■AGO On the Acari of the Family Demodecidge. 



XL VIII. — On the Origin and Affinities of the Acari of the 

 Family Demodecidse, with Brief Remarks on the Mor- 

 phology of the Group. By Stanley Hirst. 



(Published by permission of the Trustees of the British Museum.) 



The mites of the family Demodecidse are regarded by most 

 acarologists as an isolated group, and, owing to the extreme 

 elongation of the body and aunulation of the abdomen, they 

 are usually placed with the Tetrapoda (Eriophyidee), a 

 family also exhibiting these peculiarities, but otherwise very 

 dissimilar in structure. Oudemans has hinted at their deriva- 

 tion from the Sarcoptidse (Astigmata) parasitic ii'i mammals, 

 and has also drawn attention to the rather close resemblance 

 of Demodex to the Cheletid mite Psorergates belonging to 

 the order Prostigmata. There can be little doubt that the 

 Demodecidoe are closely related to the Cheletidse, and probably 

 are an offshoot from that family. The male sexual organ 

 opening is dorsal in position in Demodex, being placed well 

 forward on the cephalothorax, and this is also well known to 

 be the case in the Cheletidse, notably in the genus Myobia etc. 

 The penis is a slender elongated structure of considerable size 

 in both these families, and is attached by the posterior end, 

 the anterior end being directed forwards and upwards. The 

 mouth-parts of Demodex are very like those of Psorergates, 

 and in the former genus a pair of minute ventral stigmata 

 are present at the base of the capitulum. The respiratory 

 system is said to be absent in the family Demodecidas, but it 

 is improbable that this is really the case, and I have seen 

 internal trachea? of a very similar type to those present in 

 Cheletidaa. The nymphs of the genus Demodex differ from 

 the adults in important details of structure, being provided 

 with simple lateral plates or epimera (each of which bears 

 a pair of tubercles furnished with very minute denticles) 

 instead of the fully developed legs of the mature form. Four 

 central pairs of curious epidermal discs bearing minute spines 

 on their posterior margin are also present on the ventral 

 surface of the body. Further details are given in my revision 

 of the family Deinodecidse, which will be sent to press 

 shortly. 



