Xxii ON THE TERMINOLOGY OF DIPTERA. 



tion. As threads loosely drawn up in a frame, when strongly 

 strained by transverse threads of different length, must necessarily 

 adopt an angular direction, so do longitudinal veins, in consequence 

 of a varied situation and the length of the transverse veins. The 

 outline of the wing, the length of the longitudinal veins, the situ- 

 ation and length of the transverse veins, as well as the area of the 

 two membranes of the wing, stand in such a relation to each other 

 that the wonderful effect of their hardening after the exclusion of 

 the insect will be a surface more or less even, but in every case fit 

 for the performance of flight, the main agents of which are appa- 

 rently the anterior part of the wing, as being more stout and rigid, 

 and its posterior part, which, being in most cases united with the 

 former by the small transverse vein only, is more movable, and 

 acts as an inclined plane, propelling the insect during the motions 

 of the wings both up and down. It is in such genera only as 

 Syritta, Bombylius, Nemestrina, the habits of which require not so 

 much a rapid locomotion, as a constant hovering over a spot, that 

 a multiplied connection of the anterior and posterior part of the 

 wing by transverse veins restrains the mobility of the posterior 

 part of the wing,and renders its propelling effect less sensible. 



A correct understanding of a very intricate neuration is in many 

 cases by far not so difficult as that of a very incomplete one. The 

 latter will be best obtained by observing, that in such wings the 

 three anterior trunks of the veins are not only incompletely de- 

 veloped, but also crowded together on the anterior part of the 

 wing, an uncommonly large space being allotted to the fourth 

 trunk. In this way, the striking narrowness of the anterior and 

 middle parts of the wing and the extraordinary dilatation of the 

 posterior part, find their explanation. Sometimes a closer exami- 

 nation of the surface of the wing will yield a useful result by our 

 observing the different kind of hair peculiar to the veins, and which 

 remains, though the veins themselves are wanting. All Diptera with 

 very incomplete neuration are bad fliers, since the greater flexibility 

 of the posterior part of their wings can but imperfectly compen- 

 sate the propelling effect of this part when sustained by a stronger 

 neuration. 



The legs of diptera, like those of the other orders, consist of 

 four principal parts, called the hips (coxae), thighs (femora), 

 shanks (tibiae), and feet (tarsi). The hips consist of two joints; 

 the second, smaller one, is called trochanter. The feet are gene- 



