76 MEDICAL ENTOMOLOGY 



necessity in descriptive work, in order to be able to denote with some 

 degree of accuracy the positions of bristles, markings, etc., by means 

 of which many species are identified. Terms which have a strict an- 

 atomical significance can only be applied where the homology of the 

 part has been determined with reasonable certainty, and cannot, however 

 desirable it would be from the purely scientific point of view, be applied 

 in the present state of our knowledge. To meet this difficulty a nomen- 

 clature has been adopted, and is now in general use among systema- 

 tists, which is essentially conventional, and in the use of which no 

 anatomical definition is implied. As Osten Sacken pointed out, such 

 a conventional terminology has the great advantage of offering more 

 chances of fixity, and can well exist side by side with a more purely 

 anatomical one, which must necessarily be modified as our knowledge 

 of the structure of the Diptera increases. 



It is chiefly among the higher Diptera that difficulties are met with, 

 as in the description of these it is often essential to define with pre- 

 cision the positions of the large bristles or macrochaetae. Chaetotaxy 

 will be dealt with later, but before doing so it will be necessary to 

 describe the various regions in the current terms. The definitions 

 given by Osten Sacken, who was the first to emphasise the -im- 

 portance of the subject in descriptive work, will be followed almost 

 verbatim, as his paper, published in 1884, is not readily accessible, 

 and contains an authoritative account. At the same time the terms 

 employed will, as far as possible, be brought into line with those which 

 have been used in the foregoing pages, using Stomoxys, of which a 

 figure is given on Plate XVI, as a type. (See also Plate XIX, fig. 6.) 



The sutures which separate the various plates from one another are 

 used as landmarks. Thus, the dorsopleural suture runs from the 



humerus to the root of the wing, and separates the 

 Nomenclature , N / ,, , ' ,, 



of Thorax mesonotum (or tergum) from the pleura. The ster- 



nopleural suture is horizontal, and lies below the 

 dorsopleural and parallel to it. The mesopleural suture runs downwards 

 from the wing, and separates the mesopleura from the pteropleura. 

 The mesopleura lie between the dorsopleural and sternopleural sutures, 

 and therefore correspond to the episternum of the mesothorax, as indi- 

 cated in the figure. The pteropleura _ are situated under the insertions 

 of the wings and behind the mesopleural sutures. In the figure the region 

 will be seen to lie between the membraneous area at the wing base and 

 the posterior spiracle. It corresponds to the lateral plate of the meso- 

 thorax of Lowne, the same term being employed by Gordon Hewitt in 



