Arthropods

Dual Insect EvolutionArthropods -- a phylum that includes crustaceans, spiders, and insects -- are the most successful animals on Earth. Having conquered land, sea, and air, they account for more than 75% of all living animals. Despite their dominance, evolutionary relationships among different groups of arthropods have been consistently controversial. One generally agreed upon characteristic is that all six-legged arthropods, or hexapods -- a group dominated by insects but also including a variety of wingless six-legged groups -- arose once from a common ancestor. But a report by Nardi et al. (http://www.sciencemag.org/cgi/content/short/299/5614/1887) in the 21 Mar 2003 Science now suggests otherwise. The researchers presented mitochondrial genome sequence data that indicate that Collembola, a group of wingless hexapods considered basal to all insects, actually evolved independently and branched off much earlier from the separation of insects and many crustaceans. These results suggest that adaptation from water to land occurred at least twice in hexapods. As noted in an accompanying Perspective by R. H. Thomas (http://www.sciencemag.org/cgi/content/short/299/5614/1854), the Nardi et al. study should prompt collection of more sequence data from other taxa, spur reevaluation of morphological data, and lead to "a more solid understanding of how six-legged animals colonized and then took over the industrial world."

Most Successful Animals Largest number of species and of

individuals. Over 1,000,000 species approx. 80% of known animal species and probably only half are known.

Most Successful Animals Occupy widest stretches of territory. Found

everywhere on land and water where temp. above freezing long enough to permit breeding. This includes both polar regions, ocean depths of 14,000 feet or more and the highest mountains (a spider found at 22,000 ft. on Mt. Everest). They occupy the greatest variety of habitats of any group Arthropods are the only animals other than the vertebrates

which can fly.

 Extremely Important Ecologically Consume the largest variety and amounts of

food Are themselves the major food supply of all

larger predacious animals.

Arthropod definition.

Bilaterally symmetrical, segmented protostomes, having a chitinous exoskeleton, chitinous jointed exoskeleton, chitinous jointed appendages, a dorsal heart with ostia, a greatly reduced coelom replaced by a hemocoel and completely lacking cilia. Arthropod definition.

Tagmosis In some elongate forms, (i.e. centipedes) the

pattern of serial succession of identical segments is clear

More highly organized forms - groups of segments (tagmata) are structurally marked off from other groups and specialized to perform certain functions for whole organism. Division of labor results.

Each major group shows a specific pattern of tagmosis.

Tagmosis

Chelicerates (e.g. spiders and scorpions) - cephalothorax and abdomen

Crustaceans - head, thorax and abdomen. Head has 5-6 segments all with appendages, thorax has 8 segments with limbs, abdomen has 6 segments and all can have limbs.

Insects - head, thorax and abdomen. Head has 6 segments with appendages, if present, modified into mandibles. Thorax has 3 segments each of which contains a pair of adult walking legs. Abdomen has 11 segments and no appendages.

Tagmosis

Fusion, loss or modification of segments, as well as tagmosis, may obscure regularity of segmental pattern. Easy to modify and delete what not necessary. Aid to evolution. Arthropods no longer used coelom (or pseudocoelom) for hydrostatic skeleton.

 Jointed chitinous exoskeleton. Covers entire outside of body and fore and

hind gut; tracheal (gas exchange) tubules of insects, centipedes, millipedes, and some arachnids; and parts of the reproductive systems of some groups.

All must be shed when molting.

Layers of Exoskeleton

 Epicuticle. Thin outer, non-chitinous layer of lipids especially waxes . Provides nonwettable, impermeable coat which even protects against bacterial invasion.

  Procuticle

The procuticle has two layers; the exocuticle and the endocuticle.

Exocuticle Layer under the epicuticle. Chitin is an insoluble, horny nitrogenous complex

polysaccharide and protein bound together to form a complex glycoprotein.

In the exocuticle, the glycoprotein has been tanned. This process further stabilizes the molecular structure of the material by adding cross linkages created using chemicals called phenols.

Provides rigidity and a lightweight protective armor. The exocuticle is not tanned at joints and places

where the skeleton ruptures during molting.

Endocuticle

Thicker, more complex layer. Contains the same glycoprotein, but not tanned. Impregnated with CaCO3 and CaPO3 in the crustaceans to create a hard shell-like surface.

Exoskeleton The hard portions are linked by joints --> lever

systems of locomotion. Sites provided for origin and insertion of muscles led

to increase in nervous control and increase in sensory systems.

Specific muscle slips, crossing specialized joints provide specialized movements.

Not necessary to sacrifice mobility for protection as did mollusks.

In some places the waxy epicuticle is absent and the entire exoskeleton is thin, making it permeable to gasses and water so the cuticle covering gills of crustaceans is no barrier to gas exchange.

Exoskeleton Provides superiority over annelid movement Metamerism provided annelids with subdivided muscle system

and a hydrostatic skeleton. increased mechanical efficiency. Better off than other worms.

