fl88 Transactions of the Canadian Institute. [vol. ix 



Locality: — Pond in the vicinity of Hog's Back, on the Rideau. 

 Thy as stolli Koen. 

 Syn. Thyas stolli Koenike: Abh. Nat. Ver. Bremen, 1895, XIII Bd., 

 S. 194-196, Taf, II, Fig. 29-32. 



In 1895 I described the imago of this species that was collected in 

 Canada, but I was at that time not in a position to state the points of 

 difference between the sexes. Recently I have been able to make known 

 the male through the discovery of the penis-skeleton (Penisgeriist) in 

 a Canadian imago. It may be briefly characterized as follows: — 



Male. 



The following statements pertain to a specimen about looo/t in length. 



Including the first and last acetabula, the external genitalia measure 

 about 200/A in length, the genital valve 150/*. The latter exhibits at 

 both anterior and posterior ends a marginal obliquity, of such a form that 

 the inner side is considerably shortened. The posterior obliquity is 

 more or less emarginate, according to the position of the valves. Near 

 this oblique margin is a row of strong bristles, which increase in length 

 from before backwards (Fig, 12). The inner margin of the valve is 

 provided with a number of somewhat widely separated and very short 

 hairs. The acetabulum anterior to the valve is approximat ily ( ircular 

 and har a diameter of 27/i.. The posterior acetabula are borne upon a 

 strong chitinous appendage, which passes backwards from the under side 

 of each valve. The hindmost acetabulum is borne upon, and partly 

 sunk into the end of a very prominent process of the chitinous appen- 

 dage, which is directed backwards and outwards. It has a transversely 

 elliptical form and a longitudinal axis of 27/*. The inner of the two ace- 

 tabula lying posterior to each valve is borne at the end of a special process 

 of the appendage of the valve, though not at the same level, but deeper. 

 Its breadth is somewhat greater than that of the outer acetabulum, its 

 length about the same. Upon the inner angle of the valvular appendage 

 is a group of strong bristles. 



Behind the genital organ is seen a roundish chitinous thickening in 

 the skin (Fig. 12) which, however, is not a porous cuticular plate, such as 

 occurs e.g., in Th. barbigera Viets, but a subdermal structure, probably 

 a small chitinous body serving for muscular attachment. Such porous 

 cuticular plates as K. Viets has shown to be present in large numbers in 

 his species are entirely absent in Th. stolli.^ 



*I have recently succeeded in preparing a perfect chitinous skeleton of Thyas stolli 

 from an alcoholic male example, collected by Dr. J. B. Tyrrell in 1883. I softened 

 this specimen by heating it in a 10 per cent, solution of caustic potash over a spirit- 

 lamp, then allowing it to swell for a short time in cold water. I then boiled it I to 2 

 minutes in concentrated glycerine, after which it was transferred to glycerine and 

 acetic acid. After separating the capitulum, the internal parts were entirely removed 

 through the camerostome by pressure. 



