Allometry and functional constraints on proboscis lengths in butterflies

Summary1 In generalist nectar‐feeding insects such as butterflies, body size and proboscis length show an allometric relationship. Butterflies that deviate from this relationship and have disproportionately long proboscides can access nectar from deep flowers, which is inaccessible to species of similar or larger body size but with shorter proboscides. 2 Despite this selective advantage, few species possess disproportionately long proboscides for their body size, which indicates that there may be developmental, functional or other ecological constraints on very long proboscides. I hypothesized that species with disproportionately long proboscides had a functional cost in terms of higher handling time (amount of time spent per flower); therefore, they were at a competitive disadvantage compared to butterflies that had shorter proboscides and lower handling times. 3 I tested this hypothesis using Costa Rican butterflies. I measured body length, proboscis length and handling time on Lantana and Wedelia , two nectar plants with generalist pollination systems which attract large numbers of nectar‐feeding butterfly species. 4 There was a strong positive relationship between ‘relative proboscis length’ (proboscis length in relation to body size) and handling time per flower on both nectar plants. Species with greater relative proboscis length had up to three times longer handling time per flower. Thus, butterflies with relatively long proboscides should harvest less nectar per unit time from the same flower than butterflies with normal proboscides. 5 Reduced foraging efficiency in the face of competition from other nectarivores may thus be a functional constraint that limits the evolution of disproportionately long proboscides in generalist nectar‐feeding butterflies.