are involved in olfaction. The central body is a way-station through which nerve fibers pass from one hemisphere to the other. The nature of the signal processing that takes place here is unclear; it may be involved in visual processing (Burrows 1996). The optic lobes are laminar, that is, organized in layers, and process information from the insect's compound eyes through the layers in a way that, as in vertebrates, retains the retinotopic (spatial) map of the image. The antennal lobes process olfactory signals coming from the antennae.

This may be an appropriate place for us again to note the many parallels between vertebrate and invertebrate nervous systems. Biologists long noted the similarity of organization (eyes, mouth, digestive system, limbs, and so forth) between invertebrates and vertebrates, although they were considered analogous rather than homologous. As we have already seen in many ways, recent studies have shown similarities—sometimes close similarities—system by system, at the gene, developmental, and cytological levels. We noted in Chapter 9 the suggestion by Geoffroy St. Hilaire in the early 1800s that insects were inverted vertebrates, in that the dorsal nerve cord in vertebrates is homologous to the ventral dorsal nerve in invertebrates (and the digestive systems are in the corresponding inverted locations). There are some corresponding genetic similarities as well, but it is important to resist the temptation to extrapolate too far or to expect even true homology to be too precise at the trait level.

Overall, invertebrate nervous systems range from very simple and diffuse to relatively complex and centralized. As a result, the organisms react in ways that seem interpretable to us. Darwin went through a list of ways. Insects exhibit what appears to be fear, which can be elicited by various signals, panic (when they are upside down, for example), and exploratory behaviors, not to mention courtship (including males fighting for mates). There are many examples of learned behavior in insects as well, and they can be trained in the laboratory (Waddell and Quinn 2001). This may seem surprising, but their brains do have room for the integration of new experiences and behaviors (Meinertzhagen 2001). None of this explains how information is integrated, however, much less how the organism experiences the information. Another thing to consider is whether we are anthropomorphizing with terms like "panic" and "courtship," especially if consciousness is not involved (but is it?).

Was this article helpful?

0 0

Post a comment