Freezing the Eversible Glands: An Ultramorphological Study of Larval Nematus Japonicus (Hymenoptera: Tenthredinidae)

Eversible exocrine glands play a central role in chemical defence and signalling in insect larvae, yet their fine morphology remains poorly understood because these structures are normally retracted and difficult to observe. In sawflies (Hymenoptera: Tenthredinidae), particularly within Nematinae, chemical composition and defensive function of ventral eversible glands have been well documented, whereas ultrastructural information on the exposed gland surface has been lacking. Here, an approach was developed to stabilize the eversible gland of Nematus japonicus (Takeuchi 1921) larvae in its everted state, enabling detailed examination using scanning electron microscopy. The everted gland surface is characterized by dense cuticular folding without discrete pores or microtrichia, suggesting a thin, weakly sclerotized integument adapted for diffusion‐based release of volatile compounds. In addition, a previously unreported sensory field is documented on the larval abdomen, including rows of papilla‐like sensilla and dense arrays of sensilla chaetica surrounding the anal slit. These sensory structures are interpreted as components of a coordinated behavioural system involved in faecal expulsion and defensive posture. Together, these findings provide the first ultrastructural characterization of a nematine eversible gland and establish a morphological framework linking gland structure, sensory organization, and defensive behaviour in sawfly larvae.