British insects: Water beetles

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L. Watson and M. J. Dallwitz

Conventional key

If you are serious about identification, you should use the interactive key. It is far superior, with flexible choice of characters, character notes and illustrations, error tolerance, full taxon descriptions, and taxon illustrations.

1(0).
  • Key to the families ... 2
  • Key to the genera... 20

2(1).
  • Beetles equipped with a rostrum; antennae conspicuously elbowed; antennae with the scape much-elongated (i.e., with a single, elongated basal segment) ... Curculionidae
  • Beetles without a rostrum; antennae not elbowed; antennae without a much-elongated scape... 3

3(2).
  • Antennae filiform (including moniliform) ... 4
  • Antennae gradually expanding towards the apex ... Chrysomelidae
  • Antennae clubbed... 11
  • Antennae serrate... 19
  • Antennae pectinate ... Dryopidae

4(3).
  • Beetles ‘rowing’ and diving in water by parallel-simultaneous leg movements ... 5
  • Beetles moving in the water by alternate, walking leg movements... 7

5(4).
  • The apical segment of the labial palps more or less expanded apically; the inner parts (‘processes’) of the hind coxae constituting longitudinal plates that are fused to one another and joined with the metasternum, endowing the beetle ventrally with a flat, median longitudinal keel; the antennae of the larva 0.15-0.5 x the head width ... Noteridae
  • The apical segment of the labial palps not expanded apically; the inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel; the antennae of the larva more than 0.5 x the width of the head... 6

6(5).
  • Beetles free-swimming and diving strongly, with the main thrust delivered by the specially adapted hind legs, moving clumsily on land; prothorax with notopleural sutures; the fore-leg coxal cavities circular to longer than wide; hind coxae much expanded and extended laterally to meet the elytra; hind coxae not shaped posteriorly to receive the femur ... Dytiscidae
  • Beetles walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; prothorax without notopleural sutures; the fore-leg coxal cavities strongly transverse; hind coxae not extended laterally to meet the elytra; hind coxae posteriorly shaped to receive the retracted femur ... Scirtidae

7(4).
  • some tarsi with a tiny penultimate segment hidden by distal lobing of the fourth and fused to the fifth; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 4, 4, 4; mid-leg tarsi pseudotetramerous, with 4 obvious segments plus a reduced penultimate one; the abdomen of the larva with the spiracles on the eighth segment borne at the ends of spiracular tubes; the larvae scarabaeiform ... Chrysomelidae
  • none of the tarsi with ‘hidden’ segments; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 5, 5, 5; mid-leg tarsi pentamerous, the 5 segments all conspicuous; the abdomen of the larva without spiracular tubes; the larvae campodeiform... 8

8(7).
  • Prothorax with notopleural sutures; the fore-leg coxal cavities broadly closed internally; hind coxae not shaped posteriorly to receive the femur; the maxillae with an outer palpiform lobe (the galea) additional to the 4-segmented maxillary palp; hind coxae immovably fixed to the metasternum and dividing the first abdominal sternite ... 9
  • Prothorax without notopleural sutures; the fore-leg coxal cavities internally open; hind coxae posteriorly shaped to receive the retracted femur; the maxillae without a conspicuous galea; hind coxae movable... 10

9(8).
  • Mandibles with a reduced mola; the fore-leg coxal cavities slightly transverse; hind coxae not extended laterally to meet the elytra; eyes strongly protuberant; the hind coxae not produced behind into flat plates partly concealing the hind femora ... Hygrobiidae
  • Mandibles without a mola; the fore-leg coxal cavities circular to longer than wide; hind coxae much expanded and extended laterally to meet the elytra; eyes not strongly protuberant; the hind coxae much enlarged and produced behind into large flat plates which partly conceal the hind femora ... Haliplidae

