206 DEVELOPMENT OF THE WING-VENATION OF ODONATA, 



very close to its origin, the anterior termination of the visceral 

 trunk from the opposite side of the body. That is to say, the 

 two visceral trunks cross one another under the oesophagus, the 

 right one passing over the left, and each connects with the second 

 pedal trunk of the opposite side. These visceral tracheae lie well 

 below the dorsal tracheae, and play no part in the development 

 of the wings. 



At a point directly under the posterior part of the pleural 

 ridge, not far from the point where the abdomen joins the thorax, 

 each dorsal trunk gives off a short stout trachea, which enters 

 the base of the corresponding hindwing-case at its anal end. 

 This is the alar trunk of the hind wing. From it arise, in order, 

 the anal, cubital, median, radial, subcostal, and costal tracheae of 

 the wing-rudiment. This alar trunk is of greatest diameter at 

 its point of origin from the dorsal trunk. As it proceeds costad, 

 its diameter decreases. After giving off the six tracheae of the 

 wing, it becomes a narrow, thread-like trachea. Its course 

 through the wing-base is roughly semicircular. On leaving the 

 wing, it runs downward and slightly forward, and finally enters 

 the third pedal trachea (pa), at a point very close to the posterior 

 stigma (Sto). This third pedal trachea is itself an offshoot ven- 

 trally from the great dorsal trunk, and arises, in the ^schnidce^ 

 not far from the alar trunk of the hindwing, which may be 

 termed the second alar trunk. In the Agrionidce, owing to the 

 greater obliquity of the thorax, it comes to lie very close under 

 the origin of the^rs^ alar trunk. 



The Jirst alar trunk arises similarly to the second, from the 

 main dorsal trunk. In the ^Eschnidce, and probably in all 

 Anisoptera, this alar trunk lies very close to, and a little in front 

 of, the second alar trunk. In the Agrionidce, it is separated 

 from the latter by a greater space, and arises, as just mentioned, 

 very nearly directly over the third pedal trachea (pg). In the 

 Anisoptera, it is of somewhat smaller diameter anally than the 

 second alar trunk; in the Zygoptera, almost of the same size as 

 the latter. It passes into the forewing-base at its anal end, 

 gives off the six main wing-tracheae in the order already named 

 for the second alar trunk, makes a semicircular loop, gradually 



