ENDOTHERMIC WARM-UP IN 2 SPECIES OF SPHECID WASP AND ITS RELATION TO BEHAVIOR

The ability of two sphecid wasps, Bembix rostrata and B.zonata (Hymeno ptera: Sphecidae), to warm up endothermically is demonstrated under la boratory conditions. Mean warm-up rates of B.rostrata are comparable t o bees of similar weight. Despite endothermic ability, field observati ons reveal that B.rostrata are not active below 22-degrees-C. From obs ervations at the nectar-foraging site (clumps of Thymus vulgaris flowe rs) it is calculated that the energy resources available to wasps are sufficient to power endothermic warm-up at low temperatures. Alternati ve explanations for the absence of wasp activity at low temperatures, such as the risk of parasitism, are suggested. Endothermy may be used periodically to increase flight efficiency in response to added load, such as prey carried by females, and mates by males.