Systematics and bionomics of the apoid obligate necrophages: the Trigona hypogea group (Hymenoptera: Apidae; Meliponinae)

Abstract

A systematic revision and biological account is given of the only apoid obligate necrophages, the Trigona hypogea group, along with descriptions of Trigona necrophaga sp. nov. and the male of Trigona crassipes. Trigona necrophaga is endemic to eastern Panama and possibly the Choco; T. hypogea and T. crassipes are sympatric throughout the Amazon Basin and the Guianas. The lack of a corbicula on the hind leg and reduction of giant setae on the labial palpi, workers foraging upon carrion, lack of stored pollen in nests, and complete absence of pollen grains in larval provisions demonstrate obligate necrophagy in all three social, stingless bee species. Evolution of T. hypogea and T. crassipes from a common ancestor in the Amazon Basin is postulated, and T. necrophaga is likely to be an offshoot of T. crassipes. Morphology links T. crassipes and T. hypogea, but nest architecture joins necrophaga and crassipes. Facultative necrophagy in Trigona probably became obligate due to preadaptations including (1) massive forager recruitment via pheromone trails; (2) aggressive foraging on carrion exudates; (3) apical mandibular teeth; (4) rapid location of novel resources; (5) beneficial microbes in bee glandular secretions that metabolize protein and produce antibiotics, and (6) queen cells and food storage pots of the same size and close proximity. Accidental placement of regurgitated, partly-digested carrion in queen cells may have promoted admixture of hypopharyngeal glandular secretions and microbes with harvested liquid carrion. The most derived species, T. necrophaga, may have additional mutualist microbes because only this species has distinctive grey-green protein, and five Bacillus species in food pots and brood cells. No unusual anatomical features were found in worker bees. All three species make sweet, clear honey that contains some pollen but is of unknown origin.