Morphological Study of the Venom Apparatus and Feeding Behaviour of Predacious Heteropterans <i>Lethocerus indicus</i>, <i>Diplonychus Rusticus</i>, and <i>Laccotrephes maculatus</i>

Giant water bugs Lethocerus indicus (LePeletier and Seville, 1825), Diplonychus rusticus (Fabricius,1781), and water scorpion Laccotrephes maculatus (Fabricius, 1775) are predaceous heteropteran insects. Salivary glands play a crucial role in their predatory nature. Their venomous morphological structures consist of a complex pair of posterior main glands (PMG), anterior main glands (AMG) and accessory glands (AG). Every gland part is connected by ducts, which open at the hilus. To understand the salivary gland morphological structure, an examination was done for five male and five female insects for all three species. Their comparative feeding behavior was also observed to infer physiological and functional relationships. After dissection, AG_sac diameter, AG_sac length, PMG, AG, and AMG measurements were taken from stereomicroscope images using Fiji software. Morphologically, all three insect’s gland components showed striking similarities. However, the PMG of L. maculatus displays continuous cylindrical tapering while the other two display irregular tapering and a sudden constriction of the PMG. A greater length of salivary glands was observed in L. indicus as compared to the other two species. Interestingly, the length of AMG in D. rusticus was almost half the size of PMG, unlike other insects where AMG is smaller than PMG. This observation emphasizes the physiological and morphological evolution in smaller and larger predatory insects of the Belostomatidae and Napidae family. Males and females showed no significant difference in the proportionate gland length against body length, for all the glands except AG_sac of D. rusticus. A storage sac at the posterior proboscis, containing a small pore that discharged venom during harassment was observed in L. indicus. The three insects exhibited ten common feeding behaviours. D. rusticus displayed unique scavenger-like behavior along with live predation, whereas the other two insects had a common behavior of extending or retracting siphons for respiration when the body is submerged.