10(8).
  • The fore-leg coxal cavities narrowly separated; some of the tarsi exhibiting (one or more) conspicuously bilobed segments; antennal scape swollen; hind coxae without the steep transverse declivity characteristic of Dryopoidea; elytra soft ... Scirtidae
  • The fore-leg coxal cavities quite widely separated; none of the tarsi with conspicuously bilobed segments; antennal scape not swollen; hind coxae with a steep transverse declivity against which the femur retracts; elytra hard ... Elmidae

11(3).
  • Beetles ‘rowing’ and diving in water by parallel-simultaneous leg movements ... 12
  • Beetles moving in the water by alternate, walking leg movements... 13

12(11).
  • Beetles surface swimmers with the main thrust delivered by the specialised middle legs, moving clumsily on land (Whirligigs); mandibles without a mola; the apical segment of the labial palps more or less expanded apically; prothorax with notopleural sutures; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 5, 5, 5 ... Gyrinidae
  • Beetles walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; mandibles with a well developed mola; the apical segment of the labial palps not expanded apically; prothorax without notopleural sutures; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 4, 4, 4 ... Heteroceridae

13(11).
  • some of the tarsi with a tiny basal segment that is hard to detect ... 14
  • none of the tarsi with ‘hidden’ segments... 15

14(13).
  • Beetles somewhat waisted; antennal clubs up to 3 segmented; the claws of the mid-leg tarsi with an empodium between them (with or without bristles); the abdomen of the larva with anterior and 8th-segmental spiracles borne at the ends of a series of spiracular tubes; the labrum and head capsule of the larva completely fused, with no suture apparent ... Hydrophilidae
  • Beetles conspicuously waisted; antennal clubs 4 to 9 segmented; the claws of the mid-leg tarsi without an associated empodium; the abdomen of the larva with the spiracles on the eighth segment borne at the ends of spiracular tubes; the labrum and head capsule of the larva separated by a complete suture ... Hydraenidae

15(13).
  • The fore-leg coxal cavities medianly confluent ... 16
  • The fore-leg coxal cavities narrowly separated... 17
  • The fore-leg coxal cavities quite widely separated... 18

16(15).
  • The maxillary palps conspicuously elongated, sometimes longer than the antennae; the maxillary palps conspicuously elongated, sometimes longer than the antennae; prothorax without notopleural sutures; the maxillae with distinct galea and lacinia apically to the palp; the apical segment of the maxillary palps cylindrical to fusiform ... Hydrophilidae
  • The maxillary palps not especially elongated; the maxillary palps not especially elongated; prothorax with notopleural sutures; the maxillae with a single apical structure additional to the palp; the apical segment of the maxillary palps aciculate ... Sphaeriidae

17(15).
  • The maxillary palps conspicuously elongated, sometimes longer than the antennae; the maxillary palps conspicuously elongated, sometimes longer than the antennae; beetles somewhat waisted; antennal clubs up to 3 segmented; hind coxae not shaped posteriorly to receive the femur ... Hydrophilidae
  • The maxillary palps not especially elongated; the maxillary palps not especially elongated; beetles conspicuously waisted; antennal clubs 4 to 9 segmented; hind coxae posteriorly shaped to receive the retracted femur ... Heteroceridae

18(15).
  • Hind coxae posteriorly shaped to receive the retracted femur; beetles having ventral body cavities into which the legs fold to conform with the general body surface; the larvae only very lightly pigmented ventrally ... Limnichidae
  • Hind coxae not shaped posteriorly to receive the femur; beetles without ventral body cavities for reception of the legs; the larvae ventrally more or less heavily pigmented ... Dryopidae

19(3).
  • some tarsi with a tiny penultimate segment hidden by distal lobing of the fourth and fused to the fifth; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 4, 4, 4; antennae inserted further apart than the length of their first segment; metaventrite without a transverse groove; hind coxae not shaped posteriorly to receive the femur ... Chrysomelidae
  • none of the tarsi with ‘hidden’ segments; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 5, 5, 5; antennae inserted on the front of the head, with the distance between their bases no greater than the length of their first segment; metaventrite with a transverse groove; hind coxae posteriorly shaped to receive the retracted femur ... Psephenidae

