446 DIPTERA 



CHAP. 



second and third great divisions of the body-cavity. The pro- 

 thorax is always small, except in a few of the abnormal wingless 

 forms (Melophagus) ; in Nycteribia (Fig. 248) the mesothorax 

 forms the anterior part of the body ; the head and such parts of 

 the prothorax as may be subsequently discovered to exist being 

 placed entirely on the dorsum of the body. The mesothorax in 

 all the winged Diptera forms by far the larger portion of the 

 thoracic mass, the prominent part of it, that projects backwards 

 to a greater or less extent over the base of the abdomen, being 

 the scutellum. The first or prothoracic stigma is remarkably large 

 and distinct, and is by some called mesothoracic. Another large 

 stigma is placed very near to the halter (or balancer) ; the meta- 

 thorax being very small. An imperfect stigma is said by Lowne 

 to exist in the blowfly near the base of the wing. The number 

 of abdominal segments externally visible is very diverse ; there may 

 be as many as nine (in the male Tipula), or as few as five, or even 

 four, when the basal segment is much concealed ; the diminu- 

 tion is due to certain segments at the extremity being indrawn 

 and serving as a sort of tubular ovipositor in the female, or curled 

 under the body and altered in form in the other sex, so as to 

 constitute what is called a " hypopygium." In the female of 

 Tipulidae the body is terminated by some horny pieces forming 

 an external ovipositor. In nearly all Diptera the feet are five- 

 jointed ; the claws are well developed, there being placed under 

 each of them a free pad or membrane, the " pulvillus " ; there may 

 be also a median structure between each pair of claws, of diverse 

 form, the " empodium." 



On the surface of the body of many flies there will be seen 

 an armature of pointed bristles ; these flies are called " chaeto- 

 phorous " ; where no regularly arranged system of such bristles 

 exists the fly is " eremochaetous." In some families the arrange- 

 ment of these bristles is of importance in classification, and a 

 system of description has been drawn up by Baron Osten 

 Sacken : this branch of descriptive entomology is known as 

 chaetotaxy. 1 



The wings are of great importance in classifying Diptera ; 



but unfortunately, like the other parts, they have not received an 



exhaustive anatomical study, and Dipterologists are not agreed 



as to the names that should be applied to their parts. We give 



1 Tr. cnt. Soc. London, 1884, p. 497. 



