Control of Drosophila larval behavioral transitions by peripheral sensory neurons

Rapid growth through three larval instars prior to pupariation is key to formation of appropriately sized Drosophila adults. Since adults do not grow larger, timing of the reversal in food-associated behaviors from foraging to wandering and the resulting cessation of larval feeding is critical to final size. We demonstrate that larval food exit is preceded by a period of increased larval surfacing behavior termed the Intermediate Surfacing Transition (IST) that correlates with the end of larval feeding. The IST occurred during the larval Terminal Feeding Period (TFP), a period of constant feeding and exponential growth of the animal. IST behavior was dependent upon function of a subset of peripheral sensory neurons expressing the DEG/ENaC subunit Pickpocket1 (PPK1). Neuronal inactivation or loss of PPK1 function caused an absence of IST behavior, a consequent delay in larval food exit and an increase in final adult size presumably resulting from an extended TFP. Transgenic neuronal hypersensitization caused premature IST behavior with no change in timing of larval food exit resulting in decreased final adult size. These results suggest a peripheral sensory mechanism contributing to the length of the larval terminal feeding period and determination of final adult size.