ANATOMY. 47 



the secretion of the chitinous layer by this network 

 would be scarcely comprehensible if we did not know 

 that before the ecdysis the cells of the matrix become 

 much more numerous and form a regular close layer. 

 It will be seen from PL C, fig. 4, that this condition of 

 forming a close layer of cells exists in some of the 

 Tyroglyphidae at other times besides those immediately 

 before ecdysis; but Nalepa's network condition also 

 often occurs, and in old specimens, when the abdomen 

 is distended by the great development of the genital 

 organs and products, the endostracum seems to be 

 almost entirely lost and is difficult to distinguish at 

 all. When the endostracum is in the network condition 

 there is not any well-defined demarcation between it 

 and the subcuticular connective tissue which is found 

 in most Acari, and is often abundantly supplied with 

 fat-cells. 



The dermal appendages of the Tyroglyphidse are 

 numerous ; they consist of hairs, scales, spines, etc. 

 The epimera and sternum must also be considered 

 dermal structures, but it is more convenient to treat of 

 these in connection with the legs. The expulsory 

 vesicles (or oil-glands) appear also to be of dermal 

 origin, but it is more convenient to deal with them 

 separately when speaking of the internal anatomy. 



The hairs may be classified into three groups, viz. 

 1st, simple, flexible, setiform hairs ; 2nd, pectinated, 

 bipectinated, or plumose hairs ; 3rd, stiff rods which 

 are not pointed. Of these the very minute hairs are 

 apparently solid, the larger ones and most of the 

 spines are hollow. 



The simple setiform hairs vary from closely-set hairs 

 of such extreme minuteness that it is difficult to see 

 them, as in the case of those which are found so 

 abundantly on the tarsi of Glycyphagus spinipes (PL VII, 

 figs. 3 7), to hairs as long as the whole body, of 

 which the notogastral hairs of Tyroglyplms lonc/ior and 

 the four great hairs on the hind margin of Carpoglyplius 

 anonymus are good examples ; they may even be twice 



