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Abstract: Reproductive output is an important parameter of individual fitness. As a rule, insect offspring number is correlated with specimen size, and for a given species smaller adults lay fewer eggs than larger individuals. Whether small individuals may at the same time lay smaller eggs than larger specimens was studied for two species of Leuctra. However, direct measurements of egg size proved impractical because of continuous changes of egg size during development. The volume of many insect eggs is known to increase during development. For several stonefly species, the process is described here. In diapausing plecopteran eggs (Protonemura intricata, Amphinemura standfussi) swelling occurs in two steps. The size of freshly laid eggs increases a little after immersion; after a few days size remains stable. When development begins a pronounced size increase is observed which is a good indicator of diapause termination and of beginning development which are otherwise less easily determined. The initial swelling seems to correspond to water uptake by the egg establishing an osmotic equilibrium between the egg and its dilute surrounding medium. Subsequent egg swelling during development follows a sigmoidal pattern, and size is related to developmental stage; maximum increase occurs at katatrepsis. The swelling during development is probably also driven by osmosis, the initial equilibrium being offset by large molecular storage materials in the yolk being metabolized and broken down into smaller compounds. Space requirements and mechanical pressure of the increasingly complex structure of the embryo may be additional causes for the observed increase of egg size. Because of these permanent changes of egg size, offspring size had to be determined in freshly hatched larvae. Size of hatchlings (head capsule width, HCW) in the two Leuctra spp. was unrelated to maternal size (expressed as wing length, a good measure of maternal biomass). However, females of L. digitata produced significantly larger offspring than females of L. prima of the same size. Similarly, Protonemura nitida lays larger eggs than P. auberti, although adults are of similar size. The possible role of such differences for species' fitness is discussed.