20(1).
  • Antennae filiform (including moniliform) ... 21
  • Antennae clubbed... 71
  • Antennae serrate ... Eubria
  • Antennae pectinate... 101

21(20).
  • Beetles ‘rowing’ and diving in water by parallel-simultaneous leg movements; beetles free-swimming and diving strongly, with the main thrust delivered by the specially adapted hind legs, moving clumsily on land; hind tarsi flattened oar-like for swimming ... 22
  • Beetles moving in the water by alternate, walking leg movements; beetles walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; hind tarsi not flattened oar-like... 62

22(21).
  • some tarsi with a tiny penultimate segment hidden by distal lobing of the fourth and fused to the fifth; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 4, 4, 5; the larval fronto-clypeus much extended forwardly ... 23
  • none of the tarsi with ‘hidden’ segments; ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 5, 5, 5; the larval fronto-clypeus not extended forwardly... 50

23(22).
  • The pronotum with a single, short median-longitudinal furrow towards the outer margin on each side ... 24
  • The pronotum without a single, short median-longitudinal furrow towards the outer margin on each side... 31

24(23).
  • The pronotum with a single longitudinal furrow posteriorly on each side, these located two-thirds of the way from the mid-line and adjoining a pair of short furrows at the bases of the (closed) elytra ... 25
  • The pronotum without the pair of longitudinal furrows characteristic of Hydroglyphus, Bidessus et al. (q.v.)... 26

25(24).
  • The sutural striae extending almost the full length of the suture ... Hydroglyphus
  • The sutural striae extending no more than two-thirds towards the rear ... Bidessus

26(24).
  • Beetles predominantly black with yellow markings ... 27
  • Beetles predominantly yellow with black markings... 29

27(26).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear; elytra subquadrate ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear; elytra elongate... 28

28(27).
  • Elytra black with transverse, pale, complexly wavy markings only ... Stictonectes
  • Elytra black, with one or more pale longitudinal stripes running from the pronotum to near the tip (without the transverse wavy markings characteristic of Stictonectes) ... Graptodytes

29(26).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear; elytra subquadrate ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear; elytra elongate... 30

30(29).
  • Beetles dorsally microreticulate and hairless; upper surfaces of body glabrous or subglabrous; elytra microreticulate, with a fine impressed reticulum (this often less conspicuous in the disc than towards the apex) ... Oreodytes
  • Beetles dorsally shiny and without microreticulation beneath the covering of hair; upper surfaces of body non-glabrous; elytra without a microreticulum ... Scarodytes

31(23).
  • The prosternal process arising in a different plane from the prosternum, resulting in a distinct ridge at its origin (the neck of the prosternal process with a sharply raised edge defining a distinct step) ... 32
  • The prosternal process arising in the same plane as the prosternum, without a distinct ridge at its origin (not stepped)... 39

32(31).
  • Elytra each with a very small tooth projecting from the costa near the tip ... Nebrioporus
  • Elytra without the small tooth near the tip that characterises Nebrioporus spp.... 33

33(32).
  • Beetles only about 2 to 2.5 times as long as the maximum depth in lateral view: globular, less elongate and the elytra more rounded laterally than is usual in Hydroporus etc. ... 34
  • Beetles about 2.5 to 3 times as long as the maximum depth in lateral view: not globular, somewhat elongate, the elytra not much rounded laterally... 36

34(33).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear... 35

35(34).
  • The hind tarsal claws more or less equal in length; elytra elongate ... Hygrotus
  • The hind tarsal claws very unequal in length; elytra subquadrate ... Hyphydrus

36(33).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear... 37

37(36).
  • The middle part of the elytra with approximately equal numbers of small and large punctures scattered in the visibly reticulate surface ... Laccornis
  • The middle part of the elytra with scattered punctures all more or less of the same size, or sometimes also with lines of larger ones... 38

