130 BRITISH TYROGLYPHID^. 



embryonic development of different families of Acarina, 

 principal amongst which may be mentioned that of 

 Henkin on Trombidium,* that of Winkler upon 

 Gamasus^ and that of Wagner upon Ixodes ;J but 

 the leading work on the subject still continues to be 

 that of Claparede. This is practically the only existing 

 work which treats of the embryology of the Tyro- 

 glyphidae ; unfortunately it is more shortly dealt with 

 than that of any other family which Claparede under- 

 took. The species he studied were Tyroglyphus siro, the 

 so-called common cheese-mite, and T. longior, the far 

 commoner cheese-mite found associated with it and on 

 cheese in general, and on numerous substances. Finding 

 that the embryology was similar in the two species, 

 Claparede only describes that of T. siro; he states 

 that the egg is ellipsoidal, and covered by the yolk- 

 membrane (Dotterhaut). He says that the first 

 formation of the germ-layer (Keimhaut) escaped him, 

 and that he first detected it when it was several cells 

 thick and enveloped the whole yolk. Claparede states 

 that this germ-layer thickens rapidly on the ventral 

 side and at the cephalic end; so that the yolk in the 

 posterior half of the egg is pressed backward. The 

 appendages appear as rounded knobs on the ventral 

 side of the anterior half of the embryo ; there are 

 five pairs of them ; the anterior pair are to be looked 

 on as mandibles, the second as maxillae, and the three 

 remaining pairs as legs. As the embryo develops 

 further the maxillae gradually work forward, while 

 the mandibles remain quiescent ; thus the maxillae 

 soon come to be at the sides of the mandibles. At this 

 time the anterior part of the embryo becomes divided 

 by three transverse furrows ; each segment thus formed 



* See note, p. 127. 



f "Anatomie der Gamasiden," in ' Arb. Inst. Wien,' t. vii (1886), 

 Heft 3, pp. 317354. 



J " Zur Entwicklungsgeschichte der Milben.," in ' Zool. Anz.,' No. 

 399 (August 29fch, 1892), pp. 316320. 



" Studien an Acariden," in ' Z. wiss. Zool.,' Bd. xviii (1868), pp. 

 445546. 



