H] EXTERNAL FEATURES AND SKELETON 37 



In such a case the inferior appendages rest in front of the hind- 

 lobe of the pronotum, and may exert very little pressure. If, 

 however, the superior appendages be reduced, as in most Agrio- 

 nidae, then a high specialization of both pairs is usually found. 

 This is of such a nature that the inferior appendages, with the aid 

 of various devices of sculpture on the hind-lobe of the female 

 pronotum (grooves, ridges, tubercles, or even strongly curved 

 hooks), are able to press strongly downwards on part of the hind- 

 lobe, while the superior appendages press on it from behind. 

 Thus the hind-lobe is held in a double longitudinal lock-grip, as 

 between the jaws of two vices arranged side by side (fig. 16s). 

 In some cases the superior appendages press down over, or even 

 behind, the mesostigmatic laminae. 



Observations on the actual positions of the appendages during 

 pairing are difficult to make, since the insects nearly always dis- 

 engage when captured. A good summary of our present know- 

 ledge on the subject is given by Williamson and Calvert [201]. 

 (See also Walker [190].) 



The most primitive form of Zygopterid appendages is un- 

 doubtedly that in which the superiors were short and strongly 

 forcipate, the inferiors still shorter and straight. This form still 

 exists, with minor modifications, throughout the Calopterygidae 

 and Lestidae. The highly reduced and often stump-like superior 

 appendages of many Agrionidae are a more advanced type, in 

 which the principal part in maintaining the grip becomes shifted 

 on to the inferior appendages, aided by special developments 

 of the pronotum of the female. 



Perhaps the most remarkable appendages occurring in the 

 Zygoptera are those of Hemiphlebia mirabilis (fig. 14 L). Both 

 pairs in the male are rather long, white, and very conspicuous. 

 The inferior appendages are like a pair of white ribbons. The 

 male uses them to attract the attention of the female by waving 

 them about while at rest on a reed-stem. The female replies 

 to his signals by shewing the whitened underside of the tip of her 

 abdomen [173]. Another Australian Zygopterid, Synlestes albi- 

 cauda, hides on the dark trunks of trees in dense jungle, and can 

 only be seen by the whiteness of its appendages, which it probably 

 uses somewhat after the manner of Hemiphlebia. 



