Circulatory responses to submersion in larvae of Phaeoxantha klugii (Coleoptera: Cicindelidae) from Central Amazonian floodplains

The univoltine tiger beetle Phaeoxantha klugii survives an annual inundation period of up to 3.5 months in the third larval stage submerged in the soil. We visually determined the circulatory response to submersion of third-stage larvae in the laboratory. Heart rate depression upon submersion was about 50–67% during the first hours and reached values near zero in most larvae submerged for 2.8–6 days. Apart from a rapid first increase upon removal from the water the heart rate varied greatly within and between individuals. Longer submersion lead to stronger circulatory depression and delayed recovery (time until resuming of digging activity). The heart rate of larvae resuming digging activity was only 66% of the initial rate before submersion. Unlike the duration of recovery the heart rate relative to the initial values before submersion was not correlated with the duration of preceding submersion. The degree of circulatory depression during the first hours of submersion is less drastic than the metabolic depression of 75–95% measured as oxygen uptake in submerged larvae reported in a previous study. Visual determination of heart rate might be an useful tool to study physiological aspects of submersion and hypoxia resistance also in other insects