DIFFERENCES IN AGE STRUCTURE AMONG FIELD CRICKET POPULATIONS (ORTHOPTERA, GRYLLIDAE) - POSSIBLE INFLUENCE OF A SEX-BIASED PARASITOID

This study examined age structure in adult populations of three specie s of field cricket, Gryllus veletis, G. pennsylvanicus, and G. integer . Adults were aged by counting growth layers in cross sections of tibi ae. The study species differ in several life-history traits including the likelihood of parasitism by Ormia ochracea, a tachinid that orient s to calling males. Gryllus integer is parasitized whereas G. veletis and G. pennsylvanicus are-not. Such differences between the species sh ould result in different age patterns. Data from field collections dem onstrated that adult G. veletis and G. pennsylvanicus had similar maxi mum life-spans of about 4 weeks, and males were similar in age or slig htly older than females. The maximum age for female G. integer was als o about 4 weeks, but few males >20 days old were encountered. Moreover , male G. integer were significantly younger than females in five out of six samples. This pattern in G. integer, evident in 2 successive ye ars, could be consistent with sex-biased mortality by Ormia ochracea. The results are discussed in relation to differential longevities and the intensity of sexual selection on male mating behaviour.