itsdifferent tissues. In many winged in-sects, the tracheae are dilated in vari-ous parts of the body, forming air sacs.These air sacs are surrounded by mus-cles and form a kind of bellows systemto force air deep into the tracheal sys-tem. The spiracles, a maximum of 10on each side of the insect, are pairedand located on or between the seg-ments along the sides of the thoraxand abdomen. In most insects, thespiracles can be opened by muscular action. Closing thespiracles at times may be important in retarding waterloss. In some parasitic and aquatic groups of insects, thespiracles are permanently closed. In these groups, the tra-cheae run just below the surface of the insect, and gas ex-change takes place by diffusion.

The fat body is a group of cells located in the insectbody cavity. This structure may be quite large in relationto the size of the insect, and it serves as a food-storagereservoir, also having some of the functions of a verte-brate liver. It is often more prominent in immature in-sects than in adults, and it may be completely depletedwhen metamorphosis is finished. Insects that do not feedas adults retain their fat bodies and live on the food storedin them throughout their adult lives (which may be veryshort).

Sense Receptors

In addition to their eyes, insects have several characteristickinds of sense receptors. These include sensory hairs,which are usually widely distributed over their bodies. The

sensory hairs are linked to nerve cellsand are sensitive to mechanical andchemical stimulation. They are partic-ularly abundant on the antennae andlegs—the parts of the insect mostlikely to come into contact with otherobjects.

Sound, which is of vital importanceto insects, is detected by tympanal or-gans in groups such as grasshoppersand crickets, cicadas, and somemoths. These organs are paired struc-tures composed of a thin membrane,the tympanum, associated with thetracheal air sacs. In many othergroups of insects, sound waves are de-tected by sensory hairs. Male mosqui-toes use thousands of sensory hairs ontheir antennae to detect the soundsmade by the vibrating wings of femalemosquitoes.

Sound detection in insects is impor-tant not only for protection but alsofor communication. Many insectscommunicate by making sounds, mostof which are quite soft, very high-pitched, or both, and thus inaudible tohumans. Only a few groups of insects,especially grasshoppers, crickets, andcicadas, make sounds that people canhear. Male crickets and longhornedgrasshoppers produce sounds by rub-bing their two front wings together.Shorthorned grasshoppers do so by

rubbing their hind legs over specialized areas on theirwings. Male cicadas vibrate the membranes of air sacs lo-cated on the lower side of the most anterior abdominalsegment.

In addition to using sound, nearly all insects communi-cate by means of chemicals or mixtures of chemicalsknown as pheromones. These compounds, extremely di-verse in their chemical structure, are sent forth into theenvironment, where they are active in very small amountsand convey a variety of messages to other individuals.These messages not only convey the attraction and recog-nition of members of the same species for mating, butthey also mark trails for members of the same species, asin the ants.

All insects possess three body segments (tagmata): thehead, the thorax, and the abdomen. The three pairs oflegs are attached to the thorax. Most insects havecompound eyes, and many have one or two pairs ofwings. Insects possess sophisticated means of sensingtheir environment, including sensory hairs, tympanalorgans, and chemoreceptors.


FIGURE 46.27Scales on the wing of Parnassiusimperator, a butterfly from China. Scalesof this sort account for most of the coloredpatterns on the wings of butterflies andmoths.

Insect Life HistoriesMost young insects hatch from fertilized eggs laid outsidetheir mother’s body. The zygote develops within the egginto a young insect, which escapes by chewing through orbursting the shell. Some immature insects have specializedprojections on the head that assist in this process. In a fewinsects, eggs hatch within the mother’s body.

During the course of their development, young insectsundergo ecdysis a number of times before they becomeadults. Most insects molt four to eight times during thecourse of their development, but some may molt as manyas 30 times. The stages between the molts are called in-stars. When an insect first emerges following ecdysis, it ispale, soft, and especially susceptible to predators. Its ex-oskeleton generally hardens in an hour or two. It mustgrow to its new size, usually by taking in air or water, dur-ing this brief period.

