NOKTH AMERICAN ODONATA. 155 



the Odoiuite groups, the eyes occupy the lateral parts of the head, 

 being mainly restricted to that region in the Agrionidce, and there 

 the head is transversely elongated ; reaching farther towards the 

 mid-dorsal line in the Gomphinse, meeting at a single point on the 

 top of the head in the Cordulegasterinse ; finally, in the Libellulidi^e 

 and the Aeschninse being connected for at least some little distance, 

 and here the head is globose. It is in various genera of the Aesch- 

 ninje that their connection is proportionally greatest, and the eyes 

 themselves relatively largest, and these same genera include those 

 species whose flight is most powerful. 



The facets are hexagonal iu shape, and where the eyes meet dorsally, those 

 of the upper surface are larger than those of the lower. In such cases a gradual 

 transition in size from one to the other exists, but in many Libellulinfe the facets 

 of about the upper third of each eye are distinctly marked off in size from the 

 lower ones, the line of separation being nearly horizontal. Below this line a 

 gradual diflference in size downwards is found. In life the color of the eyes is 

 darker above and becomes paler below. 



The epicranium occupies a very large part of the area of the head. 

 In the Agrionidie, it completely separates the eyes from each other. 

 but is itself divided into right and left halves except at the vertex, 

 frons, and for a short distance above the posterior foramen. This 

 division is made — beginning at the frons and proceeding forwards, 

 downwards, backwards and upwards — by the clypeus, labrum, mouth 

 parts, gula, posterior foramen and occiput. The occiput, however, 

 although distinct in the Anisoptera, appears to be so completely 

 fused with the epicranium in the Zygoptera that no sutures are 

 visible. 



Names have been applied to different parts of the epicranium. 

 The most dorsal portion, and especially the region of the ocelli, is 

 the vertex. Between the vertex and the clypeus is the frons; the 

 place of insertion of the antennae may be considered to mark its 

 posterior boundary. Since each eye is encircled by epicranium, that 

 portion lying between the eye and the clypeus and mouth-j)arts is 

 the gena. Lastly, the hindermost part of the head is simply referred 

 to as the rear of the head, or the rear of the eyes ; the right and left 

 halves of this part unite with each other above theposterior foramen 

 and below the occiput, but the uniting suture is usually obliterated. 

 On either side of the posterior foramen is a groove in which o-Hdes 

 a tubercle of the prothorax to furnish the articulation of head and 

 thorax. 



TRANS. AM. ENT. SOC. XX. JULY, 1893, 



