Egg Hatching and Resource Partitioning in Stoneflies: The Six British Leuctra Spp. (Plecoptera: Leuctridae)

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Abstract
(1) The objective was to compare variations in egg hatching between six Leuctra spp. (interspecific variations) and between populations of the same species (intraspecific variations). There were significant interspecific, but not intraspecific, differences in adult size, adult life-span and mean number of eggs laid per female. (2) There were interspecific, but not intraspecific, differences in the percentage of eggs hatching within the temperature range 3.8-19.8.degree. C in the laboratory; optimum temperatures being 6.6-6.7.degree. C (L. geniculata, L. nigra), 8.5-8.7.degree. C (L. hippopus, L. moselyi, L. fusca) and 9.7.degree. C (L. inermis). Over 50% of the eggs of all species hatched in the range 5-15.degree. C. (3) There were interspecific, but not intraspecific, differences in the time taken for 50% of the eggs to hatch and the length of the hatching period in the laboratory; both variables were adequately represented by a negative power-function in relation to water temperature over the relevant range (3.8-19.8.degree. C). Degree-days could also be used to predict hatching time in L. nigra (363.degree. days) and L. geniculata (369.degree. days). (4) For all six species, there was good agreement between hatching times estimated from the power-function and those obtained for thirty-four batches of eggs placed in a Lake District stream. (5) Quantitative information on egg development, now available for fourteen species of European Plecoptera, demonstrates in absence of significant intraspecific differences between populations in the same or different countries. Interspecific differences in egg development in Leuctra spp. are shown to be related to their habitats and flight periods (months in which adults were present). These variations ensure that eggs of different species are hatching throughout the year so that there is a continual succession of species and hence a reduction in possible competition between closely-related species. This investigation therefore provides an example of one physiological mechanism by which complex resource partitioning can occur in closely-related species.

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