II] EXTERNAL FEATURES AND SKELETON 13 



prominent ridge with, projecting lateral tubercles. In all these 

 cases, the effect is to force the lateral ocelli forward almost in line 

 with the median one. In the Aeschninae and Libellulidae, the 

 vertex, like the occiput, becomes excessively reduced, forming 

 a small vertical tubercle or vesicle (A, vt) round which the ocelli are 

 grouped close together. 



Just in front of the antennae (p. 15) there runs a transverse 

 groove, the frontal furrow or suture. This separates the vertex from 

 the projecting anterior portion of the epicranium, known as the 

 frons (E, F, fr). In the Zygoptera, the frons is a sloping shelf, 

 usually rather narrow, and generally somewhat convex. Below it, 

 on either side, lies a further small piece of the epicranium called 

 the gena or cheek (g), bordering the eye. In the Anisoptera, and 

 in the genus Ceriagrion amongst Zygoptera, the frons becomes 

 raised up into a more or less prominent shelf, having a posterior 

 or superior horizontal portion (frj) and an anterior or vertical 

 portion (fr 2 ), forming a definite forehead. In some Aeschninae 

 (e.g. Austrophlebia, A, c, frj) the frons projects in the form of 

 an anvil, with the superior portion slanting upwards to a median 

 point. 



The Clypeus. This is a separate piece inserted below the frons. 

 It forms the chief part of the face of the insect. In all Dragonflies 

 it is divided into an upper postclypeus or nasus (pc) and a lower 

 and smaller anteclypeus or rhinarium (ac). The former embraces 

 and often overhangs the latter (D). In many Zygoptera the post- 

 clypeus forms a kind of shelf comparable with that of the frons in 

 Anisoptera (F, pc). In Rhinocypha (Plate I, figs. 4, 5) the clypeus 

 projects forwards as a kind of snout. 



The Labrum. Below the clypeus, and bounding the mouth 

 from above, lies a strong median piece with a more or less rounded 

 lower border. This is the labrum or upper lip (Ir). It is narrower 

 than the clypeus, and is bounded on either side by the strong 

 pear-shaped bases of the mandibles (md). In the Anisoptera it 

 continues the plane of the face vertically downwards; in the 

 Zygoptera it projects obliquely forwards and downwards. To- 

 gether with the epipharynx, which it carries on its inner surface, 

 the labrum forms an organ of taste (chap. vn). 



The Mouth. When at rest, the mouth is completely closed by 



