DYTISCIDAE. 67 



Metasternal epimera covered by the elytra. ii. 



Metasterual epimera triangular, exposed. Eretes. 



2. Claws of hind tarsi unequal, the inner one in certain J obsolete. :;. 



Claws of hind tarsi equal, or nearly so. 4. 



I'. Joints of hind tarsi ciliate with flattened hairs on distal margin, liind 



leys more slender, sjnirs acute. Hydaticus. 



4. Hind tarsi ciliate on the distal margin ; spurs emarginate at tip. :"). 



Hind tarsi not ciliate on the distal margin ; spurs acute. Dytiscus. 

 f). Elytra not punctured, partly aciculate in J- <I. 



Elytra densely punctured, usually 4-sulcate in 9 • Acilius. 



G. Middle thighs with long sette. Thermonectes. 



Middle thighs with short setie. Graphoderes. 



Eretes is cosmopolitan, hut in tliis. country extends only from 

 California to Kansas. With the exception of Hydaticus, which 

 occurs only in tiie Atlantic region, the other genera are distributed 

 on both sides of the continent. 



Tribe IV.— CYBISTRINI. 



This tribe is represented by a small number of species of Cy- 

 bister, of which there arc numerous species in Tropical America. 



'I'hey are easily known by the small size of the spiracles, espe- 

 cially the posterior two or tliree pairs. The hind legs are broad 

 and powerful, the tibiae short, the joints of the hind tarsi without 

 a fringe of flattened ciliaa on the distal margin, and the iiiiid claws 

 very unecpial, the inner one being obsolete or wanting in certain 

 9 5 . The spurs of the hind tibite are emarginate at tip. The 

 fro!it tarsi of the % have the joints l-o dilated into a large cir- 

 cular disk, and the cupules of the under surface are not unequal 

 as in Dytiscini, l)ut similar, and arranged in four rows. 



These insects are properly considered by Dr. Sharp as the 

 highest and most complete development of the Dytiseido type; 

 and it is also worthy of reimirk in this connection, that it is the 

 only one conspicuously better represented in the tropics than in 

 temperate or frigid regions. They are nearly undistinguishal)le 

 in specific characters, and can be separated most easily by the 

 sexual dilfereiices, which are usually quite well defined. The 

 same dilliculty in specific definition is to be discerned in the cul- 

 minating genera, groups, tribes, or families in the higher forms 

 of animal life. 



