Immature Stages of Dytiscidae
Larvae and adults of Dytiscidae are predaceous, all species are aquatic and good swimmers. They feed on aquatic animals, mostly insects, but sometimes also tadpoles or fish. However, most species live on the immature stages of other aquatic insects, mainly Ephemerida, Hemiptera and Odonata. Larvae and adults have generally the same food source, though the latter, being more agile, receive a greater choice. Although generally considered beneficial for biological control of mosquitoes, they have never been used for biological control. An early detailed review of Dytiscidae was given by Balduf (1935).
During feeding, the beetles bite away portions of the prey and swallow them a little at a time, while the larvae utilize only liquid food, a habit that is made possible by the presence of a narrow canal or channel extending from near the tip to the base on the inner margin of each mandible. Like many Coleoptera, most dytiscid larvae inject a digestive secretion into the prey's body, which to a considerable extent liquefies the solid tissues.
Adult Dytiscidae are not limited to an aquatic life, for some species are able to use their legs for running and most are able to fly extensively. However, their bodies have been modified for aquatic life, as is shown by a closer union of the body parts, a pronounced streamlining of the body, the flattening of legs, and the development of fringes of heavy hairs on the inner margins of the tarsi. Clausen (1940) noted that in incompleteness of this adaptation to aquatic life was revealed by the pupation habit. After feeding is completed, the mature larvae heave the water to form their pupation cells in mud or soil, under stones and other objects, or among trash. A distinct spherical, oval or pyriform cell is formed, which in larger species may be 5 cm. or more in diameter.
The different species show differences in their manner of locomotion. Some species in several genera move about solely by crawling over the bottom or over rocks and vegetation. Other species swim only occasionally, but many are very active swimmers and come to the surface to capture prey. The bodies of these are light which enables them to come to the surface easily. The legs of some species are equipped with fringes of hairs that facilitate swimming.
Eggs of some dytiscids are laid in masses in mud or debris at the water's edge, singly on the surface of floating leaves, or in individual incisions in submerged plant tissue. The latter behavior is typical in most species. Some damage may at times be inflicted to ornamental aquatic vegetation through excessive oviposition, which results in wilting and death of leaves and stems (Clausen 1940/1962).
There is usually only one generation annually, although adults may live for several years. The life cycle depends on temperature of the surrounding water, and egg incubation may be prolonged from one week or less to several months. Most species overwinter as adults in water, becoming active and ovipositing early in springtime. Other hibernate as larvae, and adults appear in midsummer.
Clausen (1940) referred to the interesting feature of larval dytiscids in their ability to regenerate lost body parts. Legs and antennae seem to be only partly replaced at the following molts but are fully regenerated in the pupa. Such replacement is much more complete when the parts are lost by 1st instar larvae than when they are lost by older instars.