328 



Leech and Chandler: Coleoptera 



Fig. 13:22. Gyrinidae. a, Dineutus sp., adult female; b, 



Gyrinus sp. adult; c, Gyretes sinuatus, adult, lateral view; d, 



Gyrinus sp., mud cocoon on reed (o, Borror and Delong, 1954; 



Reitter, 1909; c, Young, 1954; d, Butcher, 1933). 



The listing of Spilocryptus incertus Cresson by 

 Dimmock and Knab resulted from an error of host 

 record, perhaps the effect of contaminated cage 

 material. 



Of greater interest is the parasitization of Dineutus 

 by the larvae of Brachynus, a genus of bombardier 

 beetles. Wickham (1893, 1894) was the first to record 

 larvae of B. janthinipennis Dejean as external para- 

 sites of the pupae of D. assimilis (Aubd); Dimmock 

 and Knab (1904) described the Brachynus larva and 

 pupa in detail and illustrated them. 



Fruiting bodies of a parasitic fungus, Laboulbenia 

 yyrinidarum Thaxter of the family Laboulbeniaceae, 

 are often seen attached to the elytra and pronotum of 

 individual gyrinids, especially in the genus Gyrinus. 

 The fungi resemble minute, swollen, black bristles, 

 and tend to be near the margins of the dorsal surface, 

 whence they stick out nearly at right angles. They 

 are said not to harm or bother the beetles in any way. 



Habitat and distrioution t — Adult gyrinids are found 

 on streams, rivers, ponds, and lakes (but only tran- 

 siently on brackish or saline waters), and rarely far 

 from shore. The species of Gyretes prefer and are 

 especially adapted for swift currents such as occur 

 in rivers. Species of Dineutus are typically stream 

 inhabitants, whereas those of Gyrinus are most com- 

 mon on lakes and ponds. However, quiet pools and 

 eddies in streams and rivers are often comparable to 



still-water inhabitats, and the same species may be!] 

 found in both. Species of Gyrinus especially are! 

 tolerant of local conditions and are to be expected! 

 on almost any puddle large enough to swim in. Many] 

 gyrinids are known to fly well, and in California i 

 species of Gyrinus sometimes mistake the shiny topsi 

 of automobiles for water surfaces and rain down upon! 

 them at full speed. 



The beetles tend to congregate in large swarms or 

 schools, within which the individuals swim slowly 

 around or rest. In the eastern United States these 

 schools are often composite, containing as many as 

 five species and sometimes two genera. In the West 

 the aggregates usually contain but one species. The 

 schools are commonly formed near shore or emergent 

 vegetation, and often in a shady or sheltered spot. 

 Most species are diurnal, but in at least certain 

 unfavorable habitats some become nocturnal. I have 

 collected water beetles throughout the day in a tiny 

 stream in the Tumacacori Mountains of Arizona with- 

 out finding a single gyrinid. Yet when die same water 

 was visited with a Coleman lantern after dusk, a 

 couple of dozen Gyrinus plicifer LeConte were quickly 

 netted from the school on the surface. 



Species of Dineutus, in its various subgenera, occur 

 in the Ethiopian, Oriental, and Australasian regions, 

 as well as in the Fiji Islands and North and Central 

 America. A single species belonging to the subgenus 

 Cyclinus Kirby has been recorded from California and 

 is common in Mexico. Gyrinus is widespread and oc- 

 curs in most parts of the world except the Pacific 

 Islands beyond New Guinea, New Caledonia, and New 

 Zealand. At least a dozen species are known from 

 South and Central America, and more than forty from 

 North America. Gyretes is a Neotropical genus with 

 many species in South and Central America; two 

 species have been described from California, but one 

 is of doubtful validity. 



Taxonomic characters. — The species of gyrinids, 

 especially of Gyrinus, are monotonously alike in 

 general appearance, and in our fauna show few color 

 differences. It is often difficult to identify females 

 to species unless they can be associated by localiu 

 with males. The latter offer some clear external 

 differences, but for verification the male genitalia 

 must be extruded or dissected out. Fall (1922) has 

 illustrated the male genitalia of most of our species 

 of Gyrinus, and Roberts (1895) those of Dineutus. 

 The color of the under surface is used to distinguish 

 groups in both genera, but is difficult to apply in 

 immature specimens. In males the front tarsi are 

 broadened, and clothed beneath with dense pads of 

 sucker hairs. 



The chief external structural features of value in 

 Gyrinus are: size and convexity of the beetle; surface 

 sculpture of the elytra, not always alike in the two 

 sexes; relative sizes of the serial punctures of the 

 elytra, the extent to which they are impressed as 

 striae, and the position of the marginal series; and 

 the form of the anterior margin of the metasternum, of 

 the outer apical angles of the elytra, and of the basal 

 segment of the front tarsi of the male. In Dineutus 

 the form of the front tibiae, ciliation and dentition of 