38(37).
  • Elytra indistictly dark striped longitudinally ... Porhydrus
  • Elytra not dark-striped ... Hydroporus

39(31).
  • Beetles with sharply defined elytral spots and/or blotches ... 40
  • Beetles with elytral longitudinal stripes... 44
  • Beetles without sharply defined stripes, spots or blotches... 48

40(39).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear... 41

41(40).
  • The elytral epipleura divided into two parts by a distinct ridge at the outer corner of the elytron ... 42
  • The elytral epipleura without a dividing ridge... 43

42(41).
  • The hind tarsal claws more or less equal in length; elytra elongate ... Hygrotus
  • The hind tarsal claws very unequal in length; elytra subquadrate ... Hyphydrus

43(41).
  • The prosternal process elongate-oval, glabrous and slightly domed ... Suphrodytes
  • The prosternal process broad, flat and hairy ... Stictotarsus

44(39).
  • Elytra each with a very small tooth projecting from the costa near the tip ... Nebrioporus
  • Elytra without the small tooth near the tip that characterises Nebrioporus spp.... 45

45(44).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear... 46

46(45).
  • The elytral epipleura divided into two parts by a distinct ridge at the outer corner of the elytron ... Hygrotus
  • The elytral epipleura without a dividing ridge... 47

47(46).
  • Elytra (and pronotum) with numerous small punctures, and pits 4-5 times their diameter scattered among them; elytra not dark-striped ... Deronectes
  • Elytra (and pronotum) finely punctured, the numerous punctures all about the same size; elytra longitudinally with distinct dark stripes ... Boreonectes

48(39).
  • Beetles with the closed elytra drawn out to form a very small but distinct point at the rear ... Hydrovatus
  • Beetles with the closed elytra not drawn out to form small point at the rear... 49

49(48).
  • Beetles only about 2 to 2.5 times as long as the maximum depth in lateral view: globular, less elongate and the elytra more rounded laterally than is usual in Hydroporus etc.; the elytral epipleura divided into two parts by a distinct ridge at the outer corner of the elytron; elytra subquadrate ... Hyphydrus
  • Beetles about 2.5 to 3 times as long as the maximum depth in lateral view: not globular, somewhat elongate, the elytra not much rounded laterally; the elytral epipleura without a dividing ridge; elytra elongate ... Deronectes

50(22).
  • Eyes vertically straight edged at the front ... 51
  • Eyes with the front edge conspicuously indented above the antennal insertions... 55

51(50).
  • The apical segment of the labial palps more or less expanded apically; the inner parts (‘processes’) of the hind coxae constituting longitudinal plates that are fused to one another and joined with the metasternum, endowing the beetle ventrally with a flat, median longitudinal keel; the mesothoracic legs with 1 movable claw; the antennae of the larva 0.15-0.5 x the head width ... Noterus
  • The apical segment of the labial palps not expanded apically; the inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel; the mesothoracic legs with 2 movable claws; the antennae of the larva more than 0.5 x the width of the head... 52

52(51).
  • Scutellum present; beetles convex beneath; none of the tarsi with conspicuously bilobed segments; the posterior segments of the larva with swimming hairs; Subfamily Dytiscinae ... 53
  • Scutellum absent; beetles flat beneath; some of the tarsi exhibiting (one or more) conspicuously bilobed segments; the posterior segments of the larva without swimming hairs; Subfamily Laccophilinae ... Laccophilus

53(52).
  • The hind claws paired ... 54
  • The hind claws single ... Cybister

54(53).
  • The pronotum pale with a broad black crescent at the rear ... Hydaticus
  • The pronotum with two black bars across the middle enclosing a yellowish one ... Acilius
  • The pronotum pale with black bands on the front and hind margins (sometimes with a thin yellow line preceding the front band) ... Graphoderus
  • The pronotum dark, yellow margined either all round or at front and sides only ... Dytiscus
  • The pronotum pale, with a broad dark bat-shaped band across the middle ... Dytiscus

