The great diversity of organisms in the tropics means that it is possible to choose particularly appropriate species for answering questions about how behavior evolves. For instance, tiny orb-weaving spiders that are capable of weaving complex webs, teach about body-brain scaling relationships. The diverse array of parasitic wasps that manipulate the behavior of their spider hosts to increase the survival of their pupae make it possible to trace the evolution of the wasps’ abilities to manipulate their hosts, and gain insights into how behavioral capabilities are organized within the spiders.

The electrical activity of nerves and nervous systems is intrinsically imprecise. One possible source of new behavioral traits are such imprecisions. The more crucial the behavior, the more strongly natural selection is likely to act to correct deviations due to such imprecision. Tests of the importance of these ideas for behavioral evolution involve measurements of rates of errors in orb web construction, and rates of evolution of construction behavior in groups in which the selective importance of precise performance varies.

The hypothesis that genitalia diverge rapidly due to cryptic female choice predicts that many species-specific structures of male genitalia function to stimulate the female during copulation. This prediction can be tested in several ways: checking for otherwise mechanically superfluous movements of such structures during copulation; checking for specialized female receptors in the areas contacted by such structures during copulation; blocking or otherwise inactivating such female receptors; or eliminating or otherwise inactivating the male structures.

Comparisons among related species of spiders suggests that behavior modules have been added, subtracted, and shuffled in various ways during evolution. Selective elicitation (and repression) of particular spider behavior modules by parasitic ichneumonid wasps also favors this view. The unusually detailed knowledge of the probable behavior of some ancestral forms allows testing the prediction that such division into semi-independent units facilitated evolutionary change by allowing shuffling in lineages of spiders descended from orb weavers.

The brains of animals that evolved miniature body sizes have lower numbers of neurons and connections between them; corrected for body size, their brains are larger than those of larger relatives. Miniaturized species might be expected to be behaviorally inferior, but several aspects of the behavior of one group, tiny orb-weaving spiders, were not simpler, slower, or less precise than those of larger relatives. Nervous tissue is relatively expensive to maintain, so the relatively over-sized brains of tiny spiders are probably energically costly, with several expected ecological consequences. Careful tests of the behavioral consequences of miniaturization have not yet been performed in other groups.