Life Cycles

The way an insect or allied arthropod develops in relation to its seasonal environment constitutes its life cycle (Tauber et al. 1985). Life cycles are as varied as the kinds of animals living them. Perhaps a majority of species in arctic or temperate life zones have annual generations, that is, one complete turnover, egg to egg per year. Others have biannual or multiannual cycles. The latter implies the existence of prolonged feeding periods, often on food that is poor in nutrition (e.g., wood-boring beetle larvae), or the intercession of a period of diapause. Still others are semiannual (bivoltine) or multivoltine, with two to several generations per year. The latter are more typical of tropical or other stable environments where unfavorable drought or cold does not force temporary arrests in development. Some insects, such as pomace flies (Drosophila), develop very rapidly and repeatedly and may have almost continuous reproduction throughout the year. Some mosquitoes mature very quickly in transient water following infrequent rains but remain dormant in the egg stage for most of the remainder of the year.

Certain insect types regularly incorporate asexual reproduction in their life cycles in addition to sexual reproduction. This alternation of generations is typical of aphids, for example. When conditions are best for plant growth and therefore feeding, as in the beginning of the rainy season, emphasis is on multiplication of numbers. This is accomplished by the "stem mothers" that bear live, sterile, wingless females parthenogenetically and as rapidly as possible. As the season favorable for dispersal approaches, when there is less or no rain and winds may increase, sexually active, winged males and females appear, to mate, mix genes, and disperse to new localities. The females lay eggs that hatch into the asexual forms once again. Production of sexual forms is controlled by changes in temperature and photoperiod; under constant tropical conditions, cyclical alternation of generations may not occur.

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