55(50).
  • Elytra predominantly finely but rather conspicuously transversely striated across lines of punctured striae ... Colymbetes
  • Elytra not predominantly transversely striated across lines of puntured striae... 56

56(55).
  • Elytra reddish and elongate, distinctively sculptured with numerous, short longitudinal cuts in the reticulated surface ... Liopterus
  • Elytra not sculptured with numerous, short longitudinal cuts in a reticulated surface... 57

57(56).
  • The apical segment of the labial palps more or less expanded apically; the inner parts (‘processes’) of the hind coxae constituting longitudinal plates that are fused to one another and joined with the metasternum, endowing the beetle ventrally with a flat, median longitudinal keel; the mesothoracic legs with 1 movable claw; the antennae of the larva 0.15-0.5 x the head width ... Noterus
  • The apical segment of the labial palps not expanded apically; the inner parts (‘processes’) of the hind coxae not incorporated with the metasternum in a flat, median longitudinal keel; the mesothoracic legs with 2 movable claws; the antennae of the larva more than 0.5 x the width of the head... 58

58(57).
  • Scutellum present ... 59
  • Scutellum absent ... Laccophilus

59(58).
  • The elytral epipleura reaching to the tips of the elytra ... Platambus
  • The elytral epipleura falling short of the elytral tips... 60

60(59).
  • The undersides of the hind femora each with a short comb of bristles (sometimes only three) at the outer rear corner; Subfamily Agabinae ... 61
  • The undersides of the hind femora without bristles at their outer rear corners, or the bristles weak and not forming a comb; Subfamily Colymbetinae ... Rhantus

61(60).
  • The hind tarsal claws more or less equal in length ... Agabus
  • The hind tarsal claws very unequal in length ... Ilybius

62(21).
  • Mandibles with a well developed mola; larva with anal gill tufts ... 63
  • Mandibles with a reduced mola; larva with ventral abdominal gill tufts on abdominal segments 1-3 ... Hygrobia
  • Mandibles without a mola; larva without gills... 69

63(62).
  • The pronotum with two sharply defined longitudinal ridges ... 64
  • The pronotum without sharply defined ridges... 67

64(63).
  • The longitudinal pronotal ridges sharply incurving posteriorly, almost meeting adjacent to the scutellum ... Elmis
  • The longitudinal pronotal ridges remaining well separated throughout their length... 65

65(64).
  • Elytra with conspicuous longitudinal ribs (exclusive of the sutural margin) ... 66
  • Elytra not conspicuously longitudinally ribbed ... Limnius

66(65).
  • Elytra each 1 ribbed ... Esolus
  • Elytra each 3 ribbed ... Oulimnius

67(63).
  • Beetles characterised by two shining, hairy bumps (tubercles) towards the posterior of the pronotum, and a matching pair behind them on the shoulders of the elytra; all the legs at least as long as the entire beetle; elytra not conspicuously longitudinally ribbed ... Macronychus
  • Beetles without the pairs of pronotal and elytral shining, hairy bumps characteristic of Macronychus (q.v.); all the legs long, but none longer than the entire beetle; elytra with conspicuous longitudinal ribs (exclusive of the sutural margin)... 68

68(67).
  • Elytra each 1 ribbed ... Normandia
  • Elytra each 2 ribbed ... Stenelmis
  • Elytra each 3 ribbed ... Riolus

69(62).
  • Elytra each with a single, strong, median-longitudinal ridge ... Brychius
  • Elytra not exhibiting a single, strong, median-longitudinal ridge... 70

70(69).
  • The apical segment of the maxillary palps shorter than the penultimate one; the hind coxal plates smoothly rounded posteriorly, and leaving the last three abdominal sternites freely visible ... Haliplus
  • The apical segment of the maxillary palps longer than the penultimate one; the hind coxal plates each with a point on the posterior margin, and covering part of the sixth abdominal sternite ... Peltodytes

