250 



Annals of the South African Museum. 



Zygoptera (Text-fig. 1); covered by a special structure, the frontal 

 vesicle, in the Libellulidae and Aeschninae (Text-fig. 3) ; surrounded 

 by sculptures of various development in the other subfamilies of 

 Auisoptera (Text-fig. 2). Nothing certain is known about their 

 special function. 



The mouth-parts correspond to their masticatory function in the 

 primitive insect type ; they are of the regular three pairs — labium, 



Fig. 3. — Trithemis arteriosa. Dorsal, frontal and ventral views of 

 head. Lettering as in Text-fig. 1. occtr. Occipital triangle. 



maxillae and mandibles — the labial and maxillary palpi being much 

 reduced (or modified ?), the mandibles particularly strong. Their size 

 is relatively small in the Zygoptera (Text-fig. 1), but they are very 

 much stronger in the anisopterous line (Text-figs. 2, 3). The 

 labium is a very highly specialised structure in the family Libellulidae 

 (Text-fig. 3), being a spoon-like or mask-like organ, which covers the 

 entire mouth from beneath by the greatly developed lateral lobes, 

 their free mesial borders being connected in a long and straight line. 

 The other groups of Anisoptera (Family Aeschnidae) remain true to 



