D’AMICO, L.J.*; DAVIDOWITZ, G.; NIJHOUT, H.F.: Body size evolution in a laboratory strain of Manduca sexta : Implications for studies in insect physiology and endocrinology

The tobacco hornworm, Manduca sexta, has been used as a model organism for the study of insect developmental physiology and endocrinology for more than 30 years. Many of the connections between the growth and the endocrine physiology of the larva were established through a series of extensive studies done in the early 1970’s. The present study compares those findings from the 1970’s to our current collony when reared under identical conditions. We find that larvae are much larger now than in the 1970’s, with a corresponding increase in the critical weight of the larvae. The threshold size for determining the final instar has decreased. These changes likely result from evolutionary processes that occurred while maintaining laboratory colonies. Given the use of morphological markers in staging larvae for physiological and endocrine studies, the changes in developmental characteristics highlighted here illustrate the need for investigators to determine if their staging techniques rely on relationships originally described in the 1970’s.