CAUSES OF DEATH IN IRRADIATED ADULT INSECTS

Summary 1. Irradiation of adult insects of many orders leads to their death. There is considerable evidence that such death occurs during a limited time period, both the beginning and duration of which are relatively independent of the dose over a wide range. This suggests the operation of a specific lethal syndrome. In other groups of insects, particularly Diptera and many moths, either the absence of somatic-cell division or the short life of the imago prevents the development of this syndrome. These latter groups require much higher doses of radiation for the induction of demonstrable mortality, and the time-course of mortality is closely related to the size of the dose. 2. Superficially, at least, radiation death of adult Diptera is comparable to that of supralethally irradiated insects of the taxa which exhibit the specific lethal syndrome after lower doses. There are some observations suggesting that the nervous system becomes involved after these high doses, just as there are for mammals exposed to high doses. 3. There is also some evidence, very convincing in itself, although available only from a small number of experiments, that the specific syndrome observed after lower lethal or mid-lethal doses is a consequence of damage to the midgut, particularly to its regenerative cells. This clearly is analogous to the intestinal syndrome of mammals; as in the mammal, the processes which intervene between the cellular damage and the death of the organism remain in the realm of conjecture or controversy. This appears to be one of several areas in which more detailed investigation of the radiobiological phenomenon is certain to provide valuable information on normal physiological activities. 4. The response of insects differs greatly from that of mammals in that effects on disease-resistance mechanisms, both cellular and humoral, are not prominent. The dose required for unequivocal demonstration of such effects may be greater than the dose required to produce lethality. If endogenous infection contributes to lethality after irradiation of otherwise healthy insects, there is, as yet, no convincing evidence for it. Nevertheless, radiation may provide a useful tool for probing the mechanisms of disease resistance of insects, and for examining the regulation and the kinetics of haemocyte formation. In addition, irradiated insects might be used to select virulent strains of potential insect pathogens. 5. Quantitative information on the response of insect cells, either in situ or in zlitro, is virtually non-existent. Insect eggs have been used in a few investigations of cellular phenomena in radiation biology, but the only data applicable to dose-response relationships are those of Murakami (1969, 1970), who has taken advantage of the synchronous division that occurs during the early embryonic development of Bombyx mori. The usefulness of insect cell-cultures in delineating growth requirements and in propagating viruses has attracted far more attention than their potential for radiobiological investigations and other studies of cellular responses. This is not unlike the situation in the field of vertebrate cell-culture prior to the mid-1950s; the present time seems appropriate for developing insect cell-culture into a tool for the radiation biologist. 6. Apparently beneficial effects of radiation can be very spectacular in insects, but are not limited to insects, or, indeed, to invertebrates. The frequent demonstration of such effects in insects may stem, in part, from the greater resistance of insects than of vertebrates to the lethal effects of radiation. It must be remembered, however, that extension of the life-span is found in insects surviving mid-lethal doses of radiation. Alternatively, the lesser dependence of insects on cell-renewal systems may permit the manifestation of a phenomenon that is common to many post-mitotic tissues. If so, the causes of the extension of the insect life-span after irradiation should also be of major interest to investigators of mammalian radiation biology and indeed to all gerontologists. I am grateful to Dr Judith H. Willis and Dr Benita S. Katzenellenbogen for reading the manuscript and for making a number of helpful suggestions. My own research on radiation responses of insects was supported by U.S. Public Health Service grants GM 10208 and HD 03163. Many of my concepts of the relationship between the responses of mammals and those of insects were formulated during tenure of a Public Health Service Special Fellowship, GM 23423, at the Department of Zoology, University of Cambridge; I am grateful to the members of that Department, and particularly to Dr J. W. L. Beament and to the late Professor C. F. A. Pantin, for their generosity and hospitality in making the resources of the Department available during my stay. I am especially indebted to Dr George Salt, for his patience and wisdom, and for personal kindnesses far too numerous to record.