TJie Odonata or Dragonflies of South Africa. 251 



the zygopterous type of this organ, and thus nearer to structures 

 generally found among insects with masticatory mouth-parts. 



The frontal, vertical and occipital sclerites are mostly not easily 

 distinguished in the fully mature insect, but as a matter of con- 

 venience in all the groups the part bearing the antennae and 

 ocelli, the fronto-vertical complex, may be termed, in an abridged 

 form, " the frons " ; what is posterior to it may be named " the 

 occiput."* 



The latter especially is subject to great modification in consequence 

 of the development and disposition of the eyes. Largely visible in a 

 dorsal view in Zygoptera and Gomphinae (Text-figs. 1, 2), it is 

 reduced in this view to a mere trace, in other Anisoptera (Text-fig. 3), 

 where the occipital triangle and just a small margin on the posterior 

 side of the eye are the only parts attributable to the occiput. Its 

 ventral view is also chiefly determined by the shape of the eyes ; only 

 in the Zygoptera is the occipital foramen (Text-fig. 1) visible in a 

 strictly ventral view ; in Anisoptera the foramen is drawn forward 

 by the globose form of the head and covered from beneath by the 

 basal pieces of labium and maxillae. 



Whereas the fronto-occipital suture is obscure and often imcertain, 

 there is a very distinct suture between the frons and the sclerites 

 anterior to the frons. There are three of these transversely disposed. 

 The first, from the anterior pole, is the labrum or upper lip, which 

 partly covers the mouth-parts from above, but not entirely ; part of 

 the mandibles (or even of the maxillae) remain free and partake of the 

 colour differentiation of exposed parts. The second is the anteclypeus, 

 the third the postclypeus. Labrum, anteclypeus and postclypeus are 

 disposed very nearly in the same vertical plane in the Anisoptera, 

 which plane is continued dorsally by the frons. As a rule there is here 

 an inflection, with or without a ridge, to the horizontal plane within the 

 frons itself (only some of the Gromphiuae do not clearly show the 

 fracture in the frons), as shown in Text-figs. 2 and 3. In the 

 Zygoptera the labrum is very nearly horizontal, the auteclypeus 

 vertical, the postclypeus horizontal again ; and from the suture of 

 postclypeus and frons this latter rises mostly in a gentle curve to give 

 a nearly horizontal position to the fronto-occipital complex, the ocelli 

 looking upward. Something similar to the transverse frontal ridge 

 of most Anisoptera may exceptionally be observed in the Zygoptera 

 (Ceriagrion). Between the lateral margin of ante- and postclypeus 



* For details of the rather complicated structural homologies of the exo- 

 skeleton of the head special handbooks of anatomy must be consulted ; their 

 discussion would exceed the aims of this summary introduction. 



