Insects are remarkable for their variety of color, form, and presence in different habitats. The life cycle of many insects includes striking morphological changes (e.g., from caterpillar to butterfly) that not only affect the external form of the animal but also involve a resculpturing of the nervous system. The sensory organs of insects also display great variety and sensitivity.
Despite the diversity of insect body form (there are over a million living species), the central nervous systems of insects are remarkably similar: they vary “astonishingly little from the most primitive to the most advanced” (J. S. Edwards and Palka, 1991, p. 391). The gross outline of the adult insect nervous system consists of a brain in the head end and ganglia in each body segment behind the head (Figure 1). Bundles of axons connect ganglia to the brain. The number of ganglia varies: in some insects all the ganglia of the chest and abdomen fuse into one major collection of cells; in other insects there are as many as eight ganglia in a chain.
The brain itself contains three major compartments: two lobes of the protocerebrum and an optic lobe. The protocerebrum is the most complex part of the insect brain, and its right and left lobes are each continuous with the large optic lobe, an extension of the compound eye. Within the optic lobe are distinct masses of cells that receive input from the eye, as well as from the brain. Electrical stimulation of sites within the protocerebrum elicits complex behaviors. The relative sizes of different components of the protocerebrum differ among insects, and some of these variations may be particularly relevant to behavioral variations. For example, a portion of the protocerebrum called the corpus pedunculatum is especially well developed in social insects, and the behavior of these animals tends to be more elaborate than that displayed by solitary insects.
One prominent feature of the nerve cord of insects is giant axons—fibers much bigger in diameter than most. The large diameter of these axons means that they conduct action potentials rapidly (see Chapter 3), making them valuable for sending messages quickly. Some insects have receptive organs (the cerci, singular circus) in the tail that detect air movement; these receptors connect to giant interneurons with very large axons that ascend the nerve cord to the head. Along the way, these axons excite some motoneurons. This system originated to allow insects to escape predation by retreating rapidly. In many insects (e.g., cockroaches) this system still functions as an escape system; in other insects the cerci and the connections of the giant interneurons have been modified so that they also play a role in reproductive behavior (in crickets) or help regulate flight maneuvers (in grasshoppers). No matter how this system functions in a particular species, its basic organization and cellular composition appear to have remained the same for a very long time, perhaps as long as 400 million years (J. S. Edwards and Palka, 1991).
Let’s compare some of the features of vertebrate and invertebrate nervous systems:
Borst, A. (2014). Fly visual course control: behavior, algorithms and circuits. Nature Reviews Neuroscience, 15, 590–599.
Edwards, J. S. and Palka, J. (1991). Insect neural evolution—a fugue or an opera? Seminars in the Neurosciences, 3, 391–398.