Comparative analysis of overwintering physiology in nine species of semi‐aquatic bugs (Heteroptera: Gerromorpha)

In semi‐aquatic bugs (Heteroptera: Gerromorpha), the strategies of overwintering in a cryothermic state (i.e. at body temperatures below the equilibrium freezing point) remain largely unexplored. The present study provides an analysis of the ecophysiological aspects of overwintering in nine gerromorphan species. All nine species avoid ice formation by means of a more or less extensive supercooling of their body fluids. There is a tight correlation between the supercooling point (SCP) and the lower lethal temperature. Different species use different physiological adjustments to increase the likelihood of survival in a supercooled state. These include stabilization of the supercooled state by active antifreeze factors that cause thermal hysteresis between equilibrium melting and freezing points, the accumulation of low‐molecular weight sugars and polyols with putative cryoprotective functions, or by having a relatively high body fluid osmolality, combined with a low level of hydration. The majority of species under study overwinter only as adults, whereas Velia caprai Tamanini can overwinter either as an adult or in the egg stage. The supercooling capacity of V. caprai adults is insufficient to prevent the risk of lethal freezing. The adults therefore survive only opportunistically in suitable microhabitats, and/or during mild winters. The survival of V. caprai in winter is assured by extensive supercooling and having overwintering eggs that are highly cold tolerant.