THE TRA CREATION OF WINGS 45 



between the two dorsal lon<2;ittidinal trachea) {d. It) as described by Scott 

 (/. c.) and the one to the hind wing arises from the dorsal longitudinal 

 trachea. It will be noticed that in the side view of Lestes (Fig. 35) 

 the connection of the accessory cubito-anal trachea (a. cu-a) of the 

 mesothorax with the transverse connective cannot be seen for it is hidden 

 by the dorsal longitudinal trachea {d. It). The vestige of the true cubito- 

 anal trachea (ai-a) has doubtless been overlooked by both of the former 

 authors. 



THE INFLUENCE OF THORACIC STRUCTURE ON THE CONNECTIONS OF THE 



WING TRACHEAE 



The main tracheae of insects lie in the spaces of the body allowing for the 

 free passage of air thru their lumina. Their flexibility is a necessity for 

 tracheae entering the appendages of the body must bend readily when these 

 structures are moved and it is all important that the flexible trachea should 

 avoid contact with any structure which might at any time bring pressure to 

 bear upon their walls and interfere with the free passage of air. The course 

 of the tracheae to the wings might be expected to be direct but at the same 

 time the avoidance of rigid structures, such as those concerned with the 

 movement and articulation of the wings, among which the tracheae must 

 pass, would seem to be an important factor in determining their course. 



The material presented in this paper affords many examples of altered 

 courses of tracheae which seem to be due to peculiarities of thoracic struc- 

 ture. In the wing of the Mayfly, Epeorus, (Fig. 30) a simple example may 

 be found. There is a saucer-shaped disc, which has already been referred 

 to as lying between the medial, cubital, and transverse basal tracheae, and 

 which has been indicated in the drawing by a dotted line, around which the 

 tracheae pass. This disc with its two chitinous walls of such convexity 

 would undoubtedly be a great obstacle to the passage of air if the tracheae 

 crossed it rather than encircled it as they do, altho the latter course of the 

 tracheae is the longer. 



The case of the stone fly, Pteronarcys (Fig. 19), presents an example of 

 the modification of the medial trachea (w) which is very suggestive. The 

 medial trachea (w) will be noticed to arch caudad toward the cubito-anal 

 group of tracheae and altho there is no transverse basal trachea the medial 

 trachea might, from its position, be considered to be midway between the 

 costo-radial and cubito-anal groups of tracheae. The examination of a 

 n>Tnph which is nearly ready to emerge will show a strong process, evi- 

 dently the wing process, (indicated by a dotted line in the drawing) pro- 

 jecting between the radial (r) and medial (m) tracheae to articulate with an 

 over-lying articular sclerite, evidently the second axillary sclerite. Here 

 again it seems very clear that the course of a trachea is modified from the 

 most direct route by a mechanical obstruction. Moreover this case is very 



