Although the aging pattern of insects, like that of other organisms, is doubtless under genetic control, it is extremely flexible, depending on several biotic and abiotic factors. Life span includes the longevity of all developmental stages an individual passes through during its life cycle. Individuals of insect species (as well as those of other species) experience a number of critical points in time during their life cycles, which influence the life span from eggs over larvae up to pupae and adult stages. Without knowledge of such modifying factors the life span of an insect species is unpredictable.
Insects are ectothermic organisms. The ambient temperature is therefore a key life-span modifying factor. It is easy to manipulate the metabolic rate simply by changing ambient temperature. The mentioned work of Loeb and Northrop used this regimen as an experimental tool. The light regime also acts as a prominent abiotic stimulus together with the temperature. Both factors are essential in habitats with distinctive seasons; they determine in many species whether phases of dormancy occur or not. A phase of low metabolism can be induced experimentally by changing the photoperiod from long day (16:8) to short day (8:16). This treatment evidently interrupts the aging process (see below).
All insects dealt with in the present contribution are holometabolic. Their development from egg to adult is interrupted by a metamorphosis. Preimaginal conditions are important for the adult life course. A controlled underfeeding of larvae does not terminate the development but results in dwarf adult forms. They can be used to study the interrelationship of body size, fecundity, and longevity.
Fecundity and reproductive success are also essential for the determination of the following course of life. As we will see in the case of the blowfly Phormia, reproductive success depends not only on the availability of mates but equally on the presence of proteins as specific food for the development of eggs. This in turn is of great influence for longevity.
Physiological trade-offs between physical performance, reproduction, and longevity are also important and gave rise to numerous studies. We will refer to this in a later section.
As mentioned above, a particular situation is given in eusocial insects as wasps, bees, ants and termites. They live in colonies with cooperating members of different castes, sharing a high level of genetic similarity with the queen. Aging process and life span of the colony members are widely determined by extrinsic factors such as nutrition and kind of work. The remarkable longevity of the queen ranging from 4 to 8 years in wasps and up to 30 years in termites is of great interest (Page and Peng, 2001). This topic is further dealt with in Chapter 23, "Models of Aging in Honeybee Workers,'' and Chapter 24, "Ants as Naturally Long-Lived Insect Models for Aging.'' For a more detailed description of aging and environmental conditions in insects see Collatz (2003).
Finally, caloric restriction in general can act as a life-prolonging strategy not only in vertebrates, but also in insects. It is proposed that the basic mechanisms and effects are almost the same in such different organisms.
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