THE WINGS OF DIPTERA 77 



his description of Musca domestica. The steriiopleura are defined, as 

 those portions of the mesosternum which, from their position, form a part 

 of the pleura, or side walls of the thorax, in contradistinction to the 

 middle portion of the mesosternum. The hypopleiira are distinct pieces 

 above the two last pairs of coxae, and behind the sternopleura, from 

 which they are separated by sutures. The region is shown in Stomoxys as 

 the metasternum. The parts in this region are much compressed together, 

 and the separate sclerites are difficult to define. The metapleura lie 

 immediately above the hypopleura, and behind the pteropleura ; this 

 region is more or less convex, and lies between the root of the wing 

 and the halter. Other terms in use are as follows. Scutellar bridges 

 small ligaments on either side of the scutellum which connect it with 

 the mesonotum. Praesutural depression a depression lying in the angle 

 between the transverse suture on the thorax (between the scutum and 

 praescutum) , and the dorsopleural suture. Praealar callus a more or less 

 distinct tubercle in front of the root of the wing, and Postalar callus, a 

 similar tubercle behind the wing, between it and the scutellum. The 

 hit-merits, or humeral callus, is usually a well-marked convexity at each 

 side of the anterior end of the thorax, and corresponds to the pronotum. 



THE WINGS (PLATE XVII) 



The possession of only one pair of wings, the mesothoracic, is char- 

 acteristic of the Diptera, and, as is to be expected in insects of such 

 powerful flight, they are highly developed; 



A word as to the origin of the wings. They are not appendages 

 in the zoological sense, but arise as outgrowths from the body wall, 

 formed in the pupal stage. The small buds which first appear become 

 gradually flattened out, and as they grow their substance is penetrated 

 by tracheae or air tubes, which, when the wing has assumed its final 

 leaf-like form, remain as veins or nervures to support the delicate 

 membraneous portion. 



The wings do not vary much in shape in the different families. The 

 anterior border is straight or nearly so, the apex and posterior border 

 gently rounded. The posterior border is generally indented in such a 

 way as to divide the wing in this region into parts which have re- 

 ceived distinctive names. The indentations are found in the proximal 

 half, and are known, from without inwards, as the anal lobe, (so called 

 because when the wings are folded this portion of the wing lies over 

 the distal end of the abdomen*) the alula, the antisquama, and the 

 * This is not always actually the case. . 