71(20).
  • Ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 5, 5, 5 ... 72
  • Ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 4, 4, 4... 95
  • Ostensible tarsal segmentation formula (fore-, mid-, hind-leg: ignoring any small cylindrical segments obscured by lobes on those preceding them) 3, 3, 3 ... Sphaerius

72(71).
  • The maxillary palps conspicuously elongated, sometimes longer than the antennae; the maxillary palps conspicuously elongated, sometimes longer than the antennae ... 73
  • The maxillary palps not especially elongated; the maxillary palps not especially elongated... 91

73(72).
  • The pronotum with five full length, longitudinal furrows ... Helophorus
  • The pronotum without five full length longitudinal furrows (ignore pits)... 74

74(73).
  • Antennal clubs preceded by a cupule ... 75
  • Antennal clubs without a cupule... 84

75(74).
  • Hind tarsi with the second segment longest ... 76
  • Hind tarsi with the apical segment longer than the second ... Paracymus
  • Hind tarsi with the apical and second segments about equal in length, and all the segments short and wide... 83

76(75).
  • Hind tarsi with the apical segment only about half as long as the second ... Hydrophilus
  • Hind tarsi with the apical segment about three-quarters the length of the second ... Hydrochara
  • Hind tarsi with the second and apical segments almost equal in length... 77

77(76).
  • Antennae 5 to 8 segmented ... 78
  • Antennae 9 to 10 segmented... 79

78(77).
  • Swimming hairs confined to the tarsi; larva without gills; the abdominal apex of the larva with a respiratory chamber, formed from the 8th and 9th terga and enclosing a pair of enlarged spiracles ... Laccobius
  • Swimming hairs present on both tibiae and tarsi; larva with long and narrow lateral gills on abdominal segments 1-7; the abdominal apex of the larva without a respiratory chamber ... Berosus

79(77).
  • Elytra each with a well marked sutural stria ... 80
  • Elytra without well marked sutural striae ... Helochares

80(79).
  • Mid-leg tarsi 4-segmented ... Cymbiodyta
  • Mid-leg tarsi 5-segmented... 81

81(80).
  • Elytra longitudinally striate ... Hydrobius
  • Elytra without longitudinal striae... 82

82(81).
  • The mesoventrite keeled ... Enochrus
  • The mesoventrite not keeled, but with a dorsal point ... Anacaena

83(75).
  • Antennae 5 to 8 segmented; hind tarsi without ‘swimming’ hairs ... Chaetarthria
  • Antennae 9 to 10 segmented; hind tarsi equipped with ‘swimming hairs’ (sometimes requiring careful observation) ... Hydrobius

84(74).
  • Scutellum present ... 85
  • Scutellum absent... 90

85(84).
  • The front tibia very conspicuously asymmetrically expanded and emarginate near the attachment of the tarsus ... Megasternum
  • The front tibia more or less club-shaped, not or scarcely emarginate near the attachment of the tarsus... 86

86(85).
  • Scutellum much longer than broad; hind tarsi without ‘swimming’ hairs ... Sphaeridium
  • Scutellum not much longer than broad; hind tarsi equipped with ‘swimming hairs’ (sometimes requiring careful observation)... 87

87(86).
  • Elytra with sutural striae only ... 88
  • Elytra striate or with regular rows of punctures... 89

88(87).
  • Beetles 4 to 5.9 mm long ... Coelostoma
  • Beetles 6 to 10 mm long ... Limnoxenus
  • Beetles 10 to 20 mm long ... Limnoxenus

89(87).
  • Beetles up to 1.5 mm long ... Cercyon
  • Beetles 1.5 to 4 mm long ... Cercyon
  • Beetles 4 to 5.9 mm long ... Cercyon
  • Beetles 6 to 10 mm long ... Limnoxenus
  • Beetles 10 to 20 mm long ... Limnoxenus

