The Dytiscidae – based on the Greek dytikos (δυτικός), "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 among leaf litter.^[1] 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 Megadytes ducalis are the largest, reaching up to 4.5 and 4.75 cm (1.8 and 1.9 in) respectively.^[1][2] In contrast, the smallest is likely the Australian Limbodessus atypicali of subterranean waters, which only is about 0.9 mm (0.035 in) long.^[1] 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.^[3] 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.^[4]

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. 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.

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.^[5]

Sex-wise, 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 make the elytral structure uneven, interfering with the male’s grip. They weaken it likely with the aim of increasing the female’s control over mating. ^[6][7]

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.^[8] As they swim, diving beetles further streamline their bodies by tucking their four former legs into well-fitting grooves.^[9]

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.^[10]

Their two hind legs are much larger and wider than the other two pairs of legs so that they can use them as oars or paddles and move faster. The setae on the legs are there to help them change direction quickly while swimming. They can swim both forwards and in reverse effectively without needing to rotate. Unlike Hydrophilidae, their hind legs move in synchrony while swimming, namely during the forward and backward motions. The legs move asymmetrically while turning, in opposing directions, to steer the beetle sideways. Unlike other aquatic animals such as turtles, jellyfish, fish, and frogs, they both can stay in one direction while retreating and have a lower turning radius when they do turn. These strengths are a testament to their superior flexibility.

Diving beetles’ swimming process as a whole cycles between two actions: the power stroke and the recovery stroke. The power stroke’s function is to increase propulsion by means of maximizing the beetle’s cross-sectional area, which involves stretching the tibiae and tarsi and spreading out the setae. Because of the thrust that the hind legs give, the speed of each cycle peaks then, during a period of about 60% of the stroke. In the recovery stroke, the beetle then reduces the water resistance by rotating its tarsi 90° and folding the setae. Each cycle lasts for about 272 milliseconds, and the power stroke takes up about 47% of it. Most of the acceleration happens in the first 50 milliseconds. Likely for the purpose of escaping from predators, the acceleration is especially high when swimming backwards, with the increase in speed recorded to be from 0 to 27.3 cm/s. In 25 milliseconds, the average acceleration is 9.8 m/s², whereas the average of the whole first 50 milliseconds of each forward swimming cycle is about 1.68 m/s². The speed of the forward swimming cycle is on average about 8.74 cm/s, maximum 12.9 cm/s, and minimum 5.69 cm/s. The angular velocity of the turning cycle is on average about 8.3 rad/s, maximum 12.9 rad/s, and minimum 0.83 rad/s.^[11]

An ability specific to the smaller of the diving beetles is to rapidly blast ingested water out from the rectum. This is a solution to water surface tension impeding them from leaving the water to fly up away from it.^[12] On top of that, the rectal ampulla serves as a hydrostatic organ to regulate underwater buoyancy.^[5]

The chemical senses of diving beetles, smell and taste, are strong. These senses provide for their need to identify potential food. Their taste receptors are concentrated on the maxillary and labial palpi, and they can detect sweet, sour, salty, and bitter chemicals. Then, the antennal surface is where the receptors for smell occupy. For males, this surface doubles as a way to locate female conspecifics ready to mate. Females have the ability to secrete olfactory pheromones attracting males within an area of 20-30 cm. This form of sexual signaling has been speculatively connected with the expanded antennomeres seen in the males of many groups in Dytiscidae.

Diving beetles are attracted to alarm pheromones emitted by fish that they eat as prey. In this way, the pheromones work against the fish as kairomones.^[13]

From their pygidial gland, medium and large-sized species can secrete two types of substances: one a fluid and the other a paste-like solid. Oftentimes they go above water to groom themselves with their secretions, especially the paste, and distribute them on their body surfaces. They are an antimicrobial safeguard, protecting against bacteria and ciliates. Underwater, diving beetles apply them to sensitive body parts like spiraculi and subelytral tergal respiratory surfaces to protect them from water. Chemically, the secretion-grooming paste consists of benzoic acid, a glycoprotein, and some phenols, particularly methyl p-hydroxybenzoate and p-hydroxybenzaldehyde.^[14] Until the secretions are released, they stay in a reservoir within the pygidial gland. The reservoirs are covered with muscle layers so that the muscles can move them out when it is time. In conjunction with the pygidial glands are the prothoracic glands, another source of defensive secretions. The prothoracic glands’ reservoirs are not covered with muscle layers unlike the pygidial glands’. Instead, diving beetles use internal turgor pressure and contract their tergo-sternal muscles.  Once the secretions leave the reservoirs, they are discharged by way of one muscle that has its origin on the cervical membrane. Besides managing surface tension and buoyancy, the rectal ampulla is also a source of defense. When disturbed, diving beetle have the option to release odorous food residues from there to deter any organisms. Chemical defenses combat not only against parasites, but also predators. Steroids in the secretions can force a predator such as fish to regurgitate the beetle. Diving beetles are known to be the only insect to produce steroids.^[14] The secretions can also anesthetize or even kill predators.

