SIZE AT MATURITY AND PATTERNS OF GROWTH AND REPRODUCTION IN AN INSECT WITH INDETERMINATE GROWTH

If, in animals with indeterminate growth, fecundity is size-dependent and if a trade-off exists between reproduction and growth, current reproduction affects future reproduction via growth. and natural selection may affect age at first reproduction via mortality. We examined both conditions in the bivoltine springtail Orchesella cincta. Previous studies on this species showed mortality in the field to be high, and age at first reproduction to alternate between generations. In the present laboratory study, using spermatophore supply to manipulate egg production, we show that fecundity depends on size and that there is a trade-off between reproduction and growth. The results allow predictions concerning the relationships between optimal body size at first egg production (which maximizes life time reproductive success) and death rate, growth rate and moulting frequency. Life time reproductive success may furthermore depend on relationships between rates of production and survival. To examine trade-offs between these characteristics, in a second experiment we manipulated growth and egg production by means of quantity and quality of food. Substantial differences in somatic and egg production between groups did not result in different survivorship. However, within groups relative growth rate was negatively correlated with both final body size and longevity, which shows that individuals range from fast growing, small and short living, to slow growing, large and long living.