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]
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.[5] Diving beetles live in water bodies in various landscapes, including agricultural and urban landscapes.[6][7][8] Some species, such as Agabus uliginosus[6] and Acilius canaliculatus,[8] 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.[9][10] Some species, such as Oreodytes sanmarkii, occur in exposed areas of waters,[11] whereas many diving beetles species prefer habitats with aquatic plants,[7][9][12] especially plants with complex structures, such as sedges and bulrush.[9]
Larvae and development
Larva of the European diving beetle Dytiscus marginalis
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 pincers. 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.
Edibility
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.[13] 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.[14]
Dytiscidae sp.
Diving beetle conservation
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,[8] which consequentially has increased the distance between habitats.;[15] 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.[16] 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.[17] 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.[17]
Dytiscid adults are eaten by many birds, mammals, reptiles, and other vertebrate predators, despite their arsenal of chemical defenses.[18] 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.
Cultural significance
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.[13]
Ethnobiology
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.[13] The effect of that habit has not been tested, but it is notable that the defense glands of diving beetles contain many types of bioactive steroids.[18]
Parasites
Dytiscidae are parasitised by various mites. Those in genera Dytiscacarus and Eylais live beneath the elytra of their hosts,[19][20] those in genus Acherontacarus attach to the mesosternal regions[21] and those in genus Hydrachna attach to various locations.[22] These mites are parasitic as larvae with the exception of Dytiscacarus, which are parasitic for their entire life cycle.[19]
^ abcdG.N. Foster; D.T. Bilton (2014). "The Conservation of Predaceous Diving Beetles: Knowns, Unknowns and Anecdotes". In D.A. Yee (ed.). Ecology, Systematics, and the Natural History of Predaceous Diving Beetles (Coleoptera: Dytiscidae). Springer. pp. 437–462. ISBN978-94-017-9109-0.
^Yee, D.A. (2014). "An Introduction to the Dytiscidae: Their Diversity, Historical Importance, Cultural Significance, and Other Musings". Ecology, Systematics, and the Natural History of Predaceous Diving Beetles (Coleoptera: Dytiscidae). pp. 1–16. doi:10.1007/978-94-017-9109-0_1. ISBN978-94-017-9108-3.
^ abLundkvist, E.; Landin, J.; Karlsson, F. (2002). "Dispersing diving beetles (Dytiscidae) in agricultural and urban landscapes in south-eastern Sweden". Annales Zoologici Fennici.
^Nilsson, A. N.; Holmen, M. (1995). The Aquatic Adephaga (Coleoptera) of the Fennoscandia and Denmark. II. Dytiscidae. Leiden, the Netherlands: Brill. ISBN9004104569. ISSN0106-8377.
^ abcMiller, Kelly; Bergsten, Johannes (3 October 2016). Diving Beetles of the World: Systematics and Biology of the Dytiscidae. Baltimore: Johns Hopkins University Press. p. 20.
^ abKonrad Dettner (2014). "Chemical Ecology and Biochemistry of Dytiscidae". In D.A. Yee (ed.). Ecology, Systematics, and the Natural History of Predaceous Diving Beetles (Coleoptera: Dytiscidae). Springer. pp. 235–306. ISBN978-94-017-9109-0.
Jäch, Manfred A. (2003): Fried water beetles Cantonese style. American Entomologist49(1): 34–37. PDF fulltext
Larson, D.J., Alarie, Y., and Roughley, R.E. (2000): Predaceous Diving Beetles (Coleoptera: Dytiscidae) of the Nearctic Region, with emphasis on the fauna of Canada and Alaska. NRC Research Press, Ottawa. ISBN978-0-660-17967-4.
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