THE COADAPTATION OF FEMALE MORPHOLOGY AND OFFSPRING SIZE - A COMPARATIVE-ANALYSIS IN CRICKETS

Few studies of invertebrates have considered combinations of morpholog ical and life history traits in the context of the evolution of reprod uctive strategies. Cricket species that exploit habitats harsh with re spect to egg survival have evolved a long ovipositor, presumably becau se laying deep in the soil reduces egg mortality. Yet hatchling mortal ity increases with laying depth, and the ability of hatchlings to clim b through the soil increases with egg size. Thus a conflict may exist between survival of the egg and that of the hatchling, inducing a posi tive covariation between egg size and ovipositor length across species evolving under contrasting selective habitats. We used the phylogenet ic autocorrelation method and a path analysis to assess whether egg si ze coevolved with ovipositor length across 40 species of crickets, and whether egg size was affected by body size or ecological factors that influence egg mortality. Body size and ovipositor length were affecte d by taxonomic association, whereas common ancestry had no significant effect on egg size, diapausing strategy, and oviposition preference f or soil types. The path model indicated that 29.11% of the variance in egg size was explained by independent evolution. As expected, oviposi tor length was positively correlated with egg size, and species diapau sing in the egg stage produced larger eggs than crickets diapausing in the nymphal stage or with no diapause. Ovipositor length and diapausi ng strategy were the first and second most important traits, respectiv ely, in term of the proportion of variance in egg size explained by sp ecific values. These results support the hypothesis that the ability o f hatchlings to climb through the soil, and variation in diapause stra tegies, are general selective factors affecting the evolution of egg s ize in crickets. Phylogeny explained 51.01% of the variance in egg siz e. Egg size in a current cricket species, however, was not directly de termined by egg size in its ancestor. Instead, it was strongly related to the phylogenetic values of body size and ovipositor length. Such i ndirect phylogenetic effects of body size and ovipositor length may ha ve arisen because clades originating from ancestors with different ovi positor lengths experienced different selective pressures on egg size.