Cornell Institute for Research in Chemical Ecology, Mudd Hall, Cornell University, Ithaca, NY 14853, USA.
Female Photuris fireflies contain two different types of defensive chemicals. They sequester steroidal pyrones (lucibufagins) from male fireflies of the genus Photinus that they eat, and themselves produce the defensive betaine N-methylquinolinium 2-carboxylate. Chemical analyses of Photuris eggs showed that females that fed on Photinus males endow their eggs with both lucibufagins and the betaine, while those that did not feed on Photinus lay eggs that contain betaine, but virtually no lucibufagin. Photuris females collected in the field during the Photinus flight season laid eggs that invariably contained betaine, but lucibufagin only at times. Predation experiments showed that Photuris eggs are essentially unacceptable to larvae of a coccinellid beetle (Harmonia axyridis) and an ant (Leptothorax longispinosus), but moderately acceptable to an earwig (Forficula auricularia). When applied experimentally to palatable insect eggs, lucibufagins proved deterrent to these three predators, while betaine proved deterrent to ants and coccinellid beetle larvae only. Both types of defensive compound decreased egg predation in the field. By endowing their eggs with both exogenous and endogenous chemicals, Photuris females are essentially "maximizing their options": when feeding on Photinus, their eggs are doubly protected, but they are not entirely defenseless when the females are unable to catch Photinus prey. The dual endowment could also broaden the spectrum of antipredatory efficacy of the defense.