BODY-SIZE AND SEX-RATIO IN EMERGENT STONEFLY NYMPHS (ISOGENOIDES OLIVACEUS, PERLODIDAE) - VARIATION BETWEEN COHORTS AND POPULATIONS

The ecological significance of pattern in animal-size distribution has been receiving increased scrutiny in studies of population regulation and individual energetics. However, few studies have assessed differe nces in size variation between sequential generations or adjacent popu lations. Annual variation in body size and sex ratio of emergent perlo did stonefly nymphs, Isogenoides olivaceus (Frison) (Plecoptera: Perlo didae), were examined in populations from two Michigan mesic groundwat er rivers. Final nymphal body size was estimated from sex-specific len gth to mass regression relationships using exuviae collected from each site. Sexual size dimorphism accounted for much of the intraspecific size variation within both populations and cohorts: females were large r ((x) over bar = 24.4 mg) than males ((x) over bar = 10.0 mg). Males showed less size variation than females; unlike those for females, siz e distributions for males were significantly skewed. Secondary sex rat ios were female biased in both populations and all cohorts and ranged between 59.1 and 72.8%. Female sex bias increased with relative male s ize in both populations.