6 INTRODUCTION. 



pleural divisions do not afford so many taxonomic characters in 

 the NEMATOCERA as in some other groups, being always devoid of 

 bristles, and it is these latter which are strongly indicative of 

 affinities. 



Fig. 2. Pleural divisions of the thorax of a fly. 



a. Prothorax or propleura. e. Pteropleura. 



b. Mesopleura. /. Hypopleura. 



c. Metapleura. g. Plumula. 



d. Sternopleura. ps. Pleural or thoracic stigma. 



The three prinicipal sutures recognised generally are, (1) the 

 dorsopleural suture (or notopleural) running from the shoulder to 

 the base of the wings and dividing the dorsum (or rnesonotum) 

 from the pleurae as a whole ; (2) the sternopleural suture, 

 roughly speaking, parallel with the dorsopleural, placed about the 

 middle of the body or a little below it, and dividing the mesopleura 

 from the sternopleura ; and (3) the mesopleural suture, a more or 

 less vertical irregular line from the base of the wings downwards, 

 dividing the mesopleura from the pteropleura. 



The pleurae themselves are distributed as follows : The pro- 

 pleura, or prothorax (generally termed the latter in the present 

 work), is on the immediate anterior part of the thorax. The 

 mesopleura lies behind it, in front of the wings, bounded above by 

 the dorsopleural suture and below by the sternopleural suture. 

 The metapleura lies immediately behind the wings, above and 

 rather behind tins pteropleura, which lies directly below the wings, 

 extending downwards to between the two hinder pairs of legs. 

 The Tiypopleura is a small piece below the metapleura and imme- 

 diately over the hind coxae. The sternopleura is usually the largest 

 of all and lies below the sternopleural suture and extends down- 

 wards, occupying all the space between the front and middle legs. 

 The mesopleural suture therefore has the mesopleura and sterno- 

 pleura in front of it and the pteropleura behind it. A small piece 

 behind the metapleura is known as the plumula; and a small 

 orifice on the prothorax just below the dorsum is the thoracic 

 stigma. 



Since the NEMATOCERA are wholly eremochaetous, that is to say, 



