At first sight, insects seem to be very different from vertebrates and therefore poorly qualified to act as models in our context. A closer look, however, reveals many similarities on tissue and basic metabolic levels. Aging insect tissues, for instance, are characterized by structural changes that also occur in mammalian tissues. The deposition of the age pigment lipofuscin and the more or less abundant "whorllike" structures of mitochondrial origin are examples of common signs of aging or otherwise degenerating tissues (see below). A further peculiarity is that adult insects are composed of post-mitotic cells—except for their hemocytes and germ cells. Therefore all lifelong changes on the tissue level can easily be detected.
Most insect species are short-lived organisms. Therefore many generations can be studied in a short time. Insects can be raised in large quantities and with low costs.
The use of insects as model systems in gerontological studies has a long tradition. Loeb and Northrop (1917) were the first to use Drosophila in this way. They conducted experiments on metabolic rates and life span, and these early attempts can be acknowledged as the beginning of a comparative biology of aging. These Drosophila studies were based on results of Rubner (1908). Working with mammals, he showed that the amount of lifelong metabolic energy per gram body weight is more or less constant. These results stimulated further studies on the rate of living theory (Pearl, 1928). This old problem was then elucidated in greater detail by Sohal and coworkers using modern methods and the house fly—Musca domestica. The rate of living theory was then consequently redefined as follows: "The rate of ageing is directly related to the rate of unrepaired molecular damage inflicted by the by-products of oxygen metabolism and it is inversely related to the efficiency of antioxidant and repair mechanisms'' (Sohal, 1986). Since that time many investigations have addressed the influence of oxidative stress, free radical damage, and repair on aging and longevity in insects. Regardless of the acceptance of the rate of living theory, these factors are generally accepted as being of central importance for the understanding of aging mechanisms.
Extensive studies on aging mechanisms are mostly restricted to dipterans. The fruit fly Drosophila, of course, is the most intensively studied insect even in this sense. Other dipterans more often studied are the house fly Musca domestica and blowflies, including our own model organism Phormia terraenovae (Collatz, 1997). The scorpion fly Panorpa vulgaris and the chrysomelid beetle Gastrophysa viridula attracted particular attention in our laboratory due to special questions, which are outlined in more detail below.
For the biologist interested in comparative aging studies, large differences in life-history strategies of insects of the same genus could offer the opportunity to test phylogenetic causes of aging. In the last years special attention was given to social hymenoptera like bees, wasps, and ants with their remarkable long living queens (see below).
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