Helichus striatus is a “long-toed” beetle in the family Dryopidae. Although these beetles are aquatic, they do not swim, but cling to sticks and other floating detritus. They lack physical gills that are present in many groups of aquatic insects. Instead they create a plastron, an air bubble used for breathing.
In 1936, Microscopist, Hilda Harper, investigated the contribution of plastrons to oxygen consumption and survival underwater. The beetle, Helichus striatus, is able to survive over 40 days underwater. The beetle has branched water repelling hairs capable of holding an air bubble in place under its elytra and much of its body when submerged. The spiracles of the beetle open into parts of the plastron.
Harpster measured oxygen content of water with and without beetles and found that beetles were actively removing oxygen from the water. When she placed beetles in oxygen free water, survival was reduced significantly to between 2 and 5 days.
Having established the ability to acquire oxygen from the water, Harpster investigated the role of the plastron. Plastrons can be removed from submerged beetles by gentle brushing. Harpster found that complete removal of the plastron reduced longevity from as many as 40 days to less than 7 days. Partial removal of the plastron had an intermediate effect. Beetles without a plastron had no better survival in oxygenated water than beetles with no oxygen in the water. These experiments provided support for the hypothesis that plastrons were important for underwater respiration.
Hilda T. Harpster. An Investigation of the Gaseous Plastron as a Respiratory Mechanism in Helichus striatus Leconte (Dryopidae). Transactions of the American Microscopical Society, Vol. 60, No. 3 (Jul., 1941), pp. 329- 358.