INTERNAL ANATOMY. 105 



space within the ring becomes divided into three more 

 or less equal triangles, with curved exterior sides; 

 two of these are lateral, and are the ordinary labia. 

 The third is posterior, and is a third labium closing 

 the posterior part of the vulva, which would other- 

 wise be widely open. Glycyphagus dispar is a good 

 example of this arrangement. Ghortoglyphus nudus 

 has the third posterior labium, although the vulva is 

 not surrounded by an annular sclerite ; the third 

 labium is really only a development of what is one of 

 the commonest of the additional sclerites round the 

 vulva, besides the lateral labia, viz. a posterior central 

 supporting piece. 



In the genus Histiostoma the vulva is usually a 

 mere longitudinal slit without any sclerites whatever. 



Nalepa gives the following account of the vulva of 

 Tyroglyphus longior. The outer form of the genital 

 field is an elevated isosceles triangle, the sides marked 

 by the lateral, the base by the posterior supporting 

 pieces (sclerites). The lateral are distorted D-shaped 

 plates touching at their anterior, wide apart at their 

 posterior ends ; and presenting their edges to the ex- 

 terior. They bend inward and are more strongly chiti- 

 nized at their posterior ends ; they close the anterior 

 part of the genital opening. The posterior supporting 

 sclerite is an almost quadrangular plate, whose right 

 and left edges are joined to the lateral sclerites by a 

 hinge-like joint ; when the vagina is protruded all three 

 plates become perpendicular. In Ccurpotjlyphus <uu>- 

 nymus Nalepa says that the lateral plates would not 

 close the genital opening, and that they are less chiti- 

 nized than in T. longior, but the posterior plate is 

 largely developed and closes the vulva; it is boat- 

 shaped, convex externally, and is split into two by a 

 median flexible seam. There is an apophysis on each 

 side near the lower corner to which the occlusor muscles 

 are attached ; the hinges and flexible plates allow the 

 soft egg to pass without injury. 



Haller supposed that the external vulva in Tyro- 



