Ill] THE WINGS 45 



Development of the Wing. 



At the third or fourth larval ecdysis, the edges of the pleura 

 of the synthorax, bordering the terga, are seen to be slightly raised 

 up, forming two nearly parallel ridges, the pleural ridges. At the 

 next ecdysis, four minute swellings appear on these ridges, some- 

 what on the inner side. One pair arises from the ridges of the 

 mesopleura, the other from those of the metapleura. After the 

 following ecdysis, the forward extension of the mesopleura causes 

 the pleural ridges to become more definitely inclined towards one 

 another anteriorly. The swellings are now like tiny buds, and 

 project upwards and a little inwards. From this point on, at each 

 successive instar, the pleural ridges become more and more oblique, 

 while the wing-buds cease to turn upwards, and come to project 

 slantingly inwards and backwards. They soon take the form of 

 definite triangular flaps, fore and hind-buds lying quite close to 

 one another on either side. Also, the first pair have come to lie 

 quite close to one another along the middle line. The second pair 

 remain far apart at their bases, owing to the divergence of the 

 pleural ridges, but tend to touch at their tips, which project 

 inwards. These buds or flaps may from now on be spoken of as 

 the iving-sheaths ("wing-cases," "wing-pads," or "wing- folds"). 

 As growth proceeds, the hind wing-sheaths gradually come to 

 overlap the fore, until only a narrow strip along the costal border 

 of each of the latter is visible. Concurrently, the direction of the 

 sheaths becomes definitely backwards, so that they lie straight 

 along over the abdomen, reaching to the end of the fourth segment 

 or beyond, when fully developed, and having their costal 

 borders all parallel to the long axis of the body. The hind wing- 

 sheaths become more or less deeply pigmented. On the fore wing- 

 sheaths, owing to their being covered by the hind, pigmentation 

 is very weak or absent, except along the free costal border. 



The alar trunk, or main trachea supplying the wing, arises at 

 a very early stage as two small tracheae from the dorsal trunk. 

 The more anterior trachea passes to the costal end of the base of 

 the wing-bud, the more posterior to its anal end. The latter trachea 

 is the larger. These two tracheae soon become fused into a single 



