THE GENUS UEMODEX. 



11 



attention to the rather close resemblance of Deiiiodex to the Cheletid 

 mite Psorcrgates^ belonging to the suborder Prostigmata. There can 

 be little doubt that the Demodicidae are closely related to the Cheletidae, 

 and probably are a degenerate branch of that family. The male sexual 

 opening is dorsal in position in Deinodex, being placed well forward on 

 the cephalothorax* ; and this is also well-known to be the case in the 



TEXT-riG. 1. 



E 





v- 



<l\ 



Diagram sliowiiig affinities of the family DemodicidsD. 



CheletidfE, notably in the genus M)/o^'ia, etc. The penis is a slender 

 elongated structure in both these families, and usually of considei'able 

 size, being attached by the posterior end whilst the anterior end is 

 directed forwards and upwards. Another point of resemblance is that 

 the anus is absent in Demodex and also in the Cheletid genus Harpy- 

 rijnchus ; this is no doubt due to the fact that the food of these two 

 genera is liquid in nature and is entirely absorbed. 



The mouth-parts of Bemodex are very like those of Psorergates ; 

 and in the former a pair of minute ventral stigmata are present at the 

 base of the capitulum. It has been stated that the respiratory system 

 is absent in the family Demodicidaj, but it is improbable that this is 

 really the case, and 1 have seen internal tracheae of a very similar type 

 to those present in Cheletidae. It is probable that the mites of the 

 family Demodicidae were originally pilicolous mites like Mi/obia, etc., 

 living at the base of the hairs of the host, and have become internal 

 parasites, the very small size and elongation of the body being special 

 adaptations for this mode of life. In many respects the capitulum of 

 the Deniodicidaj is very like that of Sarcoptes in structure, for as in 

 that genus the palpi and chelicerse are completely covered both above 

 * See Ann. Mag. Nat. Hist. (ser. 9) Vol. i. 1918, p. 400. 



