THE CANADIAN ENTOMOLOGIST. 149 



THE COLEOPTERA OF CANADA. 



BY H. F. WICKHAM, IOWA CITY, IOWA. 



X. The Dytiscid^e (Dytiscini and Cybistrini) of Ontario and Quebec. 

 The species included in the above tribes are all large or moderate 

 sized insects, and among them are found those in which the perfection of 

 specialization of the ^ tarsi reaches the highest pitch. The basal disks 

 on the anterior feet often attain considerable dimensions, while occasionally 

 we find three well-marked sizes of disks on the same foot — as, for example, 

 in Acilius. These make beautiful preparations for microscopic study, the 

 method of treating them being as follows : Cut off the leg a little below 

 the knee, and place the severed piece in an aqueous 25% solution of 

 caustic potash for a i^^ff days, until it becomes nearly transparent. Then 

 take out of the potash and place in soft water for twenty-four hours, next 

 removing to strong alcohol for about the same length of time. If now 

 placed in oil of cloves for several hours, it is ready to be mounted on a 

 slide in the ordinary way for examination by transmitted light. A great 

 deal depends on carefully watching while in the potash solution ; the 

 specimen should not be so dark as to obstruct the light-rays to any very 

 great extent, nor so transparent as to render the overlying parts to be 

 separated with difficulty from the underlying, as in the latter case the 

 chitin is often rendered so pliable as to lose the original form. Each 

 species has its own arrangement of disk bearing hairs by which it may be 

 separated from its congeners, though the resemblance is sometimes very 

 close. 



For the sake of convenience, I have included both of the above tribes 

 in one generic table ; of the second, the genus Cybister is the only 

 representative, the tribal characters being given on page 72. 

 A. Length, above an inch. 



Hind tarsi with one claw, anterior $ tarsi with four rows of equal 

 disks Cybister. 



Hind tarsi with two claws, anterior $ tarsi with two large and 

 numerous small disks Dytiscns. 



AA. Length scarcely exceeding half an inch. 



b. Spurs of hind tibiae acute at tip Hydaticus. 



bb. Spurs of hind tibiae emarginate at apex. 



c. Elytra either sulcate ( $ ) or with distinct, closely-placed 

 moderate-sized punctures Acilius. 



