INTERNAL ANATOMY. 73 



stricted, forming a valve; it varies somewhat in 

 different species ; its direction is more or less down- 

 ward, sometimes it is almost perpendicular; its distal 

 end of course surrounds the anal opening, which is 

 usually a longitudinal slit in the ventral surface, but 

 may, in some species, be almost terminal ; although even 

 then it is more on the ventral than the dorsal surface. 

 The posterior portion of the rectum is attached to the 

 cuticle of the posterior end of the abdomen by a 

 number of fine muscles, and threads of connective 

 tissue which form almost a circle round the anus, but 

 are not close together (PL A, fig. 2). 



The Malpighian Vessels. These are a pair of cascal 

 tubes which arise one on each side of the narrow con- 

 striction between the colon and the rectum. They 

 may be of even diameter throughout, or may enlarge 

 at the distal ends, as in H. Robini (PL A, fig. 1, m v) ; 

 in that species they are considerably longer than the 

 rectum. Nalepa draws those of Tyroglyphus longior and 

 Oa/rpoglyphus anonymus as shorter than the rectum ; he 

 also shows them as turning sharply toward the ante- 

 rior end of the body (PL A, fig. 3, m v), and thus lying 

 one on each side of the colon about the middle (perpen- 

 dicularly) of its lateral wall. I have not seen them in 

 that position in any species which I have investigated. 

 Gudden depicts and describes them as very small in 

 T. siro ; he says he once found them filled with a yellow 

 fluid. Berlese figures them as equally small in Aleuro- 

 bius farinas.* I have not been able to detect them at 

 all in 6r. platygaster. 



Two precisely similarly situated Malpighian vessels 

 exist in Gamnsus, or at least in every species which I 

 have investigated ; although Winkler, in his admirable 

 paper,t makes them discharge almost into the centre 

 of the rectum, which I think must be one of the very 



* " Ricerclie sugli organ! e sulla funzione della digestione degli 

 Acari," Portici, 1896 ; also in * Bivist. patolog. vegetale,' 1896-7, vol. 

 v, pp. 130195, Pis. viii, ix, fig. 36. 



f " Anatomic der Gamasiden," in * Arb. Inst. Wien,' T. vii, Heft 3 

 (1888), pp. 317354. 



