Groups of Arthropods

Arthropods are classified based on the number and structure of their body segments and appendages, especially their mouthparts. There are 3 major groups: crustaceans, spiders, and insects.

Crustaceans This is the subphylum Crustacea. Most members are aquatic. This group contains crabs, shrimps, lobsters, crayfishes, and barnacles. They can range in size. Pill bugs are the only terrestrial crustacean. Crustaceans have 2 pairs of antennae, 2 or 3 body segments, and chewing mouthparts called mandibles.

It has a cephalothorax which is a head and thorax fused together It has a cephalothorax which is a head and thorax fused together. The thorax has most of the internal organs. The abdomen is the tail part of the animals. The carapace is the part of the exoskeleton that covers the cephalothorax.

They have antennae, which are sense organs. The second pair of appendages are for filter feeding or swimming. The 3rd pair are mandibles – this is a mouthpart used for biting and grinding food. Crabs, lobsters, and crayfish have 5 pairs of legs. The first pair is called chelipeds. This pair can catch, pick up, crush, and cut food. The last 4 pairs of legs are swimmerets (used for swimming).

Spiders and Their Relatives Horseshoe crabs, spiders, ticks, and scorpions are members of the subphylum Chelicerata. Chelicerates have mouthparts called chelicerae and 2 body sections, and 4 pairs of walking legs. They have a cephalothorax and abdomen. The abdomen has most of the internal organs.

Chelicerae are fangs that are used to stab and paralyze prey. Pedipalps are used to grab prey. They use book lungs or book gills. There are 2 main classes: Merostomata – horseshoe crabs and Arachnida – spiders, mites, ticks, and scorpions.

Horseshoe Crabs These are not really crabs but they have heavy armor and are more closely related to a spider. They have chelicerae, 5 pairs of walking legs, and a spikelike tail used for movement.

Spiders They can capture many different prey. Some can spin webs to help capture prey. They do not have jaws for chewing so they liquefy their food and then swallow it. A spider will inject its prey with venom then a special enzyme to breakdown tissue.

All spiders produce silk, but do not necessarily spin webs. Spiders use silk for webs, cocoons for eggs, and wrapping up prey. They have spinnerets that produces the silk.

Mites and Ticks Many mites and ticks are parasitic.

Scorpions Scorpions have claws. They also have a venomous stinger that can kill or paralyze prey.

Insects and Their Relatives This is the subphylum Uniramia and contains insects, centipedes, and millipedes. They have jaws, 1 pair of antennae, and unbranched appendages.

Centipedes Class Chilopoda More than 100 pairs of legs. Usually 1 pair of legs per body segment. Carnivores Lives beneath rocks or in the soil. They have a painful bite.

Millipedes Class Diplopoda They have 2 pairs of legs per segment. Live under rocks and decaying logs. Feed on dead or decaying plant material. They can release a stinky smell.