212 



Usinger: Hemiptera 



Fig. 7:25. a, Trepobates beck/ D. & H., female, Lake Houghtelin, near Bard, California, 

 November 13, 1951, (R. L. Usinger); b, Metrobates trux infuscatus Usinger, male, Davis, Yolo 

 County, California, October 24, 1942 (R. L. Usinger). 



the swiftest flowing parts of larger streams and 

 rivers, and Halobates are the only insects that have 

 successfully occupied the surface of the open ocean. 



Gerrids are characterized by a velvety hydrofuge 

 hair pile, long legs with preapical claws, and a single 

 scent gland opening at the middle of the metasternum 

 (omphalium). The claws are inserted before the apices 

 of the tarsi and thus do not break through the surface 

 film. The film is simply bent by the tarsi, and this 

 phenomenon may be observed in clear, shallow water 

 by the shadows cast on the bottom. 



Wing polymorphism is a characteristic of all mem- 

 bers of the family except the permanently apterous 

 marine forms (Parshley, 1919). In a single colony of 

 Gerris, specimens may be found with fully developed 

 wings (macropterous), partly developed wings (brachy- 

 pterous), and with no trace of wings (apterous). The 

 body changes that coincide with the various degrees 

 of wing development are considerable, involving a 

 totally different appearing thorax and even a dif- 

 ference in length of the abdominal spines. Wing 

 polymorphism is controlled by a combination of genetic 

 and environmental factors but has not been fully 

 investigated as yet. In macropterous specimens of 

 Metrobates and other Halobatinae the wings are often 

 broken apically, possiblv as a result of mating (Torre- 

 Bueno, 1908). 



Gerrids have been observed only rarely in flight. 



Riley (1920) reports flights at dusk and on moonlight 

 nights and presumes that they fly on darker nights 

 but are unobserved. Macropterous forms have an 

 obvious advantage where pools dry up (Parshley, 

 1922), and completely apterous forms are at no dis- 

 advantage in the permanent marine habitat. 



Most species of Gerridae are gregarious, and this 

 is especially noticed in Metrobates and Halobates. 

 The apparent immunity of these bugs from attack by 

 fish may be owing to their scent gland secretions. 



Feeding habits. — Gerrids are predaceous on a wide 

 variety of organisms, including aquatic forms that 

 come to the surface and terrestrial species that fall 

 on the water. Water striders are very agile and do 

 not hesitate to seize one of their own kind. This is 

 particularly true in confined spaces such as a drying 

 pool or an aquarium where the bugs are crowded 

 together. Individuals are particularly vulnerable when 

 they are molting. 



Life history. — Hungerford (1920) has summarized 

 life history studies for Gerris remigis and G. margin- 

 atus. The bugs overwinter in the adult stage in 

 protected places near the water. Eggs are laid 

 throughout the spring and summer. They are glued 

 to objects at the water's edge, usually to floating 

 objects which, of course, remain at approximately 

 the level of the water surface throughout the season. 

 The eggs are laid in rows, parallel to one another, 