There are two principal kinds of metamorphosis in in-sects: simple metamorphosis and complete metamor-phosis (figure 46.28). In insects with simple metamorpho-sis, immature stages are often called nymphs. Nymphs areusually quite similar to adults, differing mainly in theirsmaller size, less well-developed wings, and sometimescolor. In insect orders with simple metamorphosis, such asmayflies and dragonflies, nymphs are aquatic and extractoxygen from the water through gills. The adult stages areterrestrial and look very different from the nymphs. Inother groups, such as grasshoppers and their relatives,nymphs and adults live in the same habitat. Such insectsusually change gradually during their life cycles with re-spect to wing development, body proportions, the appear-ance of ocelli, and other features.

In complete metamorphosis, the wings develop inter-nally during the juvenile stages and appear externallyonly during the resting stage that immediately precedesthe final molt (figure 46.28b). During this stage, the in-sect is called a pupa or chrysalis, depending on thegroup to which it belongs. A pupa or chrysalis does notnormally move around much, although mosquito pupaedo move around freely. A large amount of internal bodyreorganization takes place while the insect is a pupa orchrysalis.

More than 90% of the insects, including the members ofall of the largest and most successful orders, display com-plete metamorphosis. The juvenile stages and adults oftenlive in distinct habitats, have different habits, and are usu-ally extremely different in form. In these insects, develop-ment is indirect. The immature stages, called larvae, areoften wormlike, differing greatly in appearance from theadults. Larvae do not have compound eyes. They may belegless or have legs or leg-like appendages on the abdomen.They usually have chewing mouthparts, even in those or-ders in which the adults have sucking mouthparts; chewingmouthparts are the primitive condition in these groups.When larvae and adults play different ecological roles, they

do not compete directly for the same resources, an advan-tage to the species.

Pupae do not feed and are usually relatively inactive. Aspupae, insects are extremely vulnerable to predators andparasites, but they are often covered by a cocoon or someother protective structure. Groups of insects with completemetamorphosis include moths and butterflies; beetles; bees,wasps, and ants; flies; and fleas.

Some species of insects exhibit no dramatic change inform from immature stages to adult. This type of develop-ment is called ametabolus (meaning without change) andis seen in the most primitive orders of insects such as thesilverfish and springtails.

Hormones control both ecdysis and metamorphosis.Molting hormone, or ecdysone, is released from a glandin the thorax when that gland has been stimulated by brainhormone, which in turn is produced by neurosecretorycells and released into the blood. The effects of the moltinginduced by ecdysone are determined by juvenile hormone,which is present during the immature stages but declines inquantity as the insect passes through successive molts.When the level of juvenile hormone is relatively high, themolt produces another larva; when it is lower, it producesthe pupa and then the final development of the adult.

Insects undergo either simple or completemetamorphosis.


Egg

(a)

(b)

Egg Earlylarva

Nymphs

Full-sizedlarva

Pupa

Adult chinch bug

Adult housefly

FIGURE 46.28Metamorphosis. (a) Simple metamorphosis in a chinch bug(order Hemiptera), and (b) complete metamorphosis in a housefly,Musca domestica (order Diptera).


Table 46.2 Major Orders of Insects

ApproximateTypical Number of

Order Examples Key Characteristics Named Species

Coleoptera

Diptera

Lepidoptera

Hymenoptera

Hemiptera

Orthoptera

Odonata

Isoptera

Siphonaptera

Beetles

Flies

Butterflies, moths

Bees, wasps, ants

True bugs,bedbugs,leafhoppers

Grasshoppers,crickets, roaches

Dragonflies

Termites

Fleas

The most diverse animal order; two pairs of wings; front pair of wings is a hard cover that partially protects the transparent rear pair of flying wings; heavily armored exoskeleton; biting and chewingmouthparts; complete metamorphosis

Some that bite people and other mammals are consideredpests; front flying wings are transparent; hind wings are reduced to knobby balancing organs;sucking, piercing, and lapping mouthparts; completemetamorphosis

Often collected for their beauty; two pairs of broad, scaly,flying wings, often brightly colored; hairy body; tubelike,sucking mouthparts; complete metamorphosis

