Cockroach Development

Within a cockroach species, both the number and duration of instars that precede the metamorphic molt are variable, a trait unusual among hexapods (Heming, 2003). In P. americana, for example, the length of nymphal period can vary from 134 to 1031 days (Roth, 1981a)—nearly an order of magnitude. The number of molts in cockroaches varies from 5 or 6 to 12 or 13, and may or may not vary between the sexes. Within a species, variation in cockroach development occurs primarily in response to environmental conditions: low temperature, minor injuries, water or food deficits, or poor food quality (Tanaka, 1981; Mullins and Cochran, 1987). Even in laboratory cultures in which extrinsic influences have been minimized or controlled, however, the instar of metamorphosis remains variable, even in nymphs from the same ootheca (Kunkel, 1979; Woodhead and Paulson,

1983). There can be a lag of up to 9 mon between the appearance of the first and last adult among nymphs from the same sibling cohort of Periplaneta australasiae (Pope, 1953), and "runts"—nymphs stalled in the third or fourth instar when all others in the cohort have ma-tured—have been noted in P. americana (Wharton et al., 1968). Kunkel (1979) describes the instar of metamorphosis in cockroaches as a polygenic trait with a great deal of environmental input involved in its expression. Significantly, there are records of both stationary and saltatory molts in cockroaches (Gier, 1947; Rugg and Rose, 1990). If the ancestor of the termites was like extant cockroaches, then it, too, possessed a tremendous amount of developmental plasticity prior to evolving eu-sociality.

Was this article helpful?

0 0

Post a comment