14. Insect defense
Although some humans eat insects (section 1.6), many “western” cultures are reluctant to use them as food; this aversion extends no further than humans. For very many organisms, insects provide a substantial food source, because they are nutritious, abundant, diverse, and found everywhere. Some animals, termed insectivores, rely almost exclusively on a diet of insects; omnivores may eat them opportunistically; and many herbivores unavoidably consume insects. Insectivores may be vertebrates or invertebrates, including arthropods — insects certainly eat other insects. Even plants lure, trap, and digest insects; for example, pitcher plants (both New World Sarraceniaceae and Old World Nepenthaceae) digest arthropods, predominantly ants, in their fluid-filled pitchers (section 11.4.2), and the flypaper and Venus flytraps (Droseraceae) capture many flies. Insects, however, actively or passively resist being eaten, by means of a variety of protective devices — the insect defenses — which are the subject of this chapter.
A review of the terms discussed in Chapter 13 is appropriate. A predator is an animal that kills and consumes a number of prey animals during its life. Animals that live at the expense of another animal but do not kill it are parasites, which may live internally (endoparasites) or externally (ectoparasites). Parasitoids are those that live at the expense of one animal that dies prematurely as a result. The animal attacked by parasites or parasitoids is a host. All insects are potential prey or hosts to many kinds of predators (either vertebrate or invertebrate), parasitoids or, less often, parasites.
Many defensive strategies exist, including use of specialized morphology (as shown for the extraordinary, ant-mimicking membracid bug Hamma rectum from tropical Africa in the vignette of this chapter), behavior, noxious chemicals, and responses of the immune system. This chapter deals with aspects of defense that include death feigning, autotomy, crypsis (camouflage), chemical defenses, aposematism (warning signals), mimicry, and collective defensive strategies. These are directed against a wide range of vertebrates and invertebrates but, because much study has involved insects defending themselves against insectivorous birds, the role of these particular predators will be emphasized. Immunological defense against micro-organisms is discussed in Chapter 3, and defenses used against parasitoids are considered in Chapter 13.
A useful framework for discussion of defense and predation can be based upon the time and energy inputs to the respective behaviors. Thus, hiding, escape by running or flight, and defense by staying and fighting involve increasing energy expenditure but diminishing costs in time expended (Fig. 14.1). Many insects will change to another strategy if the previous defense fails: the scheme is not clear-cut and it has elements of a continuum.
(After Malcolm 1990)