BY R. .1. TILLYARD. 211 



breathing-or^aiis utterly unlike those of any other aquatic animal, 

 speaks very strongly in favour of a special development of these 

 organs in larvae which, having lost their original marine mode of 

 respiration by taking to the land, again took to the water and 

 developed organs peculiar to themselves. 



Secondly, the persistence of thoracic spiracles, in spite of their 

 uselessness during the greater period of grcjwth, is a strong argu- 

 ment for the existence of an open tracheal system in the larvie as 

 well as the imagines of the ancestors of our present-day Odonata. 



Thirdly, a careful study of ihe peculiar connections of the leg- 

 and wing-trachese in the larvae of Odonata, must go far to convince 

 us of the same truth. Let us examine these in detail. 



The anterior spiracle (Stj) in the larva is still open, and partly 

 functional. Now p^ arises from Stj, and a branch lb passes oif 

 from it to supply the labium. On the other hand, p., arises 

 directly from DT, and receives only a small branch from Stj.* 

 This can be understood, if we suppose that this small branch was 

 the original p.,, and that, later on, a new attachment was de 

 veloped on to DT, to intercept the oxygen coming from the anal 

 end of the body. 



Again, pg arises directly from DT, but it gives off one or more 

 small trachese to the region of the closed posterior stignia(St2). 

 The same explanation would, therefore, hold here, viz., that these 

 small branches represent the remains of the original trunk of p^ 

 (now almost aborted by the complete closing of the stigma), while 

 a new attachment was formed on the dorsal trunk (DT) when 

 the larva took to breathing from its anal end. 



Let us now see whether the other trachese of the thorax sup- 

 port this exj)lanation. 



J^'irstly, the visceral longitudinal trachese (VT) cross one another 

 in a peculiar manner, and terminate each on po of tiie opposite 

 side, not far below the stigma. This is an extraordinary arrange- 



* This is the arrangement in tlie Anisopttra. In the Zygoptera, p^ 

 arises from DT in front of St^, while po connects up to Sti(Fig.20>. Either 

 of these arrangements is clearly of a secondary nature. 



