X. The Dytiscidæ (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 few 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 carefuily 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.