DYTISCIDAE Leach, 1815
This large family of unmistakable and generally abundant beetles can be found in just about any aquatic environment. A range of species are common and will quickly become familiar.
ADEPHAGA Clairville, 1806
This cosmopolitan family of water beetles includes more than 4000 described species in more than 160 genera; the classification is not settled and the distinction between many groups regarding subfamilies and tribes is sometimes ambiguous; recent systems can be found which include various versions of the rankings but generally there are up to 10 subfamilies.
At least some species occur on most freshwater habitats throughout the world and some inhabit brackish environments either as generalists or specialists, and a few inhabit more saline coastal pools. Many occur at moderately high latitudes and altitudes and some are truly cold-adapted being active in polar regions through the winter; in northwest Europe some species have been recorded above 1200m. The majority occur in shallow areas of still and slow-moving lowland water, especially where there is an abundance of marginal vegetation. Small or temporary seasonal pools, especially in woodland environments, may host large numbers of adults in the spring although some species do not inhabit pools below a certain size. One group of the Hydroporini Aube, 1836 are particularly specialized; stygobiontic water beetles spend their entire lives in cave systems or aquifers and have no contact with the outside world, only about 10 species in 8 genera are described and occur in widely unconnected areas of the world; Japan, New Zealand, Mexico, West Africa, Europe and The United States. Stygoporus oregonensis Larson & LaBonte, 1994 is a small and blind species believed to have been living in aquifers for millions of years, it has numerous sensory bristles which would prevent it from entering the water if ever it left the medium. In temperate regions many species disperse in the spring or move to overwintering sites in the autumn and so adults are likely to turn up in diverse situations, and during the summer when temporary pools dry up they may be found under logs, among moss and stones or deep within cracks in drying substrate. In general sampling for adults is straightforward with a pond net when used vigorously among marginal vegetation; better results are may be obtained if the substrate is
disturbed first, especially in moving water, and dense marginal moss etc. can be compressed underfoot so that the beetles enter the resulting depressions as they fill with water. Many species are nocturnal and will occur at lights, often in large numbers, and baited traps can be left among marginal vegetation but these will need to be looked at every morning as many specimens may arrive in a single night and predation may be high. Cattle troughs and garden ponds are always worth sampling. In temperate regions most species mate in the spring, or to a lesser extent in the autumn, and produce larvae that develop over the summer and produce late summer or autumn adults which overwinter and breed the following spring. Thus the adults, which are often long-lived, and larvae are often found together. Some species are more flexible, overwintering both as adults and larvae, and sometimes the life cycle takes more than a year to complete; overwintering in the egg stage and developing as larvae the following summer to produce adults in the autumn which will overwinter; or mating and producing larvae in the autumn which overwinter and produce adults in the summer which will overwinter and breed the following autumn. The vast majority of species are therefore univoltine.
Diving beetles either lay their eggs among the bottom substrate or aquatic foliage or they insert them into the stems of water plants using a specialized ovipositor. In most species they hatch within a week or two, an adaptation to the rapid life-cycle of univoltine species, but in some they undergo a diapauses and hatch up to a year after being laid, these must survive fluctuations in temperature and occasional drought and also, something which is common among Dytiscid eggs generally, predation and parasitism from tiny chalcid wasps. In temperate regions larval development in most species proceeds through the summer and is complete in one season, producing adults in the autumn that will overwinter. Pupation generally occurs as soon as the larvae are fully grown; they leave the water and burrow into marginal soil to pupate, this stage is brief and no species are known to overwinter as pupae. Most overwinter as adults and at least some species are long-lived, overwintering twice although some Agabus are known to survive as adults for only a few weeks. Many species overwinter in the water e.g. many Ilybius and Rhantus, and many migrate to large water bodies in the autumn for this purpose. At higher latitudes the adults of some Agabus species eclose in the autumn but remain in the pupal cell through the winter, and the life cycle of many species is modified to accommodate varying local conditions e.g. temperature, altitude or latitude.
Most species vary from black to some shade of brown, sometimes very pale, with various lighter or darker markings and some are metallic although in general only weakly so. Even the most strikingly marked species e.g. Thermonectes are cryptic when viewed in their natural environment. The U.K. fauna includes some of the largest and smallest species and most groups are represented; the family varies only in detail so that a familiarity with our list will be sufficient to recognize the wider world fauna.
The species vary widely in size from <2mm to >45mm. The body outline is continuous and always in some sense ‘boat-shaped’, and they are variously convex above and below e.g. some Hydroporus are relatively flat while members of the Hyphydrini are very convex. The body is usually more or less glabrous; there is often fine and sometimes coarse punctation and very fine pubescence, which may be dense, and scattered long hairs. Microsculpture is common, often consisting of a fine mesh within a larger one or an overall isodiametric or longitudinal cell structure, this is often more developed in the female.
