Larvae of lycaenid butterflies that parasitize ant colonies provide exceptions to normal insect growth rules

Abstract

Fourth instar larvae of Maculinea species of lycaenid butterfly live as social parasites inside Myrmica ant nests. They show highly unusual growth patterns, with small but regular growth in early phytophagous instars, followed by >10 times the growth predicted by extrapolating the early growth rate (following Dyar's rule) during the final carnivorous instar. This produces striking allometry between head and body size in full-grown larvae (ratios of 4–5% compared with 8–10%). Larvae of the Myrmica ant hosts have a similar growth. Data for c. 150 other lycaenid species showed that species with similar life-histories exhibit the same unusual growth pattern (Phengaris spp., Lepidochrysops spp., Niphanda fused); all others have regular growth throughout their larval life, including the carnivorous species that are parasitic on ants from the first instar. It is suggested that Maculinea-type growth pattern has arisen convergently in at least three unrelated lineages of lycaenids. Selection pressures might include the need for reduced early growth to produce late instars that are small enough to be integrated as brood mimics into ant social systems, combined with the need to achieve at least the same adult size as the ancestral species. Trophic pressures that operate on both sedentary ant and butterfly larvae, which must survive long periods of starvation and grow rapidly when food is abundant, may also be involved.