Small species do not have chemical defenses, so instead opt to avoid danger by reducing their activity underwater or dispersing themselves when in groups. Diving beetles can also defend themselves by playing dead (thanatosis). Species hide, escape, and bite, as well. Larger species such as Cybistrinae and Dytiscinae kick with their hind legs.^[15]

Diving beetles are the most diverse beetles in the aquatic environment and can be found in almost every kind of freshwater habitat, from small rock pools to big lakes. Some dytiscid species are also found in brackish water.^[16] Diving beetles live in water bodies in various landscapes, including agricultural and urban landscapes.^[17][18][19] Some species, such as Agabus uliginosus^[17] and Acilius canaliculatus,^[19] are found to be relatively tolerant to recent urbanization. One of the most important limiting factors for diving beetle occurrence is the presence of fish, which predate on the beetles (mostly on larvae), compete for food, and change the structure of the habitat. The presence or absence of fish can also affect habitat use and habitat selection of dytiscids.^[20][21] Some species, such as Oreodytes sanmarkii, occur in exposed areas of waters,^[22] whereas many diving beetles species prefer habitats with aquatic plants,^[18][20][23] especially plants with complex structures, such as sedges and bulrush.^[20] Oftentimes, surface color is a determiner of a diving beetle’s water environment. Having bright colors with markings is tied to clear waters with mineral substrates, and being melanin-high or green to habitats to dark substrates or dense vegetation.^[15]

A special stygobitic variety of dytiscids only live underground. Namely, these habitats are pitch-dark wells, boreholes, and caves. Most of them are in the subfamily Hydroporinae, however Exocelina abdita and Copelatus cessaina have been discovered as among the exceptions. Select Hydroporinae species live in terrestrial habitats, such as dry forest floor depressions, at least in the adult stage. Stygobitic species are prevalent in Western Australia because of the groundwater coming from its large network of paleodrainages. There, the beetles have been recorded to live in groundwater estuaries of salt lakes and shallow calcretes. Some species in Africophilus, Agabus, Fontidessus, Hydroporus, Hydrotrupes, and Platynectes are specialized for living in hygropetic habitats. Some, such as Hydroporus sardomontanus, are semi-hygropetric. Another less common environment type is interstitial or semi-subterranean habitats, such as gravel banks along rivers. Examples of interstitial species include Exocelina saltusholmesensis, Agabus paludosus, and Hydroporus bithynicus. Some of the stygobitic, interstitial, and terrestrial dytiscids have depigmentation and reduced or, in stygobitic species, none at all. Terrestrial species tend to also be smaller and have no setae on the mid and hind legs due to not swimming. Stygobitic species have fused elytra and an absence of wings. Interstitial species can have long sensory setae and reduced wings.^[24]

Similar to their wide range of habitats, Dytiscidae can be massive generalists diet-wise. Predaceous diving beetles’ diet can include both invertebrates and vertebrates. The larvae, especially, take on animals with the same or bigger size, such as fish and tadpoles. Adults readily eat both living animals and carrion, making them scavengers and water cleaners. In addition, diving beetles practice cannibalism, both within their species and outside it.^[25]

When still in larval form, the beetles vary in size from about 1 to 5 cm (0.39 to 1.97 in). The larval bodies are shaped like crescents, with the tail long and covered with thin hairs. Six legs protrude from along the thorax, which also sports the same thin hairs. The head is flat and square, with a pair of long, large, and pincer-like mandibles. It looks like a capsule due to its sclerotization. Larvae’s eyes are stemmata rather than compound eyes like the adults. During the first instar, larvae have two egg bursters on either side of the frontoclypeus. They use these to break out of their egg.

