What Bug Is That? The guide to Australian insect families.

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Dytiscidae

Overview

The predacious diving beetles are the largest and most commonly encountered group of aquatic Adephaga.

Both adults and larvae come to the surface for air, which they obtain through the terminal pair of spiracles, and the former may store air beneath the elytra. Adults are normally capable of sustained, crepuscular or nocturnal flight, which is necessary for dispersal to isolated habitats; they are often attracted to lights and may mistake artificial shining surfaces, such as glass, for water.

Dytiscids inhabit a variety of lentic and some lotic freshwater habitats, but they are most abundant in the littoral zone at the edges of lakes and ponds. They may also occur in water-filled ditches, dams, billabongs, pools in intermittent streams, and even saline inland lakes. Both adults and larvae are predators on other aquatic animals, including insects, crustaceans, worms, leeches, molluscs, tadpoles or even small fish.

Adults and some larvae are capable of taking in solid and liquid food through the mouth opening, but most larvae have a closed mouth opening and use the mandibular channels both for injecting digestive enzymes into the prey and for sucking in the resulting fluids, by means of a cibarial-pharyngeal pumping apparatus. Eggs are often deposited in slits made by the ovipositor in the stems of aquatic plants, but this habit is not universal. Pupation takes place in a cell formed by the larva in damp soil near, but out of, the water.

Description

Adults smooth and boat-shaped, with very large hind coxae (lacking coxal plates) and enlarged hind legs, which move synchronously in swimming and have flattened, paddle-like tarsi bearing a dense fringe of swimming hairs. Dorsal and ventral surfaces of body of similar convexity, with no median, flattened keel on the latter. Males in some groups have the first 3 segments of the fore tarsi greatly dilated to form adhesive pads equipped with suction discs, which are used to hold the female during copulation. Larvae with a relatively streamlined form, well-developed fringed swimming legs, large falcate mandibles, almost always deeply grooved or perforate, and 8th spiracles placed at end of elongate, siphon-like tergal process.

Distribution

All of the major groups of Dytiscidae occur in Australia, and the greatest number of species is found in the South-East. Eretes australis is a common and widespread inland species, while the largest Australian dytiscid, Cybister godeffroyi , occurs on the Cape York Peninsula. Terradessus caecus is a blind, terrestrial species known only from high altitude rainforests in North Qld (Watts 1982). [De Marzo 1979; Galewski 1971; Watts 1963, 1964, 1978.]

  • Dytiscide

  • Hydaticus parallelus

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