6-Alkyl-1,4-naphthoquinones from the defensive secretion of the tenebrionid beetle, Argoporis alutacea

Abstract

The abdominal defensive glands of the western American tenebrionid beetle, Argoporis alutacea, contain a viscous mixture of 6-methyl, ethyl-, propyl-, and butyl-1,4-naphthoquinones as well as the usual tenebrionid 1,4-benzoquinones. The naphthoquinones were isolated by preparative gas-liquid chromatography (GLC) and their structures determined by GLC, infra-red, mass, and nuclear magnetic spectroscopy of the isolated compounds and several synthetic isomers. Two additional minor components were not identified. The tubular, spirally stiffened gland reservoirs are derived from the seventh intersternal membrane and are homologous to other defensive glands in the subfamily Tenebrioninae. Upon handling, the secretion is released by protracting the sixth and seventh sternites and everting the proximal quarter of the reservoir. With its legs, the beetle wipes the secretion onto itself and the handler. The well-tracheated secretory tissue consists of strands and sheets of cells arranged along and attached by their long effluent ducts which empty along a transverse line near the reservoirs' posterior opening. The surface of the reservoir sacs is covered with a thin epidermis. Electron microscopy of the gland tissue revealed a secretory unit which consists of a long tubular cuticle-lined organelle sequentially ensheathed by three types of cells: (1) a cell with a large microvillusfilled vesicle surrounding the blind distal end of the cuticular organelle, (2) an adjacent second cell into whose central vesicle (which is delimited by small sparse microvilli) the cuticular organelle passes, becoming expanded and diverticulated, and (3) at least one tubule-sheathing cell surrounding each separate cuticular organelle from the point at which it leaves the second cell to its proximal termination at the reservoir.