12 STUDIES ON ACAEI. 



and below by a thin membranous cuticle forming the capitulum or 

 hood ; this structure is segmented in a manner roughly corresponding 

 with the internal parts (palpi, etc.). It is probable, therefore, that 

 the Trombidiidae (Cheletidae, etc.) and Deraodicidse are closely allied 

 to the Sarcoptidae (text-fig. 1). 



HABITS, ETC. 



The minute size and habit of internal parasitism of the mites of 

 this genus make observations on living specimens very difficult, if not 

 impossible. They occur very frequently in the skin of mammals, and 

 yet it is difficult to discover the way in which infection takes place. 

 1 have never seen any specimens on the outer surface of the skin, nor 

 attached to the hairs of the host. It is, of course, possible that these 

 parasites attach themselves to more active ectoparasites as a means of 

 dispersal, but there is no evidence of this. It is probable that contagion 

 is often due to actual contact or cohabitation of infected animals with 

 others of their species. 



The parasites sometimes occur in large numbers, all stages of the 

 life-history often being present together in a follicle, thus causing it to 

 become distended ; the head of the mites usually points towards the 

 bottom of the sebaceous gland or hair follicle. (For an account of the 

 nymphal stages etc., see page 31 under B. muscardini.) 



EXTERNAL MOEPHOLOGY. 



Capitulum (text-figs. 2 & 3). — Capitulum membranous and rather 

 delicate, enclosing the palpi, chelicerse, etc. It shows distinct segment- 

 ation, roughly corresponding with that of the contained palpi. Near 

 the base there is a well-marked circular linear marking giving a ring or 

 collarlike appearance to this part of the capitulum. Dorsally there is a 

 distinct triangular marking (corresponding to the epistome of the 

 nymphal stages), the pointed apex being directed forwards. Close to the 

 base, ventrally, there is a small central oval structure, and on either side of 

 it there is a very minute opening. Csokor was of opinion that these 

 are the apertures of glands, but I think it more probable that they are 

 the stigmata or openings of the respiratory system {q. v.). The palpi 

 consist of three distinct segments, of which the proximal one is much 

 the largest, the second quite short, and the third very minute ; this last 

 bears four or five rod-like setae or spines. The proximal segment is 

 convex on the outer side, and when closely apposed to that of the other 



