Great Diving Beetle

Dytiscus marginalis — the great diving beetle — is one of the most spectacular freshwater predators in Europe and one of approximately 4,000 species in family Dytiscidae (the predaceous diving beetles, cosmopolitan in fresh water). Adults are 27-35 mm long with streamlined oval bodies, dark olive-brown elytra with a yellow-orange marginal band, and dramatically modified hind legs: the tibia and tarsus carry dense fringes of hairs that function as paddles when the beetle swims. The species is fully aquatic as both adult and larva but carries air for respiration: the beetle surfaces tail-first, presses the abdomen tip against the water surface, and refills an air bubble carried under the elytra (the 'subelytral cavity') that acts as a physical gill — the bubble both stores oxygen and exchanges with dissolved oxygen in the surrounding water. A single bubble lasts 10-30 minutes underwater. Adults hunt tadpoles, small fish, aquatic insect larvae, leeches, and snails — the species can take prey larger than herself. Females have ridged elytra (males have smooth elytra), thought to provide grip during the species' notably long underwater copulations. Larvae are equally famous: large (50-60 mm) elongated 'water tiger' grubs with massive curved hollow mandibles. The mandibles are hollow tubes — the larva grasps prey, injects digestive saliva through the mandible channels, and drinks the partially-liquefied prey through the same channels (the larva has no functional mouth opening). Water tiger larvae are voracious enough to take small fish and tadpoles much larger than themselves. The species is widespread across Europe and into central Asia in still and slow-moving freshwater habitat.