The Dytiscidae, from the Ancient Greek word δυτικός (dutikós), 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. 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. 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 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 (0.4–1.0 in), and the elongate larvae range from 1 to 5 cm (0.39 to 1.97 in). The largest known adults are of the species Megadytes ducalis, at up to 4.75 cm (1.9 in), and Dytiscus latissimus, at up to 4.5 (1.8 in). At about 0.9 mm (0.035 in), 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. 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.
Use of rectum underwater
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. On top of that, the rectal ampulla serves as a hydrostatic organ to regulate underwater buoyancy. To decrease buoyancy, diving beetles ingest water. To increase it, they expel water from their rectum like they do against surface tension. Diving beetles strategically adjust their fill to the optimal buoyancy over changing conditions. The goal is for the body to have approximately the same specific gravity as the water.
Defense
Chemical
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. 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. A kind especially prevalent in diving beetles is pregnanes, such as 11-deoxycorticosterone. Experiments have shown pregnanes to possibly deter fish, protecting diving beetles from predation. Diving beetles are known to be the only family in the whole animal kingdom to produce certain kinds of steroids. For this reason, they are an important source for pharmaceutical R&D. Chemicals in diving beetle secretions can also anesthetize or even kill predators. Other chemicals produced include a variety of aromatic esters, acids, and aldehydes.
Other
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.
Habitat
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. Diving beetles live in water bodies in various landscapes, including agricultural and urban landscapes. Some species, such as Agabus uliginosus Some species, such as Oreodytes sanmarkii, occur in exposed areas of waters, whereas many diving beetles species prefer habitats with aquatic plants, especially plants with complex structures, such as sedges and bulrush. In addition, they can likely estivate over summertime dry spells.
When they need to colonize a new habitat for mating or better conditions, they fly and look for light reflections from the water surface. In urban areas, diving beetles' attraction to lights draws them erroneously to artificial lights and glossy surfaces on cars, etc. Meanwhile, on land, the gait of many adult diving beetles can appear awkward or clumsy due to their enlarged hind legs.
Specializations
A special stygobitic variety of dytiscids only live underground. Habitats include pitch-dark wells, boreholes, and caves. Most stygobitic species 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 or leaf litter, 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.
Diet
thumb|Dytiscid larva feeding on a [[Gasterosteus|stickleback]]
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. Since they can feed on mosquito larvae, they have a potential role in biological control of mosquito populations similar to Hydrophilidae.
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. In the latter case, larvae in the first instar are usually attached to a plant and independent of surface air. For them, the plant serves as their substrate. The ability of adults to cut into plants with their ovipositor is unique to the genera Agabus, Coptotomus, Cybister, Dytiscus, Hydaticus, Ilybius, Laccophilus, and Thermonectus. Mating season for most species spans 2 to 3 months, 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.
As pets
Diving beetles can be kept in a water tank as pets.thumb|Dytiscidae sp.
Diving beetle conservation
The greatest threat to diving beetles is the degradation and disappearance of their habitats due to anthropogenic activities. thus, dytiscids may be exposed to high predation risks during dispersal. The negative effects of urbanisation on dytiscid communities can be long-lasting from the temporal perspective. Some species may go extinct across an urban landscape in a long term, resulting in further losses of urban biodiversity. 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.
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. 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.
Dytiscid adults are eaten by many birds, mammals, reptiles, and other vertebrate predators, The steroids that are operative include estrone, estradiol and testosterone. Beetles in these two families are known as "yewha inat" (mother of water; Amharic የውሃ እናት) in Tanzania and rural regions of Ethiopia.
Parasites
Dytiscidae are parasitised by various mites. Those in genera Dytiscacarus and Eylais live beneath the elytra of their hosts, those in genus Acherontacarus attach to the mesosternal regions and those in genus Hydrachna attach to various locations. These mites are parasitic as larvae with the exception of Dytiscacarus, which are parasitic for their entire life cycle. analysis of taxon gene samples using the parsimony and Bayesian models. This cladogram goes down to the level of the subfamily, and only includes the 11 that are extant. The Agabinae subfamily proved paraphyletic, therefore the relationship does not apply to some of its genera, namely Hydrotrupes and Platynectes. These two deviate away from the Agabinae + Colymbetinae grouping.
Here is a version of the cladogram including the merely tentative relationships whose support from the analysis was not strong enough to be conclusive. These are the grouping of Lancentinae with Agabinae + Colymbetinae and Coptotominae with Hydroporinae + Hydrodytinae.
Evolution
As observed in both sexually antagonistic morphology (See Shape and morphology) and behaviors, females in Dytiscidae are more selective when it comes to mating. One behavior is a technique to break the male's grip. When approached by males, females in some species make fast and erratic swimming movements. As in most insects, mating tends to be more deliberate for females because offspring have a much higher energy cost for them. With this cost comes reduced fitness to mate again. Under an evolutionary lens, an evolutionary sequence called the "arms race" explains that females first evolved resistive behaviors to minimize their losses, next males' morphology evolved palettes to stop females from escaping. Then came the females' textured dorsal surface as a counter-adaptation. So, female morphological adaptations came before behaviors according to this model. It is called the arms race because it involves either sex continuously one-upping the other's previous adaptation. According to K. B. Miller's (2002) cladistics analysis, the males' palettes originated in Dytiscinae. Five groups within Dytiscinae then evolved the female dorsal surface, each independently. Iversen et al. (2019) describe a standstill in species evolution as a byproduct of the dytiscid arms race, specifically in Graphoderus zonatus, in contrast with sexual conflict normally being associated with divergence and diversification.
There is a clear consensus that diving beetles' ancestors were terrestrial and similar to ground beetles. Testing found that the body size evolution followed a model very rare among animal groups. The model consists of an early burst period of rapid morphological change in many dytiscids transitioning from lentic to lotic habitats, then a long static period of phylogenetic conservatism. However, this early burst did not seem to correlate with any species diversification. This indicates that morphology and species diversification are uncoupled in Dytiscidae.
Systematics
The following taxonomic sequence gives the subfamilies, their associated genera.
- † Sinoporus <small>Prokin & Ren, 2010</small> Yixian Formation, China, Aptian
References
- (2004): Water for a Healthy Country - Family Dytiscidae. Version of 2004-JUL-02. Retrieved 2008-AUG-04
- (2002): Chapter 26 - Eastern Asia: China, Japan, and other countries. In: The Human Use of Insects as a Food Resource: A Bibliographic Account in Progress.
- (2003): Fried water beetles Cantonese style. American Entomologist 49(1): 34–37. PDF fulltext
- (2000): Predaceous Diving Beetles (Coleoptera: Dytiscidae) of the Nearctic Region, with emphasis on the fauna of Canada and Alaska. NRC Research Press, Ottawa. .