EFFECTS OF EMERGENCE DATE AND MATERNAL SIZE ON EGG DEVELOPMENT AND SIZES OF EGGS AND FIRST-INSTAR NYMPHS OF A SEMELPAROUS AQUATIC INSECT

We examined whether or not sizes of eggs and offspring were related to emergence date or maternal size in a semelparous aquatic insect (the burrowing mayfly, Hexagenia) in which parental care is lacking and ovi posited eggs are passively dispersed. We quantified the size of males and female imagos over the emergence span at a site on the Detroit Riv er, Canada, and investigated relationships between emergence date and female size and (1) egg size and (2) size of first-instar nymphs. Alth ough size of female images (H. limbata and H. rigida combined) decline d significantly (P < 0.025) over the emergence season, there was no si gnificant relationship between body length and emergence date for male s of either species. Males were significantly (P < 0.001) smaller than females. H. limbata eggs, subsampled from three individuals from each of three size classes of female images collected on seven sampling da tes, were measured using video image analysis. Eggs (n = 100) oviposit ed by each of 63 H. limbata images were inspected daily for hatching. Newly hatched nymphs were removed, counted and measured. Egg size (P < 0.001) and size of first-instar nymphs (P < 0.001) varied significant ly with emergence date, but not maternal size. The largest eggs and ne wly hatched nymphs occurred at peak emergence of adults. The synchrono us release of larger (faster-sinking) eggs may result in reduced preda tion. Plasticity in egg development time and egg and nymph size may ac count for the ability of this taxon to recover from episodes of massiv e population reduction.