90 MEDICAL ENTOMOLOGY 



In the Pupipara, which deposit their larvae on the ground, there is no 

 longer any necessity for a long tube, and the external genitalia are 

 reduced to a simple transverse or cresentic slit, separated from the 

 anus, which is posterior to it, by a stout piece of chitin. On account 

 of the large size of the larva at the time it is deposited the slit, known 

 as the vulva, is structurally adapted to enable it to dilate at the 

 moment of parturition, and is provided with powerful muscles for 

 the purpose. Dilatation is accomplished by retraction of the posterior 

 lip of the orifice, and by separation of the two lateral halves of the 

 anterior lip, which is composed of two plates of chitin meeting in the 

 middle line but not continuous with one another. (Plate XVIII, fig. 2.) 



The genitalia of the female flies do not provide characters used in 

 classification. 



CHAETOTAXY (PLATE XIX) 



The value of the number and arrangement of the macrochaetae on the 

 exo-skeleton as characters for systematic work was first recognized 

 by Osten Sacken, and since the publication of his papers chaetotaxy 

 has been a prominent feature in all descriptions of the higher Diptera. 

 The subject is rather a difficult one, as it is often a very nice point 

 to decide as to whether a particular hair shall be termed a macro- 

 chaeta or not, and on which of the more or less arbitrary regions of 

 the thorax it is situated. As has been stated elsewhere, the reader 

 will do well to seek the advice of an expert in the determination of 

 species in such a difficult group of flies as the Muscoidea, in which 

 the chaetotaxy is extensively used. The following account will suffice 

 to give some idea of the subject, and to enable the student in most 

 cases to separate the different genera. The distinction between many 

 species in this group depends on such fine points that an actual com- 

 parison of the specimen with the type or a co-type is frequently 

 necessary in order to identify it. 



The bristles are named according to the situations in which 

 they occur, the terms used being mainly those which have al- 

 ready been enumerated in connection with the structure of the exo- 

 skeleton. 



On the head : Commencing from the front, the vibrissae are a pair 

 of stout and prominent bristles situated one on each of the prominent 

 anterior lateral angles of the epistomal orifice, called variously the 

 ' oral margin ' or ' buccal orifice '. The epistomal bristles are situated 



