Bufadienolides (lucibufagins) from an ecologically aberrant firefly (Ellychnia corrusca)

Abstract

The winter firefly (Ellychnia corrusca) is common throughout the eastern United States and Canada. It is notable among fireflies (Coleoptera: Lampyridae) for its adult activity during the winter and early spring months, and for its lack of a light-producing organ. Multiple lines of evidence suggest that E. corrusca is chemically protected, specifically its unusual phenology which results in prolonged exposure to predators, its aposematic coloration, its reflex bleeding upon disturbance, and its relatedness to toxic firefly genera. Chemical investigation of this species revealed the presence of large quantities (>500 μg/beetle) of steroidal pyrones known as lucibufagins. The structures of these lucibufagins were determined using 1D and 2D NMR spectroscopy along with UHPLC-HRMS. Four of these compounds were previously detected in fireflies of the genus Photinus, while the remaining four, which contain a highly modified D-ring of the steroid core, are previously unknown. Analysis of reflex bleeding showed that the hemolymph contained the lucibufagins and that it was primarily released via a band of cuticular pores along the costal margin of the elytra. In addition, our analysis of the eggs, larva, and pupa established that all immature stages possess lucibufagins, thus representing the first instance of a firefly species for which all life stages have been investigated chemically.