12 THE IMAGO [CH. 



With the above facts borne in mind, we may now profitably 

 study the conformation of the head itself. The various pieces or 

 sclerites of the head do not appear to be closely connected with 

 the original segmentation. With the exception, perhaps, of the 

 labrum, clypeus and gula, they cannot be assigned to definite 

 segments. Thus they are chiefly named from their position in 

 relation to the head and mouth. 



The Epicranium (epic). This is by far the largest part of the 

 head-skeleton. If we compare the head to a hollow rectangular 

 box with a lid on, the epicranium represents not only the lid but 

 the upper portions of the front, back, and sides of the box as well. 

 It is bounded laterally on either side by the eyes, which are set in 

 two large hollows with thickened rims, the orbits (A, B, orb). 

 Postero-laterally, each orbit becomes somewhat swollen at a place 

 where the eye itself is slightly concave when seen in profile. This 

 is the temple (c, t). Posteriorly, the epicranium ends in a sharp 

 shelf, the occiput (E, occ), which varies greatly in size and shape, 

 according to the extent of the eyes. It is broadest in the Zygop- 

 tera, where it usually has a wavy outline, and may be divided into 

 a median portion, the occiput proper, and two lateral postocular 

 lobes (E, pol), corresponding with the more prominent lobes of the 

 same name in the larva. These serve for the attachment of the 

 strong muscles of the mandibles and maxillae. In the Gomphinae 

 the occiput is an almost straight shelf, often hairy, and modified in 

 the female, to assist in copulation, by the presence of hooks or 

 tubercles which help to hold the complicated anal appendages of 

 the male in position. In the Aeschninae and Libellulidae, the 

 occiput becomes reduced to a small occipital triangle (A, ot). This 

 carries, in some forms, a prominent horn or tubercle. A little 

 in front of the middle of the dorsal surface of the epicranium 

 is a triangular area called the vertex (E, v), marked by the pres- 

 ence of the three ocelli, one of which is median and anterior 

 (ocm), the other two lateral, smaller and posterior (ocl). Originally 

 flat, as in most Zygoptera, we find that in Anisoptera it tends to 

 become raised up, in proportion to the restriction in the area of 

 the epicranium caused by the increase in the size of the eyes. In 

 the Gomphinae the vertex may be a small raised plateau, or a 

 strong transverse ridge with a posterior sunken portion, or a 



