46 THE WINGS [CH. 



loop, which we call the alar trunk. From this loop, six main 

 tracheae pass into the developing wing-sheath. These are the 

 costal, subcostal, radial, median, cubital, and anal tracheae respec- 

 tively, counting from in front backwards. In the later stages of 

 growth, they not only supply oxygen to the cells of the growing 

 wing, but gradually assume positions and branchings which fore- 

 shadow the main venational plan of the imago. Thus we may say 

 that this plan is the combined result of three interacting forces : 

 (1) the necessity of leading up to the imaginal design, (2) the 

 necessity of supplying every portion with sufficient oxygen for 

 development, and (3) the necessity of relying almost wholly on 

 an oxygen supply coming from the anal end of the alar trunk 

 (owing to the larval gills being situated at the anal end of the 

 body). The influence of (3) is seen in (a) the almost total loss of 

 the costal trachea, (b) a great shortening of the subcostal, (c) a 

 gain by the median over the radial trachea, (d) strong development 

 of the cubital and also of the anal trachea (in non-reduced forms). 

 How far the imaginal design has reacted on the tracheal supply, 

 and vice versa, it is not easy to say. A study of fig. 17, c and D, 

 shews that this interaction has been very considerable. 



During the last larval instar, the imaginal venation begins to 

 shew up very distinctly. A definite cuticularization of the wing- 

 sheath occurs a short time before metamorphosis, so that the 

 details of venation are permanently recorded on the sheath, and 

 are left behind on it at metamorphosis. This fact is of great 

 value when we want to establish the identity of any un-named 

 exuviae. Beneath this pattern, the imaginal venation proper 

 begins to form, by the deposition of pigment-bands along the 

 courses of the future veins. These bands are not in general de- 

 posited on both upper and under surfaces of the wing-rudiment, 

 but on one or the other alternately, according to whether the 

 corresponding trachea runs closer to one surface or to the other. 

 The margin of the wing, however, including the costa (which is 

 formed quite independently of the rudimentary costal trachea) 

 is represented by pigment bands on both surfaces. Just before 

 metamorphosis the vein-rudiments proper are formed by a weak 

 deposition of chitin along the course of the pigmented bands, as 

 well as along all the future cross- veins. Thus it comes about that 