90(84).
  • The front tibia very conspicuously asymmetrically expanded and emarginate near the attachment of the tarsus ... Megasternum
  • The front tibia more or less club-shaped, not or scarcely emarginate near the attachment of the tarsus ... Cryptopleurum

91(72).
  • Beetles ‘rowing’ and diving in water by parallel-simultaneous leg movements; mandibles without a mola; beetles surface swimmers with the main thrust delivered by the specialised middle legs, moving clumsily on land (Whirligigs); the apical segment of the labial palps more or less expanded apically; prothorax with notopleural sutures ... 92
  • Beetles moving in the water by alternate, walking leg movements; mandibles with a well developed mola; beetles walking in water or free-swimming by conventional ambulatory motion of the legs, not diving strongly; the apical segment of the labial palps not expanded apically; prothorax without notopleural sutures... 93

92(91).
  • The pronotum and elytra covered with fine hair; upper surfaces of body non-glabrous; elytra without longitudinal striae ... Orectochilus
  • The pronotum and elytra hairless; upper surfaces of body glabrous or subglabrous; elytra longitudinally striate ... Gyrinus

93(91).
  • The maxillary palps scarcely visible, being very much shorter than the antennae; beetles having ventral body cavities into which the legs fold to conform with the general body surface; hind coxae posteriorly shaped to receive the retracted femur; the larvae only very lightly pigmented ventrally ... Limnichus
  • The maxillary palps relatively conspicuous; beetles without ventral body cavities for reception of the legs; hind coxae not shaped posteriorly to receive the femur; the larvae ventrally more or less heavily pigmented... 94

94(93).
  • The pronotum with a pair of short longitudinal furrows, one on each side ... Dryops
  • The pronotum without a short longitudinal furrow on each side ... Pomatinus

95(71).
  • some of the tarsi with a tiny basal segment that is hard to detect; the front tarsi 5-segmented; mid-leg tarsi 5-segmented ... 96
  • none of the tarsi with ‘hidden’ segments; the front tarsi 4-segmented; mid-leg tarsi 4-segmented... 100

96(95).
  • Beetles collecting air at the water surface by exserting one antenna to achieve a continuum of air with the ventral plastron; beetles somewhat waisted; antennal clubs up to 3 segmented; hind tarsi without ‘swimming’ hairs; the abdomen of the larva without spiracular tubes ... 97
  • Beetles incorporating bubbles of oxygen directly into the plastron; beetles conspicuously waisted; antennal clubs 4 to 9 segmented; hind tarsi equipped with ‘swimming hairs’ (sometimes requiring careful observation); the abdomen of the larva with the spiracles on the eighth segment borne at the ends of spiracular tubes... 98

97(96).
  • Beetles 1.5 to 4 mm long ... Hydrochus
  • Beetles 4 to 5.9 mm long ... Hydrochus
  • Beetles 6 to 10 mm long ... Spercheus

98(96).
  • The pronotum narrowing towards the rear; hind tarsi with the apical segment at least as long as all the rest together ... 99
  • The pronotum not narrowing towards the rear; hind tarsi with the apical segment about equal to segments 3+4, with segment 2 very short ... Limnebius

99(98).
  • The pronotum with large pits ... Ochthebius
  • The pronotum without large pits ... Hydraena

100(95).
  • The maxillary palps conspicuously elongated, sometimes longer than the antennae; the maxillary palps conspicuously elongated, sometimes longer than the antennae; beetles somewhat waisted; antennal clubs up to 3 segmented; metaventrite without a transverse groove ... Georissus
  • The maxillary palps not especially elongated; the maxillary palps not especially elongated; beetles conspicuously waisted; antennal clubs 4 to 9 segmented; metaventrite with a transverse groove ... Heterocerus

101(20).
  • The pronotum with a pair of short longitudinal furrows, one on each side ... Dryops
  • The pronotum without a short longitudinal furrow on each side ... Pomatinus

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. British insects: water beetles. Version: 18th September 2012. http://delta-intkey.com’.

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