Predaceous Diving Beetles

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 cm (1.8 in) and 4.75 cm (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. Many diving beetles species prefer habitats with aquatic vegetation,^[7][9] while some species, such as Oreodytes sanmarkii, occur in exposed areas of waters.^[10]

Larvae and development

When still in larval form, the beetles vary in size from about 1 to 5 cm (0.5 to 2.0 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.^[11] 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.^[12]

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.^[13] Thus, dytiscids may be exposed to high predation risks during dispersal.

Dytiscid adults are eaten by many birds, mammals, reptiles, and other vertebrate predators, despite their arsenal of chemical defenses.^[14] 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.^[11]

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.^[11] 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.^[14]

Parasites

Dytiscidae are parasitised by various mites. Those in genera Dytiscacarus and Eylais live beneath the elytra of their hosts,^[15][16] those in genus Acherontacarus attach to the mesosternal regions^[17] and those in genus Hydrachna attach to various locations.^[18] These mites are parasitic as larvae with the exception of Dytiscacarus, which are parasitic for their entire life cycle.^[15]

Systematics

The following taxonomic sequence gives the subfamilies, their associated genera.^{[19][20][21][22]}

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

• Austrodytes Watts, 1978
• Cybister Curtis, 1827
• Megadytes Sharp, 1882
• Onychohydrus Schaum & White, 1847
• Regimbartina Chatanay, 1911
• Spencerhydrus Sharp, 1882
• Sternhydrus Brinck, 1945

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), Durlston Formation, United Kingdom, Early Cretaceous (Berriasian), Zaza Formation, Russia, Aptian
• † Sinoporus Prokin & Ren, 2010 Yixian Formation, China, Aptian

References

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