212 DEVELOPMENT OP THE WlNG-VENATlON OF ODONATA, 



ment. It can scarcely be doubted that these visceral trunks 

 were originally developed from Stj for direct air-breathing, and 

 that the present crossing and liitching on to pg is a secondary 

 arrangement develoj^ed when the larva took to an aquatic mode 

 of life. 



Secondly, the alar trunks, by their pecuHar arrangement, 

 suj^port the theory of an original air-breathing larva for Odonata. 

 Each ends by a slender attachment to its corresponding pedal 

 trachea (p2 or pg) not very far from the stigma. These slender 

 endings are at present of no value to the developing wing, since 

 it is quite clear, from the gradual decrease in diameter, that the 

 oxygen-supply now comes altogether from the anal end. What 

 then can they be, but the reduced remnants of the original alar 

 trunks, which arose either directly from the spiracles, or, more 

 probably, from the pedal tracheae close up to the spiracles 1 In 

 that case, there must have been oriyinally, in the air-breathing 

 larva, a supply of oxygen to the costal side of the wing-base, 

 such as is found in the larvae of all present-day insects except 

 the Odonata. Whether this suppl}' arose directly from the 

 stigma or from the pedal trachea, or even originally from the 

 main visceral or ventral trunk, it is scarcely necessary to enquire. 

 The point is that the, Protodonate wing was developed along normal 

 lines, by means of an oxygen-supply to the costal side of the 

 wing (probably also, as in Plecoptera, with a smaller supply to 

 the anal side), and that this normal method of supply was de- 

 stroyed when the larva took to fresh water. From that point 

 onwards, we must exjDect to see the gradual development of those 

 peculiar characteristics which make the Odonate wing unique. 



One mure point of interest in this question can be bi'ought 

 forward. It is well known that the whole dorsal tracheal system 

 in Odonata is of a peculiar reddish-purjole or bright coppery 

 colour, while the visceral system is silvery- white.* Now, just 

 where the main dorsal trunk passes close to the anterior stigma 



* The remarks in this paragrapli apply, as regards colouration, specially 

 to the Anisoptera. In the Zygoptera, the arrangement of the trachea is 

 similar, but the differences of colouration are not so marked. 



