Contrasting responses to desiccation and starvation by eggs and neonates of two lepidoptera

We examined the effects of desiccation on eggs and first-instar larvae of t wo species of Lepidoptera, Grammia geneura (Arctiidae) and Manduca sexta (S phingidae). Grammia geneura occurs primarily in grasslands and savannas of the southwestern United States; M. sexta co-occurs with G. geneura but also is cosmopolitan across much of the Western Hemisphere. Eggs of G. geneura exposed to 0% relative humidity (RH) lost water much less rapidly (7.6 mug d(-1); 2.4% d(-1)) than did eggs of M. sexta (79.5 mug d(-1); 5.7% d(-1)). Eggs of both species survived at rates exceeding 75% at both 0% and 85% RH. Neonates of the two species responded differently to desiccation and starv ation. In 85% RH, larval G. geneura survived at high rates (> 80%) without access to food or water up to day 17, and in 0% RH, they survived at rates exceeding 50% through the first 10 d. Larvae at 0% RH lost mass very slowly (7.2 mug d(-1); 2.9% d(-1)), which was attributable both to low rates of w ater loss and to an ability to reduce metabolic rate to low levels. Larval M. sexta, in contrast, had rates of mortality that were much higher: after 1 d, fewer than 30% were alive in either group, and by about 1.5 d, all wer e dead. Neonate M. sexta also lost mass much more rapidly at 0% RH, about 3 29 mug d(-1). Water from metabolism appeared to contribute significantly to the water budget of G. geneura but not of M. sexta. These data show that G . geneura and M. sexta can inhabit similar macroclimates via remarkably dif ferent physiologies.