ANATOMY. 49 



in the median line of the dorsal surface of the tarsus, 

 near the proximal end, in most species of the genus 

 Tyroglyphwt, and in some other species. 



The scales, which are of course only strangely 

 developed hairs, are practically of two sorts ; one of 

 these may be called the leaf-like scale, in which a 

 number of branches start from the main rachis, and 

 these are often, but not always, joined by a bordering 

 rib, or nervure, at the periphery of the leaf ; the rachis 

 and nervures often bear short spines extended between 

 the rachis and nervures, and attached to both is a thin, 

 colourless, transparent membrane ; the whole structure 

 is leaf-like in form. The best examples of this kind of 

 scale are those of Qlycyphagus palmifer (PL XIII, 

 figs. 3, 4, 5, 7) ; but good instances are also found on 

 the foreign species G. pterophorus and Histiostoma 

 phyllothricum ; in the last-named species the scales 

 have not any nervures, but only rachis and membrane. 

 The other sort of scale is a thick opaque membrane, 

 without rachis or nervures but closely set on one 

 surface with short fine hairs ; an excellent specimen of 

 this is the scale on the outer side of the femur of the 

 third leg of the female CHycypJiagus spinipes (PL VII, 

 figs. 5, 6). 



The spines are of course chiefly of the ordinary 

 thorn-like character, but attention may be drawn to 

 the great, hollow, ensiform spines of the male of 

 Histiostoma pulclirum (PL III, figs. 15, 16), the im- 

 mensely long spines of Histiostoma, spiniferum (PL IV, 

 figs. 1 3), and the remarkable spines of Crlycypliagus 

 platygaster (PL XIV) and O. dispar (PL XV). 



The Divisions of the Body, In most text-books of 

 comparative anatomy the Acarina are defined as being 

 Arachnida which have the cephalothorax and abdomen 

 fused into one common mass without any demarcation 

 between them ; if the word abdomen be used in the 

 sense in which acarologists almost universally employ 

 it this definition is entirely misleading ; it is doubtless 

 founded upon such creatures as the members of the 

 4 



