208 DEVELOPMENT OF THE WING-VENATION OF ODONATA, 



The peculiarities shown hy the winy-ve7iatio7i of Odonatd, as 

 contrasted with that of other insects, are due primarily to the 

 aquatic habits of the larvae; ivherehy, through the development of 

 rectal or caudal breathing, the oxygen-supply of the developing 

 wing is carried from the posterior end of the body, and enters the 

 wing-base at its anal end. 



The effects of the peculiar formation of the alar trunk may be 

 stated as follows. (l)The costal trachea (0) is in the most 

 unfavourable position for receiving oxygen. As a matter of fact, 

 it receives scarcely any, dwindles rapidly in size, and becomes a 

 mere rudiment in the fully developed nymphal wing. 



(2) Next to C, the subcostal trachea (Sc) lies in the next most 

 unfavourable position. It receives a certain amount of oxygen, 

 but its development is weakened and curtailed, so that it only 

 develops about half-way along the wing-rudiment. This fact, 

 originally a source of weakness to the wing, has been seized upon 

 by natural selection, and has been made a source of strength to 

 the wing by the development of the unique formation known as 

 the nodus at the end of Sc. 



(3) The radius (R) was originally the largest and longest trachea 

 of the wing (as may be seen by comparing the Odonate wing with 

 that of an Ascalaphid, an insect undoubtedly derived from the 

 same stock as Odonata, before the larvae of the latter adopted an 

 aquatic mode of life). Originally R arose at the extreme bend 

 of 1 he alar trunk at some distance away from M, and thus held 

 an excellent position for receiving the flow of gas from either 

 costal or anal ends of AT. At first, it tends to develop ahead of 

 the media (M), and at the ontogenetic period when both R and 

 M appear as fair-sized bifurcated tracheae, R is still greater than 

 M. 



(4) At this stage, M begins to gain on R, owing to its ability 

 to intercept oxygen coming from the anal end of AT. Arising 

 a little before the extreme bend of AT, M is in an exceptionally 

 favourable position for competing with R for the flow of gas. As 

 the wing develops, M begins to gain upon R, and finally moves 

 up close to it, thus sharing with R the ideal position at the bend 

 of AT, and receiving more oxygen than any other trachea in the 



