20 •. STUDIES ON ACART. 



7>. /., var. Iiominis, Megnin, Parasites, 1880, p. 2G7. 



I), f., G. Canestrini, Prospett. Acarof. Ital. 1899, part 8, p. 965. 



D.f., Berlese, Gli Insetti, 1912, vol. ii. p. 36. 



$ . Adult females of D. folliculorum (from human beings) are from 

 about 4-2 to over 7^ times as long as the width of the cephalothorax. 

 Intermediate forms between tliese two extremes are of course met with, 

 some being a little more than five times, others about 6^-6| times as 

 long as tlie cephalothoracic width. Abdomen in short forms sometimes 

 considerably less than twice the combined length of cephalothorax and 

 capitulum; in other specimens the abdomen is about twice the length, 

 and in very elongated examples 2^-3 times the length of cephalothorax 

 + capitulum. Erasmus Wilson figures some still more elongated forms; 

 Pee his figs. 1-3, 6, and especially fig. 33. This species is indeed the 

 longest of the genus, females sometimes measuring up to 390 fx., and 

 there is one specimen of this sex in the British Museum collection 

 which measures 440 /x, but it has been slightly pressed by accident and 

 is a little distorted. 



6 . Adult male specimens of D. folUculonim vary very greatly in 

 size, some specimens being very elongated whilst others are quite short. 

 The largest example examined by the author is a little less than six 

 times as long as the width of its cephalothorax, the abdomen being 

 about two and a half times as long as the cephalothorax. In very short 

 specimens the body is sometimes from only a little more than three 

 times up to more than four times the cephalothoracic width, the 

 abdomen being considerably less than twice the length of the 

 ce])halothorax. The smallest specimens of this sex have a very short 

 tail, and thus resemble i>. 2^%?/o?Wes somewhat in general appearance. 

 'Jliis short caudate form is well figured by Erasmus Wilson (see his 

 figs. 4 & 5, pi. XV. and 40-42, pi. xvii. Phil. Trans. Roy. Soc. 1844). 

 One or two specimens of this caudate form were met witli wliich 

 showed no trace of sexual structures and are, perhaps, subadult males; 

 others have the penis and sexual aperture well developed. It is 

 probable that this form is a stage through which all males of this 

 species have to pass before becoming fully developed and elongated. 

 Penis not very long; it forks to form two branches in much the same 

 way as that of I), lovis, one of the branches being apparently wider, 

 flatter, and blunter than the other. The male sexual orifice seems to 

 be much simpler than in D. hovis, being merely surrounded by circular 

 Avrinkles of the skin ; it is situated above and just behind the transverse 

 groove between the epimera of first and second legs. 



