The Smoky Stick Insect: Defensive Chemicals From a Vietnamese Phasmatodean (Necrosciinae: Neohirasea catbaensis)

Beyond their impressive repertoire of camouflage as the predominant primary defensive strategy, stick and leaf insects (Phasmatodea) can actively defend themselves chemically with deterrent substances emitted from a pair of prothoracic repellent glands. The Vietnamese Prickly Stick Insect Neohirasea catbaensis emits a conspicuous secretion when disturbed, often described as smoky, which constitutes a unique characteristic not known from any other stick and leaf insect. Here we investigate the anatomy and the chemical components of the repellent glands in this species. Via micro-computed tomography (µ-CT), we observed comparatively small sac-like glands with no ejaculatory ducts present. The anatomy of the repellent glands of both male and female is described in detail. By applying gas chromatography (GC) in combination with high-resolution accurate mass spectrometry (HRAM-MS), we found 4-vinylphenol as the major component of the secretion, accompanied by 2-methoxy-4-vinylphenol and eugenol as minor constituents. This is the first time that these molecules have been identified as repellent substances from an animal. We discuss these findings in relation to the chemical diversity and ecological function of phasmatodean repellent secretions.