Growth allometry of immature insects: larvae do not grow exponentially

1. The allometric relationship between growth rate and body mass has received considerable attention but different taxa have not been equally studied. In particular, a limited amount of information is available on growth allometry of insect larvae. 2. In life-history studies, it is often assumed that insect larvae grow exponentially. This leads one to expect that potential rewards of extending growth periods are high in terms of increased adult body masses and fecundities. Therefore, it has been a challenge to find costs of large size that counterbalance the fecundity advantage of attaining larger sizes. 3. This study examines the intraspecific growth allometry of lepidopteran larvae. Original methodology is proposed to address problems arising from the complexity of the insect growth curve, and the high sensitivity of growth rates to environmental conditions. To facilitate generalizations, larvae of 11 unrelated lepidopteran species were subjected to an identical study design. 4. Instantaneous absolute growth rates of larvae were related to body size by an intraspecific allometric exponent in the range between 0.41 and 0.88. There were significant differences between the species but values of the exponent as high as 1 (exponential growth), and as low as 0 (linear growth) could safely be excluded. 5. Instantaneous relative growth rates of larvae were typically 35% lower in their last instar when compared to the penultimate one. Using the exponential growth curve (i.e. assuming the constancy of relative growth rates) in modelling insect life-histories may therefore lead to substantially biased conclusions.