Two or more kinds of organisms living in close association to the benefit of one (com-mensalism) or all partners (mutualism) (Boucher 1982) are said to exhibit symbiosis. ("Symbiosis" is used in the broad sense here to include all forms of intimate associations, not just those of mutual benefit for which the term is used in a narrow sense.) The Neotropics provide countless, fascinating examples of this phenomenon involving insects.

Some of the best-known examples occur between insects and higher plants (Bernays 1989). Most often, the former are ants, and major examples are discussed in chapter 12, on Hymenoptera (see ants and plants). Many symbiotic relationships exist between insects and lower plants, especially fungi (Lichtwardt 1986, Wheeler and Blackwell 1984).

Nest sharing, or inquilinism, is a type of symbiosis wherein both partners are insects (Kistner 1982). Examples are certain silverfish, millipedes, beetles, mites, and cockroaches, called myrmecophiles, that share a common dwelling with ants. Other social insects are likewise visited by termito-philes, melittophiles, and sphecophiles, depending on whether they are termites, bees, or wasps, respectively. There are many variations in the degree of closeness in the association and its specific nature (Kistner 1979). In the Neotropics, army ants and leaf cutter ants are frequent hosts (see chap. 12). The visitors live on the debris, scraps of food, and even corpses of ants. This is mutualistic coexistence, the ants profiting by the nest debris removal, the guests by a reliable food source.

The invasive species finds its way into the host's colony and is accepted into its society by a variety of physical and chemical deceptions (Holldobler 1971). Staphylinid beetles may actually resemble and behave like their hosts (Wasmannian mimicry) (Akre and Rettenmeyer 1966). The trail-marking substances secreted by the ants may serve as attractants to inquilines (Moser 1964). The latter may also produce "tranquilizing" chemicals that pacify the ants or mimic the food offerings of worker ants.

The strategies of inquilines vary; some prey directly on the host, others on the immatures of the latter or on accumulated nest refuse. Others are apparently totally neutral, the purposes of their invasions remaining unknown.

A related phenomenon is exhibited by ants and other insects that have a particular fondness for the honeydew secretions produced by several groups of sap-sucking insects. Honeydew is a sugar-rich solution excreted from the digestive tract (aphids) or from special integumentary glands (scale insects, metalmark and hairstreak butterfly larvae). The solution is a food in return for which the honeydew producer receives protective, dispersal, cleaning, and other tending favors from the feeding insects. (See wax bugs, aphids, chap. 8; metalmarks, hairstreaks, chap. 10.)

Phoresy is yet another symbiotic relationship between insects. The term is used for the temporary attachment of a much smaller, relatively sedentary form to a much larger, more vagile form. The former receives transportation and is able to disperse far more widely than it could on its own. Many of the mites often found living on beetles are practicing this habit. This is also the case with the pseudo-scorpions so common under the elytra of large long-horned beetles, especially the harlequin beetle (see harlequin beetle, chap. 9). Phoresy is often a means for parasites and predators to find access to their prey (Clausen 1976).

The above examples are designated "ectosymbiosis" (Hartzill 1967) in contrast to "endosymbiosis" (Koch 1967). The latter refers to the habitation of many microorganisms in the gut, other body cavities, and tissues of insects (Boush and Coppel 1974). The best-known cases are the flagellate protozoans of primitive termites and tricho-mycete fungi associated with numerous insects (Lichtwardt 1986).

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