Water homeostasis by wild larvae of Manduca sexta

Modulation of faecal water loss is the principal mechanism by which larval Lepidoptera maintain water homeostasis in the laboratory. Is this also true of larvae in nature? We observed 12 fifth-instar larvae of Manduca sexta f eeding on Datura wrightii in the Sonoran Desert, U.S.A. The two main source s of water stress were: evaporative water loss across the cuticle, which ap peared to be promoted by increasing body temperatures and decreasing relati ve humidities during daytime observation periods; and attacks by tachinid f lies, which prompted caterpillars to defaecate large quantities of water an d to regurgitate digestive fluid onto themselves. In both cases, caterpilla rs responded by producing drier faecal pellets. A subset of caterpillars co nsumed water-rich flower buds of D. wrightii, which led to the production o f comparatively wet faecal pellets. These data demonstrate that larval wate r balance in nature is affected by a variety of biotic and abiotic factors and that larvae respond to these perturbations by modulating the loss of wa ter in the faeces.