82 THE LARVA OR NYMPH [CH. 



Variations in the Form of the Mask (figs. 29 D, 31, 32). Two 

 main types of mask are easily distinguishable, the fiat mask and 

 the concave or spoon-shaped mask. The former is clearly the more 

 archaic form. The following sub-types occur: 



(i) Gomphinae (fig. 32 D). The mentum is short and square, 

 without setae, the median lobe not usually projecting. The lateral 

 lobes have a rounded apex, a short distal margin, and a more or 

 less denticulated inner margin. The movable hook is strong. 

 Setae are absent. 



(ii) Aeschninae (figs. 29 D, 32 A to c). The mentum and sub- 

 mentum are greatly elongated, and narrowed basally. The median 

 lobe usually projects slightly. In the older forms (fig. 32 c), the 

 lateral lobes are shaped as in Gomphinae; in the rest (A, B), the 

 apex is square or pointed into a distinct hook or tooth. Setae are 

 absent, except in Gynacantha (B). In many Brachytronini two 

 pairs of tubercles on the median lobe may be remnants of galeae 

 and laciniae. 



(iii) Calopterygidae (fig. 32 E, R). The median lobe is typically 

 slightly cleft, but secondarily very deeply cleft in Calopteryginae 

 (R). The mentum may be long or short, trapezoidal, without 

 setae. The lateral lobes arc slender and carry three teeth; one 

 (the largest) at the apex, a second on the distal border (which is 

 much reduced), and a third on the inner border just below the 

 apex. Setae are absent (except a few small ones in Calopteryginae). 

 This type passes over without change from the Epallaginae to the 

 Megapodagrioninae . 



(iv) Agrionidae (excluding Megapodagrioninae) (fig. 32 r). 

 Type (iii) modified. The median lobe becomes prominent and 

 triangular. The mentum remains short. Lateral and often also 

 mental setae are developed. The distal tooth becomes flat- 

 tened into a serrated ridge; the inner tooth tends to disappear, 

 leaving a convexity only. There are many minor variations on 

 this theme. 



(v) Synlestinae (fig. 32 G). Intermediate between (iii) and 

 the older Aeschninae. Mentum and sub-mentum much elongated, 

 narrow. Median lobe prominently bilobed, with a fairly deep 

 cleft. Lateral lobes as in Gomphinae, but the apex projecting in 

 a strong tooth, supported by a second sharp distal tooth. No setae. 



