Size effects on the hygrothermal balance and foraging patterns of a sphecid wasp, Cerceris arenaria

. 1. Cerceris arenaria (L.) females nest in sandy burrows, foraging for prey weevils to stock cells in which eggs are laid. Nests are easily distinguished and observed, and tunnel diameter correlates well with the occupying females' size (range 68–128 mg). 2. Foraging depends on ambient temperature and occurs only during periods of insolation. Large females can forage earlier in the day, and usually make a greater number of successful trips per day, than smaller ones. 3. Size differences of females affect heat exchange and resultant temperature excesses (T ex ). At the highest ambient temperatures T ex for the largest wasps can exceed physiological tolerances, thus reducing foraging activity relative to the smaller females. 4. On very hot days wasps forage more frequently for nectar, probably to reduce their water deficits. Larval water balance is controlled less directly, by the architecture of burrows and the nature of cell walls and contents. 5. Temperature effects on foraging and nesting bring about a roughly two‐fold increase in success as cell provisioners (and hence egg‐layers) of larger over smaller females. Possible countering effects of very hot weather and of parasitism, either of which might favour small females in some conditions, are discussed.