Diversity and Evolution in Arthropods

  • Trilobita - oldest subphylum of arthropods and were dwellers in ancient seas
  • Chelicerata - includes spiders, ticks, mites, scorpions, and horseshoe crabs
  • Crustacea - includes familiar organisms such as crabs and shrimp
  • Uniramia - includes most arthropods; centipedes, millipedes, and all insects including bees, moths, grass hoppers, fleas, and beetles
  • first arthropod appeared in the sea more than 600 million years ago
  • since then, they have experienced several adaptive radiations
  • some colonized all parts of the sea and most fresh water habitats
  • ancestors of the arthropods were soft-bodied animals that left few fossils
  • the body form of the earliest arthropods is thought to be similar to the trilobites
  • typical trilobite's body had a thick, tough out covering and was composed of many segments
  • each appendage was divided to form two branches, one a walking leg and the other a feather like gill
  • many body parts found in their embryos fuse into larger parts during development
  • arthropod appendages have become increasingly specialized for feeding, locomotion, and other functions

Form and Function in Arthropods

  • they have a tough exoskeleton, a series of jointed appendages, and a segmented body
  • other characteristics include a brain located in the dorsal part of the head, ventral nerve cord, and an open circulatory system powered by a single heart

Body Plan

  • Exoskeleton - system of external supporting structures that are made out of the carbohydrate, Chitiin
  • some exoskeletons(most insects), are leathery and flexible
  • others(ticks, lobsters), have a hard exoskeleton
  • exoskeletons of many terrestrial arthropods are water proof, this adaptation restricts the loss of water from the body, making it possible for arthropods to live in dry environments
  • exoskeletons help arthropods to move efficiently and adapt to their environment
  • disadvantage - exoskeleton is a solid coating, not a living tissue, so it does not allow the animal to grow
  • jointed appendages
  • appendages include antennae, claws, walking legs, wings, flippers, and other specialized structures
  • all arthropods have segmented bodies
  • other arthropods such as insects, spiders, and crabs, lost their segments in the course of evolution or had the segments fuse together to form larger body parts.


  • appendages of arthropods have evolved in way that enable these animals to eat almost any food
  • every mode of feeding is seen in arthropods (herbivores, carnivores, parasites, filter feeders, detrius feeders)
  • some internal parasites passively absorb nutrients through the body wall, where as others eat away at the host from the inside
  • marine arthropods are filter feeders that use comb like bristles on their mouth parts or legs to filter tiny plants and animals from the water


  • three basic types of respiratory structures - gills, book gills, book lungs, tracheal tubes
  • few species have both book lungs and tracheal tubes
  • some species lack specialized respiratory organs
  • many aquatic arthropods have girlls that are formed from part of the same appendages that form mouth parts and legs
  • most terrestrial arthropods have another respiratory device found in no other animal
  • long branching tracheal tubes reach deep into animals' tissues
  • network of tracheal tubes supplies oxygen by diffusion to all body tissues
  • movement of their body muscles cause the tracheae to shrink and expand, pumping fresh air in and out of the spiracles

Internal Transport

  • open circulatory system
  • some spiders and insects - heart is long and narrow and stretches along the abdomen
  • lobsters and crayfish - heart is smaller and lies about halfway down the body