6 INTRODUCTION [CH. 



While the true affinities of the Perlaria remain a matter of much 

 doubt, we must maintain that no true relationships can be proved 

 between them and the Odonata, on the evidence available to us. 



Turning next to the May-flies, the position is somewhat 

 different. Their points of resemblance with the Odonata are: 

 (i) the presence 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, in the larvae 

 of the Plectoptera, and in a few archaic larvae of Odonata, of the 

 lateral abdominal appendages, in the form of gills, (v) the retention 

 of the cerci in the larvae of both groups, and in the imagines of the 

 Plectoptera and the Zygoptera, (vi) the reduced awl-like antennae, 

 (vii) the presence of only one wing-axillary, and the inability to 

 fold the wings downwards either in a flat or roof-like manner over 

 the body. With regard to the first three points, the same argument 

 may be applied as in the case of the Perlaria, especially as the 

 tracheal system of the Plectoptera is as little like that of the 

 Odonata as is that of the Perlaria. Both the latter and the 

 Plectoptera, in fact, appear to have been aquatic in their larval 

 stages long before the Odonata became so. The fourth character 

 mentioned becomes rather striking when we compare the larva of 

 Pseudophaea (fig. 87) or Cora with that of an Ephemerid. It may 

 well be that we have here some evidence of a very ancient connec- 

 tion between the two groups. As regards the retention of the 

 cerci, we must repeat the argument used above. With respect to 

 the form of the antennae, the reduction almost certainly rests upon 

 convergence, and cannot be used as an argument for relationship. 

 Similar antennae appear quite independently in other groups of 

 Insects. The last character, (vii), seems undoubtedly to have been 

 derived from a common ancestor. Thus we have in this case 

 a certa : n amount of evidence for a connection between the two 

 Orders, very long ago. 



The palaeontological evidence here helps us considerably. The 

 Protodonata of the Upper Carboniferous were admittedly the 

 ancestors of our present-day Odonata, although an unfortunate 

 gap in the Trias hides the exact line of descent from our view. 

 The Protodonata were large insects with well-developed sub-equal 

 wings. They were undoubtedly Dragonflies in facies and habits, 



