Environmental Factors Controlling Duration of Larval Instars in Tetragoneuria Cynosura (Odonata)

Abstract

Groups of Tetragoneuria cynosura larvae were subjected to combinations of experimental and natural temperatures and photoperiods until emergence. For each of the later instars, responses to the various regimens by larvae in each collection were analyzed to determine instar receptivity of these environmental factors. Final instar duration for larvae subjected to a 11—hr day varied inversely with the temperature at which the animals were maintained. For larvae collected prior to the autumnal equinox and maintained at a 14—hr photoperiod, final instar development was greatly delayed when compared to responses of short—day animals. Long—day larvae collected after the equinox responded significantly faster than their short—day counterparts. This dramatic reversal in response occurred shortly following the equinox. Significantly delayed the development attributable to long days was noted in the F—1 (penultimate) instar and to diminishing degrees in earlier in stars. For larvae at lake temperatures, long days retarded development in the F—1 and F—2 (antepenultimate) instars compared to short days. Final instar duration was compensatory to the rates of development in earlier instars since all animals at lake temperatures emerged together regardless of their photoperiodic regime. The equinox, a crucial synchronizing mechanism for this species, induced both the populations to molt synchronously into the final instar and establised a larval physiological sensitivity to spring photoperiods. Rising vernal water temperatures served as a second synchronizing element. Animals collected prior to the equinox were in a radically different physiological state from larvae that had experienced the equinox in nature _kw Tetragoneuria cynosura