Hygrothermal determinants of insect activity patterns: the Diptera of water‐lily leaves

. 1. The numbers and types of flies resting on water‐lily leaves varied through the diurnal cycle. The different patterns of occupancy were recorded in relation to time of day and to microclimatic conditions at the leaf surface. 2. Overall fly activity on the leaves varied according to gross climatic patterns on particular days. The timing of activity differed according to species. 3. Smaller, darker flies (especially ernpids and Notiphila ) were common on the leaves early and late in the day, with the more reflective dolichopodids occurring only towards midday and in full sun. Hence there were clearcut correlations between both the size and reflectance properties of flies, and the climate (in particular radiation received at the leaf surface); though this pattern was superimposed on a diurnal activity effect. 4. The timing of insect activity on the leaves could therefore be related to hygrothermal considerations, particularly the raising and maintenance of body temperature and avoidance of heat stress. Different types of fly used the favourable microclimate around the leaves as a resting site at times appropriate to their own biological requirements.