104 BRITISH TYROGLYPHID^E. 



against each other in the central line, particularly at 

 their anterior ends ; they thus cause the external 

 opening to be almost a longitudinal slit. These labia 

 are almost perpendicular to the ventral surface ; that 

 is to say, the breadth of each labium is perpendicular, 

 while its length lies along the ventral surface. In each 

 labium is sunk a chitinous plate or blade, which some- 

 times fills the entire labium ; the two plates lie against 

 each other at their anterior ends, and continue to do so 

 for a distance which varies with the species ; toward 

 the posterior end, however, the edges of the plates 

 usually curve away from one another, forming the 

 arms of the Y ; between these arms there is ordinarily 

 a slight depression, which is in fact a portion of the 

 opening of the vulva. A section through the vulva 

 snowing the labia (le) pressed nearly against each other 

 is shown at PL B, fig. 10. The plates in the labia are 

 often connected with, or surrounded by, other sclerites 

 of which no general description can properly be 

 attempted because they appear to vary in every 

 species ; although they probably do not vary so widely 

 in principle as those surrounding the external genital 

 organ of the male. 



The vulva is often also protected by an external 

 chitinous bar, or sclerite, usually more or less horse- 

 shoe-shaped, with the convexity forward ; it is gene- 

 rally placed a little anterior to the vulva itself ; it is 

 much more general and better developed in the 

 Sarcoptidae among the bird's-feather parasites than in 

 the Tyroglyphidse ; in the former family Robin called 

 it the " sternite : " in the latter it may be seen more 



t/ 



or less developed in Hericia Robim, Qarpoglyphus 

 anonymus, and in some species of the genus Glycy- 

 phagus. It may become a continuous ring entirely 

 surrounding the vulva (Glycyphagm Crameri, PL XVII, 

 fig. 6), and the epimera of some of the legs may be 

 joined to it (Glycyphagus dispar, PL XV, fig. 2). 

 Where it forms a ring the stalk of the Y is apt to 

 be much shortened or entirely abolished, and then the 



