Defensive Glands of the Darkling Beetle Mesomorphus villiger Blanchard (Coleoptera: Tenebrionidae)

Massive home invasion by the darkling beetle Mesomorphus villiger Blanchard 1853 (Coleoptera: Tenebrionidae) during monsoon season make it a nuisance pest in many regions of south India. Morphology of defensive glands and mode of release and dispersal of the defensive secretion were analysed. Defensive glands were separated from the abdominal sternites by cutting along the posterior margin of the seventh sternite. Glands are evaginations of intersegmental membrane between the seventh and eighth sternites consisting of two long sac-like reservoirs, and glandular secretion is released by exudation and spread through epipleural gutter of elytra. Gradual release of the secretion is a strategy to repel the predators for a longer duration. 1. Introduction Darkling beetle, Mesomorphus villiger Blanchard 1853 (Coleoptera: Tenebrionidae: Opatrini), is of cosmopolitan distribution with occurrence in Indian subcontinent, Afghanistan, Siberia, Australia, and Africa (Madagascar) [1–3]. Nibbling and gnawing at the base of the stem of newly transplanted tobacco seedlings lead to the death of the plants. Hence, they are referred as tobacco ground beetle in tobacco growing belts in India [1, 4]. However, in the Kerala state in south India, they are present in the litter of rubber (Hevea brasiliensis ((Willd. ex Adr. De Jus) Müll. Arg. 1865)), mango (Mangifera indica, Linnaeus 1753), cashew (Anacardium occidentale, Linnaeus 1753), and rain tree (Samanea saman (Jacquin) Merrill 1916) and have strong feeding preference towards fallen tender leaves (personal observation). Home invasion of huge aggregation of M. villiger into residential buildings with the onset of monsoon season, their nocturnal movements and release of an irritating, odoriferous quinonic secretion that causes mild skin burns, makes it a nuisance pest in many regions of the South Western Ghats. Similarities in morphology and the aggregation pattern by M. villiger often lead to its misidentification as rubber litter beetle Luprops tristis (Fabricius, 1801). No data exists on the structure of defensive glands of the genus, and the present study analyses the structure of defensive glands and mode of release and dispersal of the defensive gland secretion in M. villiger. 2. Materials and Methods Aggregated beetles were collected from a residential building at Calicut (11°15′N, 75°50′E), in south India, during the monsoon season. Adults of both sexes were killed using diethyl ether and pinned to a wax tray. Elytra and abdominal tergites were removed to expose the internal structures and observed under a stereo