The head is transverse and generally hidden within the prothorax to the level of the posterior margin of the usually prominent eyes. Antennae always 11-segmented and generally glabrous, usually with all the segments elongate but in some tropical species the basal segments may be modified in the male. The clypeus is usually fused onto the frons and the labrum is transverse with a central excavation bearing prominent setae. The palps are slender with all the segments elongate; the labial palpi are 3-segmented and the maxillary palpi are 4-segmented. The eyes are usually nearly circular and convex; either entire or with the anterior margin notched. Mandibles usually well-developed; short, curved and sharp. Pronotum relatively large and generally transverse with protruding anterior angles and a sinuate hind margin; in some groups this is produced medially over the scutellum. The lateral margins narrow from the generally acute hind angles to the anterior margin and are margined in many groups. The surface is generally even although there may be transverse series of large punctures parallel to the anterior and posterior margins; sculpture is unusual although some groups have an impressed stria next to the lateral margin or in the basal half which may connect at the base with another short stria on the elytra. The prosternum is usually produced into a long process between the anterior coxae which extends into a notch on the anterior margin of the metasternum; this process is sometimes more deeply impressed than the surrounding cuticle. Metasternum well-developed and laterally produced into triangular processes which reach the epipleura. Metacoxal process well-developed, extending back from the large metacoxal plates, in most species a curved line extends from the inner coxal margin onto this process. The relative sizes and forms of the various ventral characters are often useful in identification. Abdomen with 6 visible sternites; the first divided by the metacoxal process and the apical segment often modified and sexually dimorphic; excavated or rugose. The elytra are generally smooth and not, or only narrowly, explanate although there are exceptions, variously punctured and pubescent and often with 3 or 4 series of setiferous punctures set in longitudinal grooves. In some groups they are randomly and quite strongly punctured throughout. Females tend to be dull when compared to the males due to their stronger microsculpture and in some groups they are sulcate or polymorphic. Sutural stria usually absent. The epipleura generally narrow strongly around the middle and in some groups there is an oblique raised line under the humeral margin. Hind wings usually well-developed and most species are capable of flight. The front and middle legs are generally more slender and less flattened than the hind legs. All tibiae have a pair of apical spurs although these may be tiny on the front legs, and the external edges, and sometimes also the internal edges, are fringed with long ‘swimming hairs’; these are most strongly developed on the middle and hind legs, these are used for swimming while the front legs are used to grab prey and in mating etc. Tarsi 5-5-5, the metatarsal segments frequently lobed and with various arrangements of setae. In the male the protarsi are generally modified in some way, with some segments dilated, sometimes greatly so in the Dytiscinae, and equipped with adhesive setae ventrally.
The larvae are characteristic; elongate and crescent or S-shaped with a large head, long appendages and palps, and prominent urogomphi, the apex of the eighth segment is often produced into a siphon. The mandibles are long and curved with the inner margin grooved, an adaptation for the partial external digestion of prey. They hunt by clinging to the bottom substrate, vegetation or by hanging from the water surface, and remaining still until food passes, they then lung and secure the prey with the front legs and mandibles. Adults are strong swimmers that actively pursue their prey, they are broadly carnivorous and will also feed on carrion whereas larval prey depends much more upon body size; the majority consists of small crustaceans and insect larvae but some species are more specialized; Cybister larvae feed on odonata larvae while some Dytiscus species predate caddisfly larvae while still in their cases. Adults carry air beneath the elytra and renew this, abdomen first, at the water surface, this bubble or ‘plastron’ presents a large surface area for gas exchange with the water and so enables the beetle to remain submerged for long periods, this may be especially important for adults overwintering. Most larvae surface to renew their air supply but it is thought that a significant proportion of gas exchange occurs through the cuticle. Larger and heavier larvae tend to live in shallower marginal environments where access to the surface is easier. Dytiscids always swim with synchronous leg movements while two other aquatic families, Paelobiidae and Haliplidae, swim with alternate leg movements. The superficially similar Noteridae are distinguished from Dytiscids by the modified antennal segments.
Scutellum not visible, size smaller than 6 mm.
Scutellum visible, size larger >5.5mm.
Segments of the hind tarsi lobed beneath; all tarsi obviously 5-segmented.
Segments of the hind tarsi simple; front and middle tarsi apparently 4-segmented as the fourth segment is very small.
Eyes notched behind the antennal insertions.
Hind coxal lines strongly sinuate, almost right angled and approaching the centre-line.
Hind coxal lines more widely separate and not approaching the centre-line, the angle behind the coxal lobes obtuse.
Posterior apical angle of the hind femur with a small ‘comb’ of closely-arranged setae, sometimes only a few but always arranged in a straight line.
Posterior apical angle of the hind femur with at most a few scattered setae, and if present these are not arranged in a straight line.
Beetles of Britain and Ireland vol. 1
Andrew G. Duff
Provides keys and accounts on all the UK species.
Water Beetles of Britain and Ireland
G. N. Foster & L. Friday
Keys to family and species level.
The aquatic Adephaga of Fennoscandia and Denmark
Detailed accounts of Dytiscidae and other families.
British Water Beetles
Classic account of all British water beetles.