I] INTRODUCTION 5 



POSITION OF THE ORDER IN THE CLASS INSECTA. 



The Dragonflies occupy such an isolated position within the 

 Class, that it is a difficult task to indicate any other groups as 

 having any real relationship with them. Morphologically, they 

 have been considered to shew relationships with (a) the Stone-flies 

 or Perlaria, (6) the May-flies or Plectoptera. 



The points of resemblance with the Perlaria are : (i) the pres- 

 ence of three ocelli, (ii) the large number of Malpighian tubules, 

 (iii) the aquatic habit of the larvae, and the fact that both breathe 

 by means of tracheal gills, (iv) the retention of the cerci alone of 

 all the abdominal appendages in the larva (except in a few genera). 

 It seems very probable that not a single one of these characters 

 has been inherited from a common ancestor. As regards the 

 ocelli, fusion of two out of an original four to form a single large 

 median ocellus takes place in a number of widely separated groups 

 of Insects. The Malpighian tubules of the Dragonfly are arranged 

 in groups of five or six together. The young larva has only three 

 tubules. Thus the large number of these organs is again a 

 secondarily developed character. The larvae of the Perlaria have 

 no stigmata, and their tracheal system appears to be a primarily 

 closed one. The larvae of Odonata, on the other hand, possess 

 stigmata, and their tracheal system is only secondarily closed. 

 This points to the supposition that the Perlaria were aquatic long 

 before the Odonata, and that the aquatic habit is not to be traced 

 back to a common ancestor. Lastly, as regards the cerci, these 

 appendages in the Perlaria remain as undifferentiated filiform 

 organs, and are so carried over into the imago. In the Odonata, 

 they are either reduced, or else developed in the form of caudal 

 gills. Only in the Zygoptera are they carried over into the imago. 

 Combining this with the facts that a few Odonate larvae still 

 possess their lateral abdominal appendages, and that the gill- 

 systems in the two Orders are absolutely different, we cannot be 

 far wrong in assuming that the retention of the cerci has been 

 brought about by gradual and incomplete parallel reductions of 

 the original complete set of abdominal appendages once possessed 

 by the ancestors of all Insecta, and does not indicate any special 

 affinitv. 



