Phylum Arthropoda

It doesn’t get any bigger than this!

Major Features

• An exoskeleton (external skeleton) made out of chitin.

• Must molt (shed their skeleton) in order to grow.

• Have jointed appendages– Serve a variety of roles (walking, swimming, repro,

eating, sensing)• Three body regions – head, thorax, abdomen.

Features cont’d

• Well-developed nervous system– Includes a brain and a ventral nerve cord– Includes sense organs (antennae, compound eyes)

Diversity

Classification

• Broken into subphyla.– Subphylum Crustacea– Subphylum Uniramia– Subphylum Chelicerata

Subphylum Crustacea

• “Crustaceans” – barnacles, shrimp, lobster, crab, crayfish… marine arthropods.

• The sow bug is a terrestrial example.

Crustacea cont’d

(We’ll use crayfish as our representative)External structure:• Cephalothorax – skeleton over head and thorax

are fused.• Head has compound eyes, antennae, and pairs of

mouth appendages.• Thorax has 5 pairs of limbs (4 pairs of walking legs,

1 pair of pincers [chelipeds]).• Abdomen has swimmerets, uropods and telson.

Crustacea cont’d

Crustacea cont’d

Internal structure:• Digestive system – 2-chambered stomach,

digestive glands, intestine. Green glands for excretion.

• Cardiovascular system – heart pumps blood into the space surrounding the internal organs (the “hemocoel”).– Blood contains a blue pigment (hemocyanin).

Crustacean circulation

Crustacea cont’d

Internal structure cont’d:• Nervous system – brain, ventral nerve cord,

and ganglia in several segments.– Receptors line antennae – include chemical

receptors and force receptors.• Reproduction – separate genders.– Sperm transferred by 1st pair of swimmerets.– Females carry fertilized eggs on their swimmerets.

Crustacea cont’d

Subphylum Uniramia

• “Insects” – most diverse group on Earth.

General structure:• Head has compound or simple eyes,

antennae, and mouth appendages.• Thorax has 3 pairs of legs– Sometimes includes wings.

• Abdomen stores the internal organs.

Insects cont’d

We will use grasshoppers as our representative.External structure:• 3rd pair of legs adapted for jumping.• Has two pairs of wings.• Females – have a posterior structure called an

ovipositor for digging holes to lay eggs in.• Has tympanum – a thin membrane – on

abdomen for hearing.

Grasshopper Exterior

Insects cont’d

Internal Structure:• Digestive system – complete, with mouth, stomach,

intestine, and anus.• Excretion – uses structures called Malpighian

tubules that release uric acid into intestines for disposal.

• Respiration – uses openings in the exoskeleton called spiracles that that lead into a trachea.– Air is pumped by the contraction and relaxation of the

body wall.

Spiracles and Malpighian Tubules

Insects cont’d

• Circulation – a heart pumps hemolymph into the aorta, which empties into a hemocoel (open space around organs).– Hemolymph in insects is NOT used to carry O2, so

it has no pigment.• Reproduction – Fertilization is internal.

Genders are separate.– Fertilized eggs are ejected into the ground.

Circulation and Reproduction

Insects cont’d

• Metamorphosis: a change in physiology and anatomy that occurs as an insect matures from a larva to an adult.– (Many insect larvae, including grasshoppers, are

called nymphs).

Subphylum Chelicerata

Arachnids (spiders, ticks, scorpions, mites).General Features:• Cephalothorax– Has 6 pairs of appendages attached to it• 4 pairs are walking legs• 1 pair are pedipalps – these sense and hold prey• 1 pair are fangs (chelicerae)

• Abdomen – stores internal organs

Arachnid structure

More arachnids!

Arachnids cont’d(Representative organism = spiders)• Capable of delivering venom to prey through

fangs.• Digestion: prey is injected with digestive

juices. Juices gradually digest prey.– Spider will “suck up” liquefied prey to complete

digestion.

Arachnids cont’d

• Respiration: Uses “book lungs” – folds of tissue inside the body wall.– The folded surface provides plenty of room for gas

exchange.