EXTERNAL STRUCTURE : VENTRAL PLATES 575 



and posteriorly by a raised margin of the ventral integument. Ex- 

 tending from each side of the genital orifice there are two well 

 marked grooves, the genital grooves (fig. 3), which pass backwards and 

 outwards to the posterior margin of the body ; these furrows are 

 of some importance in the identification of species. The anal orifice 

 (fig. 3) is situated in the median line posterior to the last pair of 

 legs ; it consists of a longitudinal slit which is guarded by two lateral 

 chitinous valves. The groove which surrounds the 



anus is of great importance in the grouping of the Anal onflce and 



. . . . groove 



several genera, as it varies in its position and shape. 



In the genus Ixodes (Plate LXXVI, figs. 1, 5 and 6) it surrounds the anal 

 aperture anteriorly, its two arms extending backwards in a divergent, 

 convergent or parallel direction, towards the posteior margin. In several 

 species of the Ixodes, however, the anal groove instead of being rounded 

 anteriorly is pointed like a gothic arch ; it is then spoken of as being 

 ogival (Nuttall and Warburton). This type of anal groove is seen for 

 instance in the species Ixodes minor, Neum., Ixodes sculptus, Neum., 

 and others. In the genera Haeinaphysalis, Dermacentor, Hyalomma, 

 Amblyomma and Aponomma (see figs.) the anal groove, instead of extend- 

 ing around the anal opening anteriorly, curves round it posteriorly, the 

 two limbs then passing forwards towards the genital grooves, with which 

 they often unite. In the genus Margaropus (see figs.) the anal groove is 

 obsolete, and it is here that the classification of the genera based on 

 the position of the anal groove breaks down. On the ventral surface of 

 many ticks a distinct postero-median groove is frequently present, and 

 may be united with the anal groove ; this median groove extends to the 

 posterior end of the body. 



In the males of certain species of ticks there are flat pieces of chitin, 

 the ventral plates, embedded in the skin and either slightly or con- 



siderablv raised above the surface ; they are of great 



. ., . . , . T ,, Ventral plates, 



importance in the identification 01 species. In the , aaues shields and 



genus Ixodes (Plate LXXVI, fig. 1) there is a Pre- caudal appendage 

 genital plate situated in front of the genital orifice, a 

 median plate lying between the genital and anal openings, and an anal 

 plate, in the median line, on which the anus is situated. An adanal 

 plate lies on each side of the anal plate, and the two may unite behind 

 it near the posterior border. The epimeral plates lie outside the genital 

 grooves, extending backwards towards the fourth legs. Any or all these 

 plates may be absent, as in the genera Dermacentor and Haeinaphysalis. 

 Plaque is the name given to a small piece of chitin situated on the 



