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Modern insects and crustaceans diverged from an ancestor over 500 million years ago, but their neural circuitry retains many common features
A taxonomist viewing insects or crustaceans tends to see tough, jointed exoskeletons and elaborate limbs as their salient features-so salient, in fact, that the taxonomists who named the phylum to which these animals belong called it Arthropoda, or "jointed foot." The same biologist will also notice that the arthropod body plan is that of a modified worm, where the worm's more-or-less uniform series of segmental units becomes well differentiated along the length of the arthropod body. This gives arthropods distinct head, middle and tail regions.
As neurobiologists considering evolution from simple to more complex animals, we are interested in the changes in the neural circuits serving locomotion and sensory coding that accompanied the transition from worm to arthropod. Such evolution is surely crucial to the emergence of advanced mobile animals. Because brains and body plans must have evolved in step from their simpler antecedents, comparative studies of neural development and of the mature nervous systems of animals promise to reveal much about arthropod evolution. In arthropods almost every nerve cell grows in a particular pattern and makes specific synaptic connections to form the elaborate neural circuits that give an animal its mobility and control; these features make each cell a recognizable identity.
Just as vertebrae and ribs are serially repeated, sets of neurons are repeated in each segment of an insect's body, or beneath each facet of the compound eye. Using identified neurons, anatomists can compare the development and structure of nervous systems from different arthropods much as they can compare vertebrate skeletons. In addition, we can record the electrical activity from a single neuron, which allows us to compare physiological function in different lineages. So far, most work has been done on insects. But comparisons can also be made between insects and crustaceans. Such comparisons tell biologists how these arthropods, with their jointed exoskeletons and specialized limbs, evolved from simpler ancestors and how they have subsequently diversified during evolutionary history.
The Emergence of Arthropods
Among a variety of arthropod-like animals, three main classes are clearly present in the fossil record by the Middle Cambrian Period, around 520 million years ago. These include...