218 



Usinger: Hemiptera 



Fig. 7:32. Hydrometra marfini Kirkaldy, female. Imperial Dam, 

 Colorado River, Calif., Nov. 14, 1951 (R. L. Usinger). 



Family HYDROMETRIDAE 



Marsh Treaders 



Hydrometrids are perhaps the most fragile of the 

 surface water bugs, with very slender bodies and 

 threadlike legs (fig. 7:32). In our species the head is 

 subequal in length to the abdomen. The swollen 

 anterior part of the head above has two pairs of small 

 brown pits in which are inserted long, erect bristles 

 or trichobothria, and another pair is present at the 

 base of the head. Brachyptery is common, the hemely- 

 tra sometimes being reduced to virtually unrecogniz- 

 able stubs or rounded elevations. The body surface 

 is more or less granular and variously pitted. Unlike 

 most surface dwellers, the minute claws of Hydrometra 

 are terminal, and this no doubt renders them less 

 efficient in open water than such masters of the 

 surface film as the Gerridae and Veliidae. 



Life history.— Martin (1900) and Hungerford (1920) 

 have given excellent accounts of the biology of our 

 commonest species, Hydrometra martini Kirkaldy. 

 Adults overwinter in the adult stage. Eggs are laid, 

 at least in the eastern and midwestern United States, 

 in the spring. The eggs are spindle-shaped and are 

 attached by a slender stalk to solid objects at or 

 just above the surface of the water. According to 

 Hungerford, the average durations of the various 

 stages are: egg, 7 days; 5 nymphal instars, 2 days each. 



Habitat and distribution* — Hydrometra is an inhabit- 

 ant of quiet waters where it dwells in the protection 

 of various kinds of plants. Emergent grasses, duck 

 weed, and cattails provide support and protection. In 

 the Colorado River drainage system it was found most 

 commonly in small seepage pools choked with vegeta- 

 tion and without fish. Whether or not its scarcity in 

 larger bodies of water is due to the predatory pressure 

 of fish is not known, but it is a fact that Hydrometra 

 lacks the thoracic scent glands which are so char- 

 acteristic a feature of other surface bugs. 



The geographical distribution of the genus in Cali- 

 fornia is unusual. H, lillianis has never been reported 

 since the original collection at Santa Barbara, in 

 spite of the fact that a diligent search was made for 

 it in habitats that appeared to be ideal. H. martini 

 has been found only along the Colorado River under 

 what could best be described as subtropical condi- 

 tions. This would not be remarkable but for the fact 

 that the only other record on the Pacific Coast is 

 Corvallis, Oregon (Drake and Harris, 1928). Negative 

 evidence for the intervening territory is, of course, 

 not conclusive, but might be owing to the arid condi- 

 tions which prevail during the summer months in 

 most of California. Careful comparison of a Corvallis 

 specimen loaned by V. Roth with Colorado River 

 specimens shows no significant differences. Else- 

 where H. martini occurs over most of eastern North 

 America and well into Mexico. 



Feeding habits. — Hungerford (1920) reports that 

 Hydrometra will feed upon insects which fall onto the 

 water surface, but that the normal inhabitants of the 

 surface film are much preferred. He cites ostracods 

 and mosquito larvae and pupae as favorite foods. 

 The role of hydrometrids as predators of Anopheles 

 larvae, which spend most of their time at the under 

 side of the surface film, led F. X. Williams to con- 

 sider introducing them to areas where they do not 

 normally occur as a possible means of biological 

 control. 



Taxonomic characters. — Hydrometra species are 

 distinguished by the relative proportions of antennae, 

 rostrum, and legs, by the ratio of anteocular to post- 

 ocular parts of the head, by average size, by the 

 shape of the clypeus, by the arrangement and number 

 of pits on the acetabula, and especially by the shape 

 of the genital segments in both sexes, and by the 

 presence or absence and form of a pair of projections 

 or carinae near the base of the sixth ventral segment 

 in the male. The principal taxonomic works on Hydro- 

 metridae are by Torre-Bueno (1926) and Hungerford 

 and Evans (1934). 



