Dytiscidae


The Dytiscidae, from the Ancient Greek word δυτικός, 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 long, though much variation is seen between species. The European Dytiscus latissimus and Brazilian Bifurcitus ducalis are the largest, reaching up to 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 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.

Shape and morphology

Habitus

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.

Structure

Head

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:
  • A transverse upper lip with a notch in the middle of the anterior edge
  • The mandibles' apex forked into two
  • Large, sickle-shaped lacinia with apex pointed and covered with dense strong hairs along the inner edge
  • Narrow galeae, with two segments as in most other Adephaga
  • Maxillary palpi with four segments and a developed palpiger, and labial palpi with three
  • Large submentum apically expanded into lateral projections
  • Transverse, concave mentum with wide, upward-curved lateral projections meeting the prementum
  • Transverse prementum with the anterior margin moderately notched and covered with long setae
  • Notch on the mentum's anteromedial edge forming a median projection
  • Another notch on the projection

    Abdomen

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.
The male aedeagus is symmetrical, although Dytiscidae is among the beetle families whose aedeagus experiences retournement over development. The female ovipositor includes a pair of appendages connected with the hypopygidium called gonocoxites and basal sclerites called valvifers. The structure is set into the abdomen.

Legs

The four anterior legs are rather short, and the front pair is slightly flattened, whereas the hind legs are enlarged and strongly flattened for swimming. Generally, the legs' front tibiae lack spines. The tarsi have a tarsal formula of 5-5-5 and either one or two claws. Species with two claws have their mesoscutellum large and exposed. The claws are not toothed and can either be equal or unequal in size. The fore and middle tarsi have four segments, with the fourth sometimes very small and concealed between the third segment's lobes.

Identification

Unlike Noteridae and most Hydrophilidae, the dorsal surface is not more strongly convex than the ventral. Dytiscidae also differ from Noteridae in their mandibles, since they lack enlarged portions there of the molars. The antennae are glabrous and number up to 11 depending on the species. Dytiscid antennae are threadlike, generally longer than the head's width, and look like their palps except longer, whereas hydrophilid and gyrinid antennae are clubbed. Their eyes are flat rather than protruding out, and, unlike Gyrinidae, are not divided. Unlike Carabidae, they do not have a transverse suture on their metasternum. They also lack the keel-like feature between the legs of many hydrophilids. All known diving beetles except ones in the genus Celina have the scutellum concealed, with only a minuscule part seen from the surface. This does not mean that the scutellum is invisible as in Noteridae.

Sexual dimorphism

Many species in Dytiscidae are sexually dimorphic. Males have suction cup-like palettes on their legs to help them keep grip of females during copulation, and, in many species, females have furrows on their elytra and variously the pronotum and base of the head. In males, these parts are smooth. The furrows of the female uneven the elytral structure, interfering with the male's grip. They weaken it likely with the aim of increasing the female's control over mating.

Size

The length of adults averages at 1 to 2.5 cm, and the elongate larvae range from 1 to 5 cm. The largest known adults are of the species Megadytes ducalis, at up to 4.75 cm, and Dytiscus latissimus, at up to 4.5. At about 0.9 mm, meanwhile, Limbodessus atypicali is likely the smallest.

Color

Most are dark brown, blackish, or dark olive in color. Diving beetles in some subfamilies sport golden highlights on the dorsal borders, others variously-colored spots or bands. Sometimes, diving beetles are reflective with a metallic appearance.

Swimming and water navigation

Role of body shape

Diving beetles' shape is optimized to ease navigation through water by reducing drag and improving stability while swimming. No segment moves or bends off balance due to being integrated with the others. This reduces form drag in two ways. One, it minimizes frontal resistance, which can upturn a swimmer with an uneven position. Two, it minimizes eddies, or waves, from eddy resistance that excessive, especially jerky, movement can incur. As they swim, diving beetles further streamline their bodies by tucking their four former legs into well-fitting grooves. Their bodies' water resistance differs considerably by hunting type, such as search or ambush.

Oxygen

Like other water beetles, adult Dytiscidae get their oxygen while swimming by storing air in a space between their elytra and abdomen. At the same time, they can also diffuse dissolved oxygen from the water. The former ability keeps Dytiscidae alive underwater for about 30 minutes, whereas the two combined can give them around 24–36 hours' worth of oxygen in one go. Some Dytiscidae have an additional way: using their elytra as a respiratory organ. One of them, Deronectes aubei, has been recorded to survive 6 weeks without atmospheric oxygen. Deronectes aubei and other smaller Hydroporinae have specialized setae on the elytra, pronotum, and ventral side that act as tracheal gills. The setae form a layer that keeps out water and traps air. Smaller species like these can also stay underwater for weeks because they can live off oxygen from natural vegetation. Another feature acting as a gill in diving beetles is a small air bubble pressed out from the subelytral cavity and held by the hydrofuge hairs at the tip of the abdomen. The bubble shrinks over time, requiring the beetles to surface periodically due to gas exchange decreasing. This behavior of alternating between the surface and high depths is why they are known as diving beetles. When bigger species break the surface, they "hang" on it with their rear end protruding slightly. Along with regaining their bubble, they can also bend their abdomen slightly downwards for gas exchange in the subelytral opening. Since larvae lack ventral gills, larvae instead have a siphon at the tip of the abdomen that they draw in air with.