Description & Statistics
There is a much greater variety in behavior among members of this family than in the wholly aquatic Coleoptera. Most species are aquatic, but a number of the subfamily Sphaeridiinae are terrestrial in all stages, living in moist soil or associated with animal wastes (Legner et al. 1980, 1981). Adults are commonly known as "water scavenger beetles." Balduf (cited by Clausen, 1940) provided an extensive early account of hydrophylid behavior.
Aquatic hydrophilids are most often found in ponds, especially those having extensive vegetative growth, although they also may be found along streams. Adult beetles are principally scavengers, consuming decaying animal matter and also living on dead plant tissue. Many species feed mostly on algae and other lower forms of plant life. Although most larvae are predaceous, there are some exceptions. Predaceous forms feed on various worms, snails, insect larvae and pupae, Entomostraca, small fish, crayfish and tadpoles. They may actually swallow bits of solid matter, but seem to prefer body fluids of their prey.
Several species of the genus Dactylosternum are predators of crop pests. Dactylosternum dytiscoides F., D. hydrophiloides M'Leay and D. cycloides Knisch. attack larvae of the sugarcane beetle borer, Rhabdocnemis obscura Boids., in the Philippines. These species were imported to Hawaii during 1925-1926 for biological control. D. abdominale and D. dysticoides were imported to Jamaica for biological control of banana borer, Cosmopolites sordida Germ.
Oviposition behavior is distinctive because of the silken case within which the eggs of many species are contained. Several more primitive subfamilies lay their eggs singly, with little or no covering. In a few genera, the cases are attached to the body of the parent by silken strands, while in others they are enclosed in a folded leaf, placed on foliage underneath the water, or float free on the surface (Clausen 1940/62). The larvae of many aquatic species are unable to swim, but rather move about by crawling along the bottom or on vegetation, and may be found only partially submerged. Larvae of most species construct their pupal cases out of the water, near the water line in mud, under various objects, or on plants above ground. Enochrus is reported to form its case from floating strands of Spirogyra.
Hydrophilidae have more than 500 species. They are cosmopolitan. Although primarily aquatic beetles, many species have terrestrial habits. Diagnostic characters of these are the long and slender maxillary palpi, as long or longer than the antennae. Antennae have 6-10 segments with the apical segments shaped into a distinct club. All the tarsi have the same number of segments; the prosternal sutures are distinct; the gular sutures are double and the hind coxal craters do not divide the first ventral abdominal segment.
The larvae of Hydrophilidae are voracious predators on a variety of insects and other aquatic animals; while the adults are scavengers on decaying plant and animal matter (Legner et al. 1980). Several species have been introduced for biological control with no reported success.
There are 1-2 generations annually, and overwintering is as adults (Clausen 1940/62).
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