Arthropod levers more efficient. Less waste of energy. Greater precision. Circular and longitudinal muscles in annelids contrasted with flexors and extensors in arthropods. Increased force can be exerted more precisely against the environment per unit volume of muscle, therefore the arthropod system is faster, more precisely directed and more efficient in terms of energy expenditure.

http://www.pbs.org/kcet/shapeoflife/episodes/conq_explo2.html

Epidermis

Consists of ectodermal cells which secrete cuticle. Has stout basement membrane to which muscles can be attached.

Strengthened by intracellular fibers which extend into the cuticle. Morphogenesis and physiology very interesting. No time.

Jointed Appendages

Primitively one pair to each segment, but number often reduced.

Four basic functions:  locomotion - swimming or walking feeding - creation of a water current, straining

food, food capture, mandibles. respiration - particularly if they form flat surfaces

they can act as gills sensory organs bearing receptors.

Jointed Appendages Primitively the appendages were involved in

all four functions, but in more advanced arthropods the limbs are more specialized for  only one or two of them.

To operate the appendages there is a complex muscular system with attachments to the exoskeleton, striated muscles for rapid actions, and smooth muscles for visceral organs.

No cilia are found anywhere in arthropods.

General features Although very diverse externally, arthropods

are remarkably similar in many internal features.

Reduced coelom (in embryo find series of mesodermal cavities, in adult these are reduced to the cavities of the gonads). The green glands (excretory organs in the antennae) of the decapod crustaceans are believed to be coelomoducts as are the coxal glands of chelicerates.

General features Hemocoel. Most of the body cavity consists

of hemocoel (sinuses, or spaces, in the tissues filled with blood).

Blood can be pigmented (hemocyanin or hemoglobin) or colorless. Often find wandering amoebocytes in the blood.

Respiration by body surface, gills in aquatic species, tracheae (air tubes) in insects, book gills or book lungs in chelicerates.

Digestion

Mouthparts modified from appendages and adapted for different methods of feeding.

Entirely extracellular Foregut (ectodermal) for mechanical

processing Midgut (endodermal) for chemical processing

and absorption Hindgut (ectodermal) for forming fecal pellets

Circulatory system

Open system with dorsal contractile heart with ostia (holes) and few, if any lateral blood vessels.

Blood goes from heart to arteries, to sinuses where it bathes tissues directly. No veins.

Heart is neurogenic - requires nervous system to maintain beat, and both acetylcholine and adrenaline increase heart rate in crustaceans. Not antagonistic as in the vertebrates.

Excretion Paired excretory glands = coxal (some

chelicerates), antennal or maxillary glands (in crustaceans)

Malphigian tubules found in spiders and insects (among others) = tubules which open into the digestive tube and allow excretory material to be passed out through the anus.

Terrestrial arthropods usually excrete uric acid crystals to conserve water.

Sense organs Greater variety than in any other phylum. Eyes, antennae (touch and chemical

receptors) statocysts, auditory organs, sensory bristles (hairs, spines, scales and pits) wind sensors for flight, etc.

Reproduction Most are dioecious, use modified appendages for

copulation. Fertilization is internal in terrestrial forms and both

internal and external in aquatic. Often there is metamorphosis and a few forms are

parthenogenetic. Eggs have a nucleus surrounded by a small island

of cytoplasm and large yolk mass. This causes the protostomatous spiral cleavage to be modified. Cleavage is superficial and usually get 8-9 divisions before cell membranes form. Development resembles chick development.

Molting (ecdysis) -- steps Epidermis secretes inactive form of enzymes at the

base of the skeleton. Epidermis then detaches from skeleton and secretes new epicuticle.

The inactive enzymes now become activated and digest the untanned endocuticle. Ca and proteins from old cuticle are reabsorbed.

New procuticle secreted beneath new epicuticle. Longitudinal rupturing of old cuticle along dorsal or

lateral sides of body. Animal pulls self out

Molting Growth possible because new cuticle is soft

and pliable. Animal takes in water or air to expand it. Gradually grows new tissues to fill new

armor. Growth is therefore gradual. Instars or stadia are growth periods between

molts. Segments are added and pairs of appendages.

Metamorphosis occurs at various molts.

Molting Disadvantage. Animal vulnerable. Dead white and

conspicuous until the cuticle hardens (soft shell crabs). Movement restricted because of soft skeleton.

Behavioral changes associated with molting. Most hide. Some build web. Occasionally reverse normal reflex patterns

(e.g. light gradients) Land crabs need privacy or won't molt.

Hormones get fouled. Sensory input from disturbing factors increases secretion of inhibitory hormone from brain and prevents secretion of molting hormone. Other crabs eat molting crabs. Can't even trust your relatives.

 Neuro-endocrine controls in crustaceans.

Hormonal control summary  X-organ in eye stalk produces neurosecretory

hormone which inhibits molting and is stored in the sinus gland.

Y gland located in antennary or 2nd maxillary segments produces molting hormone. Degenerates in animals which reach terminal stage and no further growth occurs.

When the CNS receives a stimulus that it is time to molt, the sinus gland stops releasing the molt inhibitor and the Y-organ is free to release its ecdysone and the molting process begins.

Similar in insects, but different organs secrete the various hormones.

