SPERCHEIDAE Erichson, 1837
Filter-Feeding Water Beetles
Our single British member of this small and distinctive family of water beetles is probably now long extirpated, but remains locally common in Europe, and so could occur here again.
POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
Spercheus Kugelann in Illiger, 1798
S. emarginatus (Schaller, 1783)
A small family containing a single genus with around 20 species. They occur throughout the world except for the Nearctic. The widest diversity is found in the Oriental and African tropical regions. Two species occur in Australia; S. wattsi Hebauer, 1999 and S. platycephalus Macleay, 1825 which also occurs on some islands of Southeast Asia. At least two have been described from South America. One from Iran; S. babylonicus Hebauer, 1997, and one from Madagascar; S. hovanus Fairmaire, 1903. There is only a single Palaearctic species; S. emarginatus (Schaller, 1783).
Adults are very characteristic and should not be confused with any other group, even though in the field they are often covered in mud and detritus. The genus is characterized by the following combination of characters:
Head strongly contracted behind the eyes.
Clypeus widely dilated and dimorphic, being variously excised on the front margin in the male.
Antennae 7 segmented; segment 3 small and segments 4 to 7 large.
Mentum large and transverse.
Empodium large and mulitsetose.
Galea long and thin.
Characters useful in identifying the species include any denticular arrangement on the tibiae, ridges or tubercles on the elytra, and the lateral margins of the pronotum which may be lobed, undulate or in other ways uneven.
The larvae are distinctive with a large head and strongly tapered abdomen with lateral processes on the first 8 segments. The labrum is fused to the head capsule. Labial palps 1 segmented. Apex of mandibles bidentate. Gular sutures separate. First segment of the maxillary palps with a large digitiform appendage. Abdominal segments 8 and 9 form a spiracular chamber. Worldwide the larvae are filter feeders of detritus and algae etc. although at least one Australian species predates snails, worms, small crustaceans and insect larvae.
The family is morphologically distinct and clearly monophyletic possessing many unique characters in both adult and larval stages. Most of these are adaptations to a filter feeding way of life. They occur in stagnant water with a high density of vegetation, and often in the mud at the sides of small pools. Adults and larvae are among the very few coleopteran filter feeders. Neither stage swims but may be found hanging upside down from the surface while filtering food. Adults feed on algae near the surface while larvae feed on organic remains of small invertebrates from above or below the surface. Eggs are carried in a cocoon by the female, attached to the metatibiae and carried below the abdomen, in hydrophilids it is attached directly to the abdomen. The cocoon is discarded once the larvae hatch. Larval development is usually rapid and they may be found alongside cocoon carrying females. Pupation occurs in a cocoon on soil at the water margin. In temperate regions the enclosed adults overwinter in the cocoon.
Spercheus emarginatus (Schaller, 1783)
This is the only Palaearctic species of the family; it occurs throughout central and parts of northern Europe but is very local and seems to be nowhere common. During the nineteenth century it occurred locally in southeast England but there is only a single twentieth century record, from Suffolk in 1956 and it is now presumed to be extinct. Adults are varying shades of brown with darker markings and carry an air bubble on the ventral surface. They are fully winged but rarely fly. Both adults and larvae are aquatic, living along the margins of muddy, eutropic stagnant water. Both stages typically feed upside down at the water’s surface on algae and decomposing vegetable matter, the larvae also predate small insect larvae etc. Females take care of the offspring, carrying a cocoon containing around 60 eggs underneath the body. This cocoon is abandoned after the larvae hatch. The full grown larvae are dark with a large, square head, convex dorsal surface and flat underside. They often remain alongside the female until fully grown. Pupation occurs in late summer in a cocoon on the water margin and the eclosed adults overwinter.
5-7mm. Head strongly contracted behind the eyes. Eyes prominent and asymmetrical. Centre of frons convex and then concave either side towards the eyes. Surface very uneven and with obscure large punctures. Clypeus dilated and notched at centre of front margin, anterior corners angled in male. Antennae 7 segmented but may appear 6 segmented as 3 very small, 5-7 form a weakly expanded club. Segments 3-7 pubescent. Palps longer than antennae. Pronotum transverse, convex on disc and explanate towards lateral margins. With large, dense punctures, shining between and microsculptured within. Front margin bordered and sinuate, produced at centre. Lateral margins evenly rounded. Hind margin weakly sinuate and produced at centre. Scutellum elongate with surface microsculptured. Elytra very convex, flanks almost vertical, declivity steep. Surface randomly punctured and with 4 or 5 narrow, longitudinal unpunctured and slightly raised lines; 2, 3 and 5 continued almost to apex but the strength of these varies. Side margins finely bordered and narrowly explanate. Sutural margin and adjacent ridge raised just before apex. Tibiae with raised ridges lined with small, stiff spines, and with combs of small spines at apex. Tarsi 5, 5, 5. Segments 1-4 lobed below. Terminal segment long and curved, as long as 1-4 combined. Claws large and evenly curved, without teeth. Empodium large, prominent and with long setae at apex. Abdomen with 5 visible sternites. Underside with close hydrofuge pubescence.