Often social, known to many by their sting; two pairs of transparent flying wings; mobile head and well-developed eyes; often possess stingers; chewing andsucking mouthparts; complete metamorphosis

Often live on blood; two pairs of wings, or wingless;piercing, sucking mouthparts; simple metamorphosis

Known for their jumping; two pairs of wings or wingless; among the largest insects; biting and chewing mouthparts in adults; simple metamorphosis

Among the most primitive of the insect order; two pairsof transparent flying wings; large, long, and slender body;chewing mouthparts; simple metamorphosis

One of the few types of animals able to eat wood; twopairs of wings, but some stages are wingless; social insects;there are several body types with division of labor;chewing mouthparts; simple metamorphosis

Small, known for their irritating bites; wingless; smallflattened body with jumping legs; piercing and suckingmouthparts; complete metamorphosis

350,000

120,000

120,000

100,000

60,000

20,000

5,000

2,000

1,200


Chapter 46 Summary Questions Media Resources

46.1 The evolution of jointed appendages has made arthropods very successful.

• Jointed appendages and an exoskeleton greatlyexpanded locomotive and manipulative capabilitiesfor the arthropod phyla, the most successful of allanimals in terms of numbers of individuals, species,and ecological diversification.

• Traditionally, arthropods have been grouped intothree subphyla based on morphological characters butnew research is calling this classification of thearthropods into question.

• Like annelids, arthropods have segmented bodies, butsome of their segments have become fused intotagmata during the course of evolution. All possess arigid external skeleton, or exoskeleton.

1. What are the advantages of anexoskeleton? What occursduring ecdysis? What controlsthis process?2. What type of circulatorysystem do arthropods have?Describe the direction of bloodflow. What helps to maintainthis one-way flow?3. What are Malpighian tubules?How do they work? What othersystem are they connected to?How does this system processwastes? How does it regulatewater loss?

• Chelicerates consist of three classes: Arachnida(spiders, ticks, mites, and scorpions); Merostomata(horseshoe crabs); and Pycnogonida (sea spiders).

• Spiders, the best known arachnids, have a pair ofchelicerae, a pair of pedipalps, and four pairs ofwalking legs. Spiders secrete digestive enzymes intotheir prey, then suck the contents out.

4. Into what two groups arearthropods traditionally divided?Describe each group in terms ofits mouthparts and appendages,and give several examples ofeach.


• Crustaceans comprise some 35,000 species of crabs,shrimps, lobsters, barnacles, sowbugs, beach fleas,and many other groups. Their appendages arebasically biramous, and their embryology isdistinctive.

5. On which parts of the bodydo crustaceans possess legs?6. How do biramous anduniramous appendages differ?


• Centipedes and millipedes are segmented uniramia.Centipedes are hunters with one pair of legs persegment, and millipedes are herbivores with two pairsof legs per segment.

• Insects have three body segments, three pairs of legs,and often one or two pairs of wings. Many havecomplex eyes and other specialized sensorystructures.

• Insects exhibit either simple metamorphosis, movingthrough a succession of forms relatively similar to theadult, or complete metamorphosis, in which an oftenwormlike larva becomes a usually sedentary pupa, andthen an adult.

7. How are millipedes andcentipedes similar to each other?How do they differ?8. What type of digestive systemdo most insects possess? Whatdigestive adaptations occur ininsects that feed on juices low inprotein? Why? 9. What is an instar as it relatesto insect metamorphosis? Whatare the two different kinds ofmetamorphosis in insects? Howdo they differ?


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• Arthropods

• EnhancementChapter: ArthropodTaxonomy, Sections 1 and 2

• EnhancementChapter: ArthropodTaxonomy, Section 3

• EnhancementChapter: ArthropodTaxonomy, Section 4

• Student Research:Insect Behavior

chapter 46 biology book

Documents

major arthropod groups

functional groups

array of appendages

chitinous exoskeleton

cowthe exoskeleton

exoskeleton ismade

classification of arthropods

class merostomata