Achievement of a critical body size provides the proximate stimulus for the metamorphic molt in Oncopeltus and other hemimetabolous and holometabolous insects. Effects of size—dependent stimuli on metamorphosis vary in detail among different insects, although the adaptive significance of this variation is as yet unknown. The phenomenon is important to life histories where growth rates are highly variable, since it allows larvae to delay eclosion until a prerequisite body size is attained. In Oncopeltus, variation in growth rate occurs where nymphs develop on individual host plants which differ greatly in nutritional quality, depending on the presence or absence of seeds. This variation in growth rates results in marked variation in adult size. The size at which nymphs eclose is determined by opposing selective forces in the adult and larval stages. When growth is slow, nymphs which must attain a larger size exhibit longer developmental times and lower survival to eclosion. However, bugs which eclose at a larger adult size survive longer under starvation conditions, and presumably could disperse for longer periods in search of food sources. This is particularly important for females eclosing on nonfruiting plants, since they must locate a supply of seeds before reproduction can commence.