n] 



EXTERNAL FEATURES AND SKELETON 



17 



and are barely visible when the mouth is closed. The basal joint 

 or cardo (c) is somewhat twisted. The distal joint or stipes (st) is 

 longer and wider. It carries on its distal end two freely movable 

 processes or lobes. Of these, the outer lobe or palp (p) is curved, 

 hairy, and only moderately pointed. It is obviously an organ of 

 touch. The inner lobe (il) represents the lacinia and galea of 

 more typical mandibulate insects, fused together. It has a rather 

 flattened base, from which project six large, sharply pointed and 

 somewhat curved teeth, occupying the outer two-thirds. The 

 inner third forms a small lobe or tubercle clothed with long 

 sensory hairs. The inner lobe is probably both prehensile and 

 tactile in function. Mastication is performed almost entirely b^ 7 

 the mandibles. 



mi- 

 ll 



Fig. 5. Labium of Aeschna brevistyla Ramb. ( x 10). eh end-hook; II lateral 

 lobe; m mentum; mh movable hook; ml median lobe; sm sub-mentum; 

 sq squame. Original. 



The Labium (figs. 5, 6). In all insects this organ is formed by 

 the fusion of the second maxillae. It is attached basally to the 

 gula, and represents the paired appendages of the sixth or last 

 head-segment. In the Odonata, both larvae and imagines, it is 

 of a highly specialized form. The question of the homologies of 

 its parts has given rise to much controversy. The conflicting 

 views are dealt with in the account of the larval mask on p. 80. 



In the imago, a considerable variation of form is seen in the 

 labia of different groups. In all cases, however, we can recognize 

 the same principal parts. There is a broad short base with a 

 projecting middle portion, at the sides of which are articulated 



T. D.-F. 2 



