IT] EXTERNAL FEATURES AND SKELETON 11 



1-3, bearing the clypeus and labrum, have become pushed dorsally 

 upwards into their present positions. 



The Compound Eyes (see chap. vn). In the Dragonflies, as is 

 the case also in a somewhat less degree with their Vertebrate 

 analogues the Birds, the shape of the head, and the sizes and 

 relationships of its parts, have become profoundly modified in 

 correlation with the extraordinary development of the sense of 

 sight. The orbits have become expanded to an enormous diameter, 

 in order to accommodate the huge compound eyes (e), which, in 

 many forms, occupy by far the largest part of the whole head. 

 In the Aeschninae and Libellulidae this development culminates in 

 the meeting of the two eyes mid-dorsally on the head in a long 

 line of contact, the median eye-line (A, me], the true nature of which 

 is explained on p. 146. The stages leading up to this result are 

 preserved for us in a long line of existing Anisopterid forms. Of 

 these, the Gomphinae and Petalurinae lie nearest to the primitive 

 type. Their eyes are rounded, not excessively large, and separated 

 by a width of epicranium less than the dorsal diameter of the eye. 

 The eyes of Meganeura (fig. 156) and other early fossil forms seem 

 to have been very similar to this. In the Chlorogomphinae the 

 eyes become somewhat more transversely elongated, and take on 

 a more oval shape. They just fail to touch one another mid- 

 dorsally. In most of the Cordulegastrinae a further slight advance 

 is seen, the eyes just meeting in a point. In the Petaliini there 

 is another slight advance, the eyes meeting for a very short distance. 

 In the Brachytronini the eye-line becomes of moderate length; 

 while in most of the Aeschnini and Libellulidae it is so long as to 

 obliterate the greater part of the epicranium, and approaches in 

 length the diameter of the eye itself. 



In the Zygoptera (E, F) the eye has remained rounded and 

 comparatively small. The tendency has been towards improving 

 the power of sight by a progressive movement of the eyes laterally 

 outwards. Thus the highest forms in this suborder have the head 

 transversely elongated to form a kind of cross-stalk carrying the 

 button-like eyes at its two ends. The distance between the eyes 

 then becomes much greater than the dorsal diameter of a single 

 eye. In the older forms (Calopterygidae] the transverse elongation 

 of the head is not so noticeable. 



