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Leech and Chandler: Coleoptera 



Fig, 13:6. Dytiscidae. a, Dytiscus sp., larva taking air at 

 surface of the water; b, Coptoiomus sp., larva, to show abdominal 

 gills; c, Acilius sp., adult renewing air supply at surface of 

 water; a*, Matus bicarinatus, tibia and tarsus of middle leg of 

 larva showing chela (a, c, Wesenberg-Lund, 1912; b, Peterson, 

 1945; d, J. Balfour-Browne, 1947). 



forms are considered to be derived from terrestrial 

 adephagids which took to the water and became 

 adapted to conditions found there. Representatives 

 of the ancient families Amphizoidae (hind legs not 

 modified for swimming; larvae C arabu s-like in appear- 

 ance) and Hygrobiidae (hind legs moved alternately 

 in swimming; mandibles of larvae not channeled) are 

 more primitive than the Dytiscidae; the Hygrobiidae 

 (Great Britain, western and southern Europe, western 

 China, Australia) are probably most like the ancestral 

 group from which the Dytiscidae and others arose. 

 However, we have no proof that all these families 

 had a common ancestor, nor that all the aquatic 

 families result from a single successful venture from 

 land to water. 



Respiration. — The Dytiscidae are almost glabrous 

 underneath and lack the ventral film of air, held by 



hydrofuge pubescence, found in the Hydraenidae, 

 Hydrophilidae, Elmidae and others. Like adults of 

 the other Hydradephaga they have their main reservoir 

 of air under the elytra. To fill this subelytral chamber 

 with fresh air, the beetles must break the surface film 

 with the tip of the abdomen. When undisturbed they 

 often float slowly upward, tail first, since the center 

 of gravity is near the ventral anterior part of the body. 

 But when in danger or perturbed they swim up quickly, 

 do a half somersault, poke the end of the abdomen out 

 (fig. 13:6c), and dive almost immediately. 



The hairs on the last dorsal segment (pygidium) are 

 kept oiled by unicellular hypodermal glands and are 

 not wetted when the abdominal tip breaks the surface 

 film. In conjunction with the elytral apices this pygi- 

 dium forms an opening through which stale air can be 

 forced out of the subelytral chamber by an up-and-down 

 movement of the flexible dorsal surface of the ab- 

 domen. The large last pair of abdominal spiracles 

 opens to the fresh air at the pygidium, enabling the 

 tracheal system to be filled directly; the other spir- 

 acles connect the two lateral longitudinal tracheae 

 and the thoracic air sacs with the subelytral reservoir. 

 The adults of many species of Dytiscidae hibernate 

 during the winter; because of their inactivity and the 

 low temperature, their oxygen requirements are doubt- 

 less very small. Other species remain active, even 

 under ice, and probably obtain air from the layer 

 trapped beneath the ice as it rises from aquatic plants. 

 The larvae of Dytiscidae have two longitudinal 

 tracheae, but no enlargements or storage areas. As a 

 result, most of them are forced to swim or crawl to 

 the surface at intervals to renew their air supply. 

 This they do by breaking the surface film with the 

 abdominal tip or the hairy caudal cerci, by which they 

 hang suspended (fig. 13:6a); only the large caudal 

 spiracles can take in air. According to the literature 

 the other spiracles (except perhaps the mesothoracic 

 pair, through which air may be forced out) are closed 

 and nonfunctional except during each larval molt, 

 and at maturity when the larvae leave the water to 

 pupate. 



The larvae of Coptotomus spp. have lateral ab- 

 dominal gills, a pair on each of the first six segments 

 (fig. 13:66). They are able to remain continuously 

 beneath the surface, obtaining their air from the 

 water. The larvae of the Hydroporinae do not usually 

 come to the surface, and are also able to obtain air 

 from the water though they have no gills. The venters 

 of these and some other dytiscid larvae have a net- 

 work of tracheae near the surface, so they may be 

 able to breathe through the skin. 



Life history. — In the north temperate zone there is 

 but one generation a year, though larvae of various 

 sizes, pupae, and teneral adults may all be present 

 at one time in late summer. Most species, if not all, 

 pass the winter as adults; some hibernate, others 

 remain active even under the ice. In certain regions 

 which lack summer rain, such as the coast of northern 

 California, some species of the Hydroporinae can be 

 found only during the winter, at which time they breed 

 in the ephemeral ponds formed by the winter rains. 

 Some species of dytiscids overwinter both as adults 



