276 PTILOTA : IMAGO — EXTERNAL ANATOMY. 



variations of these organs serving more especially as organs 

 of defence to the true wings. In their most simple form, as 

 in some of the minute Hymenoptera, especially the genus 

 Psilus (fig. 128), the wing consists but of the two layers of 

 membrane, without any visible air-tubes, or veins, which, 

 in the more advanced structure of the wing, are developed, 

 dividing it into more or less numerous divisions, like the 

 frames of a window. This formation is more and more com- 

 plete, and the divisions more and more numerous, until we 

 find the wing exhibiting a network of meshes too numerous 

 to be counted J this is the case with the th-agon-fly (fig. 129). 

 In the Hymenoptera and Diptera, the wings are essentially 

 similar in the construction ; but the nervures, especially of 

 the posterior wings of the former, are less numerous, form- 

 ing but few spaces, or cells, as they are termed. The second 

 wings of the Hymenoptera are smaller than the anterior, 

 whilst in the dragon-flies, and some other neuropterous 

 insects, they are of equal size. If, therefore, we regard the 

 developement of vnngs as the chief characteristic of the 

 Ptilota, or winged insects, those species which exhibit the 

 wings of equal consistence and size, must be regarded as 

 the types of the tril)e. 



The external margin of the posterior wings exhibits, also, 

 in many insects with naked wings, minute hooks for retain- 

 ing the wings on each side together during flight. These 

 are especially found in the Hymenoptera, and are called 

 hamuli. In the lepidopterous insects, another structure pre- 

 vails for the like purpose ; the margin of the 230sterior wing 

 being furnished at its base with a long and curved bristle, 

 which is received into a little hook on the under suiface of 

 the anterior wings, in which it plays. The wings in this 

 same order off'er another peculiarity, since, instead of being 

 naked and transparent, they are clothed with a double layer 

 of minute scales, somewhat resembling those with which 



