Activity budgets, habitat use and body size in two coexisting species of sand wasps ( Microbembex: Sphecidae, Hymenoptera)

. 1. Two superficially similar species of Microbembex, M.monodonta (Say) and M.cubana Bohart (Hymenoptera, Sphecidae, Nyssoninae), occur syntopically on Staniel Cay, a small Bahamian island (15 km 2 ). 2. Nesting (female) and territorial (male) aggregations of M.cubana were smaller and more numerous than those of M.monodonta in the study site and on the island as a whole. 3. Both species had regular patterns of daily activities that required them to use a variety of microhabitats in the study area. 4. Males of both species defended territories from c. 09.00 to 12.00 hours EST (in May) and the left to feed on nectar until they retired to sleeping burrows, typically from 13.00 to 15.00 hours. Male M.monodonta were active longer each day and intermixed feeding and territorial defence more than did male M.cubana. 5. Females of both species were active for longer each day, from c. 08.30 to 15.30 hours, and intermixed their respective activities – hunting for provisions, feeding, and nest digging and maintenance – during each day more than did males. Female M.monodonta were active longer each day, rising earlier and retiring later, than female M.cubana. 6. In M.monodonta , females were larger (length, X =11.00 mm) than males (10.5 mm). In M.cubana , males were larger (11.1 mm) than females (10.4 mm). Although mean differences were small, they were highly significant ( F =22.42; P <10 −4 ) for the species‐sex interaction. 7. Differences in activity budgets, habitat use, and sexual size dimorphism appear to be related. A combination of factors, particularly thermal constraints, possible competition, and different mating systems, could explain these patterns.