Dytiscidae

The Dytiscidae, from the Ancient Greek word δυτικός (dystikos), meaning "able to dive", are the predaceous diving beetles, a family of water beetles. They occur in virtually any freshwater habitat around the world, but a few species live in terrestrial habitats such as among leaf litter. The "diving" in their common name comes from their cycling between underwater and the surface to replenish oxygen like a diver. The adults of most are between 1 and 2.5 cm (0.4–1.0 in) long, though much variation is seen between species. The European Dytiscus latissimus and Brazilian Bifurcitus ducalis are the largest, reaching up to 4.5 and 4.75 cm (1.8 and 1.9 in) respectively, although the latter is listed as extinct by the IUCN. In contrast, the smallest is likely the Australian Limbodessus atypicali of subterranean waters, which only is about 0.9 mm (0.035 in) long. Most are dark brown, blackish, or dark olive in color with golden highlights in some subfamilies. The larvae are commonly known as water tigers due to their voracious appetite. They have short, but sharp mandibles, and immediately upon biting, they deliver digestive enzymes into prey to suck their liquefied remains. The family includes more than 4,000 described species in numerous genera. The oldest of the species is †Palaeodytes gutta, from the Late Jurassic according to Karabastau Formation fossils.

Species employ diverse techniques and traits to source their oxygen underwater. Dytiscidae are adept swimmers, thanks to their enlarged, flattened hind legs with setae and smooth, streamlined, and solid body. Dytiscidae boast distinctive chemical properties, such as defensive secretions containing steroids not known in any other animal. For this reason, diving beetles have been a source for pharmaceutical company R&D. In different parts of East Africa, young girls and boys prompt bites from the beetles for pubertal benefits, and for boys, to help them learn to whistle.

Dytiscidae have also attracted study for notable parts of their evolution, including a sexual arms race, and their body size evolution following a rare early burst model. Ecologically, dytiscids' main limiting factors are anthropogenic activity, fish, and parasitic mites. Surface color and a sufficiency of aquatic plants are other influences on diving beetles' habitats. Due to being most common in unpolluted water, they can be a good water quality indicator. They can potentially control mosquito populations by feeding on larvae, as well. They are able fliers so that they can colonize different habitats. Some species live up to several years, and most are univoltine with 2-3 month breeding periods. Various species overwinter, estivate, or enter diapause. In culture, the diving beetle is prominent in a Cherokee creation story.

Like most other water beetles, adult Dytiscidae have an oval habitus, often tapering toward the head with the pronotum widest at the base. Generally, it is smooth, flattened, and solid. Many species are only smooth macroscopically, though. The dorsum often has microsculpturing, including a mesh of loops, shagreen, lining, dotting, hatching, or granulation in combinations varying by species.

The head, thorax, and abdomen are all streamlined; that is, they are integrated into a single, overall cohesive oval, as opposed to the three visibly articulate sections of some Carabidae like Brachinus. The elytra are so prominent that they conceal the abdominal sclerites.

The head is prognathous both in the adult and larval stages and wider than tall in shape. To a degree, it is drawn into the pronotum, whose pointed anterior, lateral angles abut the head's edge. The clypeus and frons of the head are fused.

Diving beetles largely conform to one mouthpiece model, with the following characteristics:

Diving beetles across subfamilies have a set of six abdominal segments, or ventrites, visible on the belly. They are lined up roughly perpendicularly to the sagittal plane, one after the other. The segments can occupy much of the abdomen's ventral side, from the anterior to the posterior. All the visible segments have different lengths in alignment with the oval shape's curvature. The last ventrite, on the anus, is the hypopygidium. It is modified to end in a notch, tubercle, or keel at the apex, and its surface texture is often wrinkly. Sometimes, the hypopygidium receives a process of the elytra's epipleura inserted into the former's pit. Accompanying the ventrites are 8 tergites and 8 variously-sized pairs of spiracles for breathing. Also an identifying feature of the belly is the prosternal process. It usually projects back to the mesocoxae in a spear shape.