307 

 Leech and Chandler: Coleoptera 



and as larvae; adults of certain kinds aestivate in 

 the soil when ponds dry up during the summer. 



The kind of ovipositor a female dytiscid has is an 

 indication of where she will lay her eggs. There are 

 three major types: (1) those in which the genital 

 valves are somewhat dorsoventrally flattened, blunt, 

 weak, or short, and not adapted for piercing, but 

 often hairy and doubtless tactile (Hydroporinae; var- 

 ious unassociated genera such as Colymbetes); such 

 beetles lay their eggs externally on plants and other 

 underwater objects, or in soft mud; (2) those in which 

 the valves are suited for piercing (Dytiscus, Cybister) 

 or sawing (llybius, Laccophilus) plant tissue; the 

 eggs are usually laid within the stems and leaves of 

 plants growing in the water; and (3) a greatly elon- 

 gated ovipositor with which eggs are laid in cracks 

 and protected places under loose bark, in moss or 

 among grass roots, sometimes out of the water though 

 still in damp places (Acilius). It should be noted that 

 oviposition habits cannot always be inferred from the 

 form of ovipositor. Some species with nonpiercing 

 genital valves are reported to lay their eggs in plant 

 tissues, in holes first made with the mandibles. 



The eggs hatch in about five to eight days, though 

 in Dytiscus they may take weeks or even months, 

 depending on the water temperature. There are three 

 larval instars. The larvae are predaceous and canni- 

 balistic; they find their prey — chiefly the larvae and 

 adults of aquatic insects, shrimps, worms, leeches, 

 snails, tadpoles, and even small fish — either by 

 hunting actively, or by waiting in ambush. The jaws 

 are sharp, and channeled; when the prey is caught, 

 the jaw tips pierce through the body wall and a brown- 

 ish digestive fluid is injected through the channels. 

 This serves both to kill the prey, and to start preoral 

 digestion of the body contents, all but the insoluble 

 parts of which are then sucked back through the 

 mandibles. 



What appears at first glance to be a remarkable 

 raptorial development of the front and middle legs of 

 the larva of Matus bicarinatus (Say) was reported by 

 J. Balfour-Browne (1947). The serrated inner edge of 

 the tarsus opposes the marginally serrate edge of a 

 prolongation of the tibia (fig. 13:6rf), forming an ob- 

 vious chela. By watching living larvae, Balfour-Browne 

 found that they never used the front legs in a raptorial 

 manner, but did use them most successfully to burrow 

 in mud. The tibial projection acted both as a pro- 

 tection to the tarsus, and as a digging scoop. It is 

 thus a fossorial adaptation otherwise unknown in the 

 family Dytiscidae, and quite unlike the short broad 

 fossorial legs of the Noteridae. 



Mature larvae crawl up the shore and burrow in a 

 suitably damp spot, forming a pupal chamber in the 

 soil. Some species tunnel into the mud near the water 

 line; others crawl under logs or stones, then dig 

 down; a few build turretlike cells above the soil (see 

 Matheson 1914, fig. 4) in sheltered places. Beetles 

 freshly emerged from the pupal skin usually stay in 

 their pupal cells for a week or more, while the integ- 

 uments harden and attain full coloration; they break 

 out of the cells about a month or six weeks after 

 having formed them. 



Parasites. — Few, if any, parasites have been 

 recorded from the Nearctic Dytiscidae. Both Larvae 



and adults of Old World species arc known to Buffer 

 from the effects of Protozoa (internal and external 

 forms), hairworms (Gordius and allied genera), ne- 

 matodes, and mites. Species of several genera of 

 proctrotrupid and chalcid Ilymenoptera parasitize 

 dytiscid eggs, even those laid in the underwater 

 stems of plants. 



Dytiscidae, particularly adults, fall prey to various 

 aquatic animals. There are numerous records of finding 

 them in the crop and stomach contents of reptiles, 

 amphibians, fishes, birds, especially ducks and 

 wading birds, as well as land birds such as swallows, 

 and animals such as raccoons and skunks. 



Habitat and distribution. — Dytiscid beetles occur 

 in almost any spot damp enough to be called wet, 

 except the open sea and extremely hot or saline 

 waters. Streams, rivers, ponds, lakes, acid bogs, 

 salt marshes, stagnant pools, all have their char- 

 acteristic species. Just as these habitats grade into 

 or resemble one another, the shallow weedy bays of 

 lakes and the quiet backwaters of rivers being com- 

 parable to ponds, so the species typical of ponds 

 may be often found in rivers or lakes. Some species 

 are restricted to specialized habitats such as hot 

 springs or saline desert pools, so that their popula- 

 tions are small and isolated. Other forms may be 

 remarkably tolerant; for instance Deronectes stria- 

 tellus (LeConte) may be found from sea level to an 

 altitude of 10,000 feet, in clear, cold mountain 

 streams, in ponds choked with decaying sawdust, 

 and in the pools of a drying up stream where the 

 water is turbid and so warm that the fish are dying. 

 Large species, such as those of the genera Dytiscus 

 and Cybister, tend to be in the larger, though not 

 necessarily deeper, bodies of water, whereas small 

 members of the vilis section of the genus Hydroporus 

 may be in isolated moss-covered seepages no bigger 

 than one's hand. 



Keeping in mind the microhabitats within major 

 habitats, and the overlapping of species into several 

 habitats, one may generalize about Nearctic species 

 as follows. In fast-flowing streams and rivers: Bides- 

 sus, Deronectes, Oreodytes, Agabinus, Agabus. In 

 slowly flowing waters: the same plus Laccophilus, 

 Hydrovatus, Desmopachria, Hygrotus, Hydroporus 

 Hydrotrupes, llybius, Matus, Copelatus, Rhantus, 

 Colymbetes, Dytiscus, Hydaticus. In ponds and lakes: 

 Macrovatellus, Laccophilus, Desmopachria, Bidessus, 

 Celina, Hygrotus, Hydroporus, Deronectes, Agabus, 

 llybius, Matus, Copelatus, Coptotomus, Neoscutopterus, 

 Rhantus, Dytiscus, Hydaticus, Acilius, Thermonectus, 

 Graphoderus, Eretes, Cybister. In saline pools: 

 Hygrotus, Deronectes. In hot springs (up to about 

 40° C): Bidessus, Hygrotus. 



Those species which occur in fast water usually 

 crawl among stones or stay in the slack water behind 

 them. In quiet water, weedy shallows are more pro- 

 ductive than those without much vegetation; very 

 few species (though sometimes many specimens) are 

 found where there is a dense growth of filamentous 

 algae. 



