The first arthropods probably diverged from their primitive annelid (worm-like) ancestors about 600 million years ago. The entire phylum may have developed from a single common ancestor (monophyletic hypothesis) or animals with arthropod-like traits may have had several independent origins (polyphyletic hypothesis). In any case, these first arthropods were distinctive because they had a chitinous exoskeleton, jointed appendages with claws, and a well-defined head capsule with specialized feeding appendages.
There are three main branches on the arthropod's family tree:
- Trilobita -- marine arthropods that were abundant in prehistoric times but became extinct about 245 million years ago. Very little is known about their ecology.
- Chelicerata -- includes spiders, ticks, mites, scorpions, and horseshoe crabs. These are predatory animals with fang-like mouthparts (chelicerae) that contain a poison gland.
- Mandibulata -- includes crustaceans, myriapods, and insects. These animals have mandibles and maxillae among their mouthparts. They represent a wide range of ecological adaptations.
One group of Mandibulata (class Crustacea) became adapted to marine life -- they currently populate a wide range of habitats throughout the world's oceans. Another group, the ancestors of myriapods (e.g. millipedes and centipedes) became adapted for life on land. Gradual reductions in the number of trunk segments and walking legs eventually led to the appearance of six-legged animals (hexapods) with three distinct body regions: head, thorax, and abdomen. This group includes all of the insects (class Insecta) and three other closely related classes (Protura, Diplura, and Collembola).
The most primitive living insects (Archeognatha and Thysanura) are wingless as adults (apterygote) and undergo no metamorphosis as they grow (ametabolous development). The immatures are similar in appearance to the adults. More advanced insects (e.g., Odonata and Ephemeroptera) undergo incomplete metamorphosis (hemimetabolous development); they have wings which develop gradually as they mature (exopterygote), but the wings cannot be folded down against the body (Paleoptera).
In more advanced insects (Neoptera), the wings can be folded against the body when at rest. The most primitive neopterans are the stoneflies (Plecoptera) and the webspinners (Embioptera). Early in the Carboniferous period, this branch of the insect's family tree diverged into three major lineages:
- The Orthopteroid line includes at least nine living insect orders that have relatively unspecialized mouthparts. Most of these insects (except Mantodea and Mantophasmatodea) are herbivores or scavengers.
- The Hemipteroid line includes four orders with mouthparts that exhibit some degree of specialization for rasping or piercing/sucking. Most of these are herbivores, but some have adopted a lifestyle of predation or parasitism.
- The Endopterygota includes all insects that undergo complete metamorphosis (holometabolous development). These insects have four stages in the life cycle (egg, larva, pupa, and adult). Larvae are quite different in appearance from adults. Wings and other adult structures develop internally during the pupal stage. The nine holometabolous orders include about 4/5 of all living insect species. These insects represent a wide range of ecological diversity -- scavengers, herbivores, predators, and parasites.