EFFECTS OF TEMPERATURE AND PHOTOPERIOD ON LARVAL SIZE AND SURVIVORSHIP OF A BURROWING MAYFLY (EPHEMEROPTERA, EPHEMERIDAE)

There is great size variation in naturally occurring and laboratory-reared populations of larvae of Hexagenia species. We investigated differences in mean larval size and survivorship of Hexagenia limbata and Hexagenia rigida reared at different temperatures (12, 17, and 22-degrees-C) and photoperiods (24 h L, 24 h D, 12 h L : 12 h D) over two time intervals (60 and 120 d) in the laboratory. Owing to space limitations, two sets of replicates (3 replicates per set) were conducted in sequence for the 60-d trial. Larvae were hatched from eggs collected from imagoes at the Detroit River near Windsor, Ontario, and stored at 8-degrees-C. Of the factors examined, temperature alone influenced larval size after 60 d of growth. None of the factors had any significant effect on larval survival after 60 d. Larvae from the second 60-d replicate set (hatched from eggs incubated for 75 d longer than the first set) were larger and exhibited lower survivorship than larvae in the first set. This finding suggests that larval size may not be a good indicator of larval age, especially if eggs have been exposed to low temperatures for varying periods. Temperature and its interaction with photoperiod significantly affected both size and survivorship of larvae reared for 120 d. At 22-degrees-C, larvae reared under the 12 h L : 12 h D regime were larger than those reared under either constant tight or dark conditions: no discernible trend in photoperiod was evident at the lower temperatures. Larval survivorship was lowest for treatments exhibiting conflicting environmental cues (12-degrees-C and 24 h L, and 22-degrees-C and 24 h D).