Reasons for arthropod success Exoskeleton. Tough, resistant, insoluble in water,

alkalis, and weak acids. Provides protection. Prevents desiccation Permits

joints to form. Serves for muscle attachments. Often lines foregut and hindgut, lines and supports

the trachea and can be adapted for biting mouthparts, sensory organs, copulatory organs, and ornamental purposes. (Problem with it is necessity to molt and limitation on size caused by weight of muscles necessary to operate it as arthropod gets larger)

Reasons for arthropod success Segmentation and appendages for more efficient

locomotion. Typically each segment is provided with a pair of jointed appendages, but this arrangement is often modified, with both segments and appendages specialized for adaptive functions.

The limb segments are essentially hollow levers moved by internal muscles, most of which are striated for rapid action. Jointed appendages are equipped with sensory hairs and have been modified and adapted for sensory functions, food handling, swift and efficient walking legs and swimming appendages, and respiration.

Reasons for arthropod success Air piped directly to cells. Highly efficient

for mall terrestrial animal, but limits size. = Major restriction on size of terrestrial arthropods. Aquatic arthropods use efficient gills.

Highly developed sensory organs so they are keenly aware of what is occurring in their environment.

Reasons for arthropod success Complex behavior patterns. Exceed most

other invertebrates in the complexity and organization of their activities (e.g. social behavior in some insects). Much of their behavior is innate because of limitations on size. Have small brains with no room for redundancy. Miniaturize system and develop social behavior to compensate. Learning important in some.

Reasons for arthropod success Reduced competition through

metamorphosis. Many arthropods pass through metamorphic changes, including a larval form quite different from the adult in structure. The larval form is often adapted for eating a different kind of food from that of the adult, resulting in less competition within a species.

Living Subphyla

Chelicerates. Spiders, scorpions, ticks, mites, horseshoe crabs, etc. Large subphylum with thousands of species, many of which are economically important. Two outstanding traits set them apart from other arthropods.

They have no antennae, They have no mandibles. Instead, the first

pair of appendages are primitively pincer-like chelicerae.

 Cephalothorax Anterior region is never divided into separate

head and thorax regions, but is made up of the head, mouthparts and segments bearing the walking legs. = cephalothorax.   Second pair of appendages are pedipalps,

sometimes leg-like and sensory, but sometimes claw-like and prehensile.

Next four segments usually complete the cephalothorax. Each segment bears a pair of walking legs.  

Abdomen Posterior tagma has reduced or modified

appendages. = abdomen. Has maximum of 13 segments and a telson (= terminal structure of the posterior end, containing sting of scorpion) Abdominal appendages are always reduced or highly modified.

Abdomen First segment is pregenital segment which is

always reduced or modified in some manner. Second segment is genital segment, which

contains the gonopores. Next segments may bear appendages

modified for respiration and converted into book gills or book lungs. These are replaced by tracheae in some arachnids.

 Excretory organs

May be highly modified nephridia = coxal glands, or may be Malpighian tubules, attached to the digestive tube at junction of midgut and hindgut.

Classes of Chelicerate Subphylum Merostomata. Includes the horseshoe crab.

www.enn.com/.../1999/06/062199/ horseshoecrab_3773.asp

Classes of Chelicerate Subphylum Pycnogonida. Very

modified group, but placed with chelicerates because of brain structure. Most only few mm long. Highly specialized browsers which creep over the surface of colonial hydroids and live on or in clams. Grasp part of zooid by chelicerae and suck it into the mouth.

http://www.dc.peachnet.edu/~pgore/students/s98/aprylle/arthropod2.htm

Class Arachnida Most successful chelicerates. Include scorpions,

tarantulas, spiders and mites.  Well over 50,000 species. Compensate for small size by large numbers, and are ecologically important in most terrestrial envirs. Make up 18 percent of the arthropod macrofaunna of Illinois deciduous forests and are predominant in soil. About 20,000 mites per cubic meter are reported from soil 10 to 20 cm below the surface in tropical and temperate forests; over 160,000 mites per cubic meter have been found in disturbed grassland soil in England. First arachnids to invade land.

Subphylum Crustaceans

More than 30,000 species. Include lobsters, shrimp, crabs, crayfish, barnacles, etc. Mostly marine, some fresh water and a few terrestrial. Major ecological and economical importance.

Defining Characteristics

Mandibles derived from modified appendages.

Two pairs of antennae Biramous appendages (at least primitively) Respiration (gills usually), excretion. Nervous

system and circulation follow basic arthropod plan.  Compound eye is typical of phylum

SubphylumUniramia (myriapod - insect line). Mandibles One pair of antennae Uniramous appendages

SubphylumUniramia Respiratory system is through body and

tracheal system which delivers air directly to the tissues. Simple, direct and easy to molt.

But limits size (problem only in terms of brain size) and makes them susceptible to aerosol poisons.

Compensate for brain size problem by: miniaturized cells, but 10,000,000 is maximum for

large active social wasp. Social behavior --> super-organism

SubphylumUniramia

Can fly Social insects show fantastic behavioral

adaptations to problems of arthropod body plan.