Number of larval instars and sex-specific plasticity in the development of the small heath butterfly, Coenonympha pamphilus (Lepidoptera: Nymphalidae)

Enrique García-Barros

doi:10.14411/eje.2006.007

Abstract

The number of larval moults, larval head capsule width and pupal weight were investigated in both direct-developing and diapausing individuals of a South-West European population of Coenonympha pamphilus. The frequency distributions of head widths of successive larval instars overlapped, partly due to variation in the number of larval moults. The larvae that entered diapause went through five instars, instead of the four reported from this species. The evidence indicates that the five instar developmental pathway represents a plastic response rather than an example of compensatory growth. This alternative growth pattern was expressed in response to short photoperiods in parallel with, or as a consequence of, larval diapause. On average, the larvae with five instars had larger heads than their normal siblings. This resulted in comparatively heavier male pupae, while the opposite trend occurred in females. It is concluded that the variation in the number of larval instars is a plastic response to diapause when temperatures remain mild and that it might have an adaptive value in areas with mild winter climates. The sexually dimorphic expression in the larval growth patterns, in terms of pupal weight, may well imply different patterns of allocation of larval resources to adult structures, although sex-dependent differences in investment into purely larval structures cannot be discounted.

Highlights

Phenotypic plasticity in insect life histories provides a way of optimising phenotype to the environment, which may substantially contribute to individual fitness in seasonal habitats (Tauber & Tauber, 1981; West-Eberhart, 1989; Nylin & Gotthard, 1998)
There were no significant differences between the mean head capsule width of the first instar of both groups (F1, 401 = 0.22, P = 0.64), while that of second instar larvae that underwent five instars was smaller than that of larvae with four instars (F1, 401= 12.46, P = 0.0005) (Table 1)
The results indicate that larval head width is not a reliable tool for identifying instar in this butterfly

Summary

Introduction

Phenotypic plasticity in insect life histories provides a way of optimising phenotype to the environment, which may substantially contribute to individual fitness in seasonal habitats (Tauber & Tauber, 1981; West-Eberhart, 1989; Nylin & Gotthard, 1998). Recent studies on plastic responses in the phenology of butterfly life histories have often focused on three crucial traits, i.e. size, development time and growth rate (Nylin, 1994). Pupal weight is an accurate expression of the total weight gain achieved by the larvae. This “composite” interpretation of the role of the pupal stage may mask some complex underlying relations, such as sex-linked differential investment of larval-gathered resources into adult structures (the more evident example of this being sexually dimorphic adult to pupal weight ratios). A further insight into plastic responses during the larval stage may be gained by using measurements of larval size that are independent of weight and show a variable response to environmental factors

Methods

Results

Conclusion