Every Buggy Pees: Changing Morphometrics of Osmoregulatory Organs in Aquatic and Terrestrial Insects and Changes in Ion Transport Properties in Response to Environmental Changes and Pharmacological Inhibition of Ion Transport

Abstract

Terrestrial and aquatic insects such as crickets, caterpillars, and mosquito larva utilize specialized osmoregulatory organs known as Malpighian tubules to regulate water and ion levels in their hemolymph, a parameter crucial for proper digestion, absorption, and excretion. Structures such as anal papillae are present in aquatic insects to additionally absorb ions from the environment and excrete waste. To examine the effects of dietary ion content on insect osmoregulation, caterpillars were raised on control, high-water and high-K+ diets for several experiments. Changes in dietary ion availability did not increase hemolymph osmolality and did not perturb the transepithelial potential Vte or basolateral membrane potential Vbl in the Malpighian tubules. Interestingly, the number of secondary cells present in the MTs of caterpillars raised on a high-water diet was significantly lower. Malpighian tubule function was additionally examined in crickets using simple dye accumulation assay. Inhibition of active ion transport with DNP, as well as treatment with ouabain (a specific Na+/K+-ATPase inhibitor) and incubation with Na+-free saline decreased the MT function. In mosquito larva, anal papillae serve as both excretory and osmoregulatory organs. To examine whether the size of the anal papillae would change under increasing external ion concentrations, we measured the length and the width of this organ to find that under increasing environmental salinity, there was no significant change in the width of the anal papillae however there was a decrease in length. Although they do not diminish completely because the anal papillae have other functions in mosquito larva in which they play a role in excretion and respiration. These results proposed that the insects have osmoregulatory organs that can change in size in response to environmental and dietary ion availability.