204 DEVELOPMENT OF THE WING-VENATION OF ODONATA, 



Rs never succeeded in getting under more than the first branch 

 (Mj) of the median trachea. Forms like Diphlebia, which show 

 a peculiar oblique vein under Mj far distal from the subnodus, 

 need also a thorough investigation; since in such cases the vein, 

 at present taken to be M,, may eventually prove to be none other 

 than Rs itself. There, for the present, we must be content to 

 stop, resting in the certaint}^ that the many forms of Calojjterygid 

 larval wings will one day yield a wonderful harvest to tlie fortu- 

 nate student who has a chance of investigating them. 



Part ii. 

 The study of the source of the Oxygen-supply of the Wing-trachese. 



Section A.— Description of the tracheal system of the thorax. 

 The tracheal system of the thorax can only be studied in Anisop- 

 tera by means of very careful dissections, or by serial sections. 

 Dissection is difficult, owing to the great thickness of the pleural 

 ridges and their underlying muscles, in which it is very difficult 

 to follow the course of the alar trunk. Eight larvae of ^Eschnida^ 

 were dissected, but the complete result was obtained only by 

 piecing together the separate points made out in diffiirent larv?e. 



The most satisfactory method of study is to select a Zyyopterid 

 larva immediately after its final larval ecdysis, and study it 

 directly, under a low power of the microscope. If a green or 3^ellow 

 transparent larva be chosen (Lestes, Ischnura, or Caliagrion) and 

 submitted to strong transmitted light, the whole tracheal system 

 stands up clearly in black, against a background of transparent 

 pale greenish or yellowish. 



The following description embodies the general results obtained 

 for both Anisoptera and Zygoptera. 



In all the specimens examined, the great dorsal trunks pass 

 into the thorax from the abdomen with an upward curving, and 

 also distinctly converging towards one another. Thus they come 

 to lie very close under the pleural ridges, and pass forwards and 

 upwards to a point very nearly vertically above the second coxae, 

 where they attain their maximum convergence, and are connected 

 by the short stout thoracic anastomosis (T A) (a very short con- 

 necting trachea from which two small diverging branches pass to 



