Comparative analysis of larval growth in Lepidoptera reveals instar-level constraints

Abstract

Juvenile growth trajectories evolve via the interplay of selective pressures on age and size at maturity, and developmental constraints. In insects, the moulting cycle is a major constraint on larval growth trajectories. Surface area to volume ratio of a larva decreases during growth, so renewal of certain surfaces by moulting is likely needed for the maintenance of physiological efficiency. A null hypothesis of isometry, implied by Dyar's Rule, would mean that the relative measures of growth remain constant across moults and instars. We studied ontogenetic changes and allometry in instar-specific characteristics of larval growth in 30 lepidopteran species in a phylogenetic comparative framework. Relative instar-specific mass increments (RMI) typically, but not invariably, decreased across instars. Ontogenetic change in RMIs varied among families with little within-family variation. End-of-instar growth deceleration (GD) became stronger with increasing body size across instars. Across-instar change in GD was conserved across taxa. Ontogenetic allometry was generally non-isometric in both RMI and GD. Results indicate that detailed studies on multiple species are needed for generalizations concerning growth trajectory evolution. Developmental and physiological mechanisms affecting growth trajectory evolution show different degrees of evolutionary conservatism, which must be incorporated into models of age and size at maturation.