Reference: Biol. Bull., 155: 499-510. (December, 1978) SIZE-DEPENDENT TIMING OF METAMORPHOSIS IN MILKWEED BUGS (ONCOPELTUS) AND ITS LIFE HISTORY IMPLICATIONS NIGEL BLAKLEY 1 AND SUSAN ROSS GOODNER Department of Zoology, University of loica. loiva City. Iowa 52242 The allocation of time and resources to the conflicting demands of growth and reproduction has been an important theme in evolutionary explanations for the diversity of life history patterns among living organisms (e.g., Gadgil and Bossert, 1970; Schaffer, 1974). In examining the problem of how this allocation is achieved and adaptively modified under different ecological conditions, studies of organisms with discrete life stages are particularly important, since growth and reproductive phases of the life cycle are often clearly delineated. Further, an understanding of this problem requires the integration of ecological theory with developmental and physiological considerations. These points are well illustrated by Wilbur and Collins' (1973) attempt to model control of the neurohormonal processes underlying developmental plasticity in amphibians and to account for variation in amphibian life cycles. A salient feature of their model is the role of hypothetical mechanisms which allow larvae to monitor both their rate of growth and body size ; information on which the decision to metamorphose is assumed to be based. Although the existence of such mechanisms in amphibians remains to be demonstrated, there is evidence to suggest that analogous models may be relevant to the study of adaptation in insect life cycles. Mechanisms which influence development in response to changes in body size have been described in studies of the blood-sucking bug, Rhodnius prolix us ( Wigglesworth, 1934; Van der Kloot, 1961; Steel and Harmsen, 1971). Molting occurs only after the ingestion of a blood meal, and these studies provide strong evidence that distension of the abdomen is detected by stretch receptors which initiate the molt by triggering the release of brain hormone. Further evidence of size-dependent regulation of insect development has come from experimentally manipulating growth of larvae of a holometabolous insect, Manduca sc.vto. Nijhout and Williams (1974a, b) have shown that 5th-instar larvae molt only when they attain a size corresponding to a weight of about 5 g. When this critical size is reached, the corpora allata cease to secrete juvenile hormone, initi-ating the endocrine events leading to the molt. The determination of which molt will lead to a pupa is also apparently made on the basis of size (Nijhout, 1975). Here evidence is presented that molting in Oncopeltits nymphs is also initiated only when a critical size is attained. Viewed in an ecological context, it is sug-gested that size-dependent regulation of development in Oncopeltus represents a life history adaptation to unpredictable conditions for larval growth. 1 Present address : Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado 80309. 499
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