SIZE-DEPENDENT TIMING OF METAMORPHOSIS IN MILKWEED BUGS (ONCOPELTUS) AND ITS LIFE HISTORY IMPLICATIONS

¹ Department of Zoology, University of Iowa, Iowa City, Iowa 52242

Experimental alteration of growth of Oncopeltus nymphs suggests that adult ecdysis occurs only after a critical size (indexed by weight) is attained. Nymphs which exceed this size at the outset of the 5th (final) instar metamorphose 6 to 8 days later. Nymphs which attain this size later in the 5th instar subsequently metamorphose 5 to 7 days later.

Studies of O. c. cingulifer and O. fasciatus suggest that interspecific differences in adult body size provide an indication of differences in the critical size for metamorphosis. Further, sexual dimorphism in body size also indicates a dimorphism in the critical size for metamorphosis.

The ability of Oncopeltus nymphs to assess their size provides a means of adjusting development in response to variable conditions affecting growth. Delayed metamorphosis in slow-growing nymphs provides additional time for further growth. This effect may be advantageous, since reproductive success declines with decreasing adult body size.