Along with the mandibles, the mouthpiece is also made up of a maxilla and a labium. The maxilla is further made up of a cardo, stipes, a palp of three palpomeres, and a palpiform galea. The labium has a postmentum on the base, prementum on the apex, and, attached to the prementum via a small palpiger, a pair of labial palps.

The dorsal surface is usually distinctly sclerotized, like the head, but not the ventral surface. There, sclerotized plates only appear sometimes on the most posterior segments, while the rest of the surface is mostly membranous. Sclerites’ pigmentation makes them often stand out from the rest of the body. The thorax has three segments, the pro-, meso-, and metathorax, whereas the subcylindrical abdomen has eight visible segments. Each of the thorax’s segments have a pair of articulated legs, a large tergite and, in most specimens, a pair of smaller laterotergites associated with each leg attachment. On the abdomen, the first 1-7 are relatively uniform in appearance while segment 8 is modified for respiration in varying ways. This last segment ends in a pair of urogomphi.^[26][5]

When hunting, they cling to grasses or pieces of wood along the bottom, and hold perfectly still until prey passes by, then they lunge, trapping their prey between their front legs and biting down with their pincers. The larvae are also known to partially consume prey and discard the carcass if another potential prey swims nearby. Their usual prey includes tadpoles and glassworms, among other smaller water-dwelling creatures. As the larvae mature, they crawl from the water on the sturdy legs, and bury themselves in the mud for pupation. After about a week, or longer in some species, they emerge from the mud as adults. Adult diving beetles have been found to oviposit their eggs within frog spawn in highly ephemeral habitats, with their eggs hatching within 24 hours after the frogs and the larvae voraciously predating on the recently hatched tadpoles.

Adult Dytiscidae, particularly of the genus Cybister, are edible. Remnants of C. explanatus were found in prehistoric human coprolites in a Nevada cave, likely sourced from the Humboldt Sink.^[27] In Mexico, C. explanatus is eaten roasted and salted to accompany tacos. In Japan, C. japonicus has been used as food in certain regions such as Nagano prefecture. In the Guangdong Province of China, the latter species, as well as C. bengalensis, C. guerini, C. limbatus, C. sugillatus, C. tripunctatus, and probably also the well-known great diving beetle (D. marginalis) are bred for human consumption, though as they are cumbersome to raise due to their carnivorous habit and have a fairly bland (though apparently not offensive) taste and little meat, this is decreasing. Dytiscidae are reportedly also eaten in Taiwan, Thailand, and New Guinea.^[28]

Diving beetles can be kept in a water tank as pets.^[29]

The greatest threat to diving beetles is the degradation and disappearance of their habitats due to anthropogenic activities.^[1] For example, urbanisation has led to the decreasing quantity and quality of dytiscid habitats,^[19] which consequentially has increased the distance between habitats.;^[30] thus, dytiscids may be exposed to high predation risks during dispersal. Urbanisation has complex effects on the inter- and intraspecific variation in dytiscid traits. Some flight-related traits of Acilius canaliculatus and Hydaticus seminiger, such as body length and hindwing traits, were found to change along the urban gradient at different scales, whereas the traits of Ilybius ater exhibited no change.^[31] Brownification, which refers to the change in surface water colour towards yellow–brown hues caused by recent climate change and land-use change, can also drive changes in dytiscid communities.^[32] As some species, such as Dytiscus marginalis, are tolerant to brown water, whereas some species, Hyphydrus ovatus, tend to occur in clear water, brownification may threaten dytiscid species that are intolerant to highly coloured waters.^[32] Drainage can have adverse effects on their populations. For example, species such as Rhantus bistriatus and Graphoderus bilineatus went extinct in Britain likely because of the drainage of the Whittlesea Mere. Drainage affects dytiscids mostly due to disturbing their breeding cycles, as demonstrated through how dytiscid populations dramatically increased in East Asian paddy fields. Their flourishing started after rice producers switched from the conventional method of draining the land midseason while it is flooded, to no-till.^[33]

Dytiscid adults are eaten by many birds, mammals, reptiles, and other vertebrate predators, despite their arsenal of chemical defenses.^[34] But by far the most important predator of diving beetles are fish, which limit the occurrence of most diving beetle species to fishless ponds, or to margins of aquatic habitats. Although the larvae of a few dytiscid species may become apex predators in small ponds, their presence is also often incompatible with fish. Therefore, the main focus of water beetle conservation is the protection of natural, fish-less habitats. In the European Union, two species of diving beetles are protected by the Berne Convention on the Conservation of European Wildlife and Natural Habitats, and thus serve as umbrella species for the protection of natural aquatic habitats: Dytiscus latissimus and Graphoderus bilineatus.

The diving beetle plays a role in a Cherokee creation story. According to the narrative, upon finding nowhere to rest in the "liquid chaos" the beetle brought up soft mud from the bottom. This mud then spread out to form all of the land on Earth.^[27]

Adult Dytiscidae, as well as Gyrinidae, are collected by young girls in East Africa. It is believed that inducing the beetles to bite the nipples will stimulate breast growth.^[27] The effect of that habit has not been tested, but it is notable that the pygidial and prothoracic defense glands of diving beetles contain many types of bioactive steroids.^[34] In Uganda, girls do not use Dytiscidae, but only the smaller Gyrinidae, since it is believed that the Gyrinidae are the females and the Dytiscidae are the males of the same species.^[35] Beetles in these two families are known as “yewha inat” (mother of water; Amharic የውሃ እናት^[36]) in Tanzania and rural regions of Ethiopia.^[35] For the opposite effect, young boys in Tanzania’s Njombe Region use the same technique. They do so to mitigate the breast growth that can temporarily arise during the period of puberty before testosterone levels go up. Meanwhile, in other areas of East Africa such as Zimbabwe, diving beetles are an aid for boys learning to whistle. In this case, the beetle bites the tongue.^[37]

Dytiscidae are parasitised by various mites. Those in genera Dytiscacarus and Eylais live beneath the elytra of their hosts,^[38][39] those in genus Acherontacarus attach to the mesosternal regions^[40] and those in genus Hydrachna attach to various locations.^[41] These mites are parasitic as larvae with the exception of Dytiscacarus, which are parasitic for their entire life cycle.^[38]

The following taxonomic sequence gives the subfamilies, their associated genera.^{[42][43][44][45]}

Subfamily Agabinae Thomson, 1867

Subfamily Colymbetinae Erichson, 1837

Subfamily Copelatinae Branden, 1885

• Agaporomorphus Zimmermann, 1921
• Aglymbus Sharp, 1880
• Copelatus Erichson, 1832
• Exocelina Broun, 1886
• Lacconectus Motschulsky, 1855
• Liopterus Dejean, 1833
• Madaglymbus Shaverdo & Balke, 2008
• Rugosus García, 2001

Subfamily Coptotominae Branden, 1885

• Coptotomus Say, 1830

Subfamily Cybistrinae

Subfamily Dytiscinae Leach, 1815

Subfamily Hydrodytinae K.B.Miller, 2001

• Hydrodytes K.B.Miller, 2001
• Microhydrodytes K.B.Miller, 2002

Subfamily Hydroporinae Aubé, 1836

Subfamily Laccophilinae Gistel, 1856

Subfamily Lancetinae Branden, 1885

• Lancetes Sharp, 1882

Subfamily Matinae Branden, 1885

• Allomatus Mouchamps, 1964
• Batrachomatus Clark, 1863
• Matus Aubé, 1836

Subfamily †Liadytiscinae Prokin & Ren, 2010

• † Liadroporus Prokin & Ren, 2010 Yixian Formation, China, Early Cretaceous (Aptian)
• † Liadytiscus Prokin & Ren, 2010 Yixian Formation, China, Aptian
• † Mesoderus Prokin & Ren, 2010 Yixian Formation, China, Aptian
• † Liadyxianus Prokin, Petrov, B. Wang & Ponomarenko, 2013 Yixian Formation, China, Aptian
• † Mesodytes Prokin, Petrov, Wang & Ponomarenko, 2013 Yixian Formation, China, Aptian

Subfamily Incertae sedis

• † Cretodytes Ponomarenko, 1977 Doronino Formation, Russia, Early Cretaceous (Barremian), Kzyl-Zhar, Kazakhstan, Late Cretaceous (Turonian)
• † Palaeodytes Ponomarenko, 1987 Karabastau Formation, Kazakhstan, Late Jurassic (Oxfordian), Zaza Formation, Russia, Aptian
• † "Palaeodytes" incompletus Ponomarenko, Coram & Jarzembowski, 2005 Durlston Formation, United Kingdom, Early Cretaceous (Berriasian) (undescribed genus)^[46]
• † Sinoporus Prokin & Ren, 2010 Yixian Formation, China, Aptian