120 HORTENSE BUTLER. 



Orthoptera and Plectoptera a condition exists analogous to this of 

 the embryonic stage. Notably in Dlaphemorera and Prisopus we 

 find the lamime externce slipping up over the lacinje in such a man- 

 ner as to partially surround them, much as is the case in the embry- 

 onic labium of J.Ha.c. It will further be observed that the lateral 

 lobe articulates with a segment quite separate from that with which 

 the ligula articulates, and that this piece seems in every way to cor- 

 respond to the palpiger. From this evidence it seems reasonable to 

 conclude that, in the so-called lateral lobe of the labium of the 

 Odonata, we have represented merely the palpus of the typical max- 

 illa, while the ligula represents the consolidated lacinse and galeae. 



IV. - Comparative Anatomy of Nymphal and Adult 



Labia by Groups. 

 The forms of the labia are exceedingly variable in the various 

 families of tlie Odonata, both during the nymphal life and later 

 after transformation. The variations in a single group are, however, 

 comparatively slight, and each group can be adecjuately represented 

 by a single typical specimen together with details from several of 

 those showing the most marked differences. 



Adult Labia. 

 L.IBKL,L.UL,