Migration

Intraspecific variation in the wing form of insects is usually associated with migratory flight, that is, dispersal or migration from the habitat, as opposed to trivial flight, activity associated with routine behavior such as feeding, mate finding, or escaping from enemies. As such, the environmental cues known to influence wing form are those that signal seasonal habitat deterioration (photoperiod, temperature) or less predictable, density-dependent habitat changes (poor nutrition, stress, crowding) (Travis, 1994; Masaki and Shimizu, 1995). High population density is known to induce a number of morphological and physiological changes in studied cockroach species, for example, Blab. craniifer (Goudey-Perriere et al., 1992) and Eublaberus distanti (Rivault, 1983), but to date, wing form has not been one of them.

Mass migrations and dispersals have been recorded in cockroaches, though not in wing-polymorphic species. Surface activity in C. punctulatus occurs following rainfall, during daylight hours in spring (Nalepa, 2005). Soil-

burrowing Australian Geoscapheini undertake spectacular pedestrian migrations after rains—sometimes seen by motorists crossing roads every few yards for 32 km at a stretch (Monteith, pers. com. to LMR). There are two intriguing reports of possible long-distance movement by flight. On a sunny morning in Venezuela at an elevation of 1100 m, Beebe (1951) observed a "flurry" of at least 30 Blaberus giganteus fluttering slowly up a gorge used as a flyway for migrating insects. Under the hot sun in an Arizona desert, Wheeler (1911) watched two separate swarms of male Homoeogamia subdiaphana alternately flying and quickly running over the sand in a southwesterly direction; he likened their quick movements to those of tiger beetles (Cicindelidae). Overpopulated buildings or sewers have been known to spawn natural migrations in several species of urban pests (Roth and Willis, 1957). It is unusual that many of these movements occur during daylight hours in otherwise nocturnal insects. Stein and Haschemi (1991) report that German cockroaches emigrating from a garbage dump used solar cues for orientation. Most walked directly toward the sun, with their bearing shifting from east to west over the course of the day.

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