Depth selection by larval Ischnura verticalis (Odonata: Coenagrionidae): effects of temperature and food

SUMMARY. 1. Water temperatures in a shallow, weedy bay of a reservoir in southern Ontario were related to depth; mid‐day temperatures in summer were approximately 30°C at the water surface and 20°C below 25 cm. 2. In the laboratory, larval Ischnura verticalis released in a thermal gradient (warmest water near the surface) did not remain near the surface any more than larvae in isothermal conditions. However, larvae in both isothermal and gradient conditions tended to spend more time close to the surface than expected by chance. 3. Larvae provided with food at all depths spent more time near the surface than did larvae with no food. Larvae provided with one feeding site in the coldest water remained near the food as much as did larvae provided with a single feeding site in the warmest water. 4. Results suggest that selection of microhabitats is based primarily on food availability and secondarily on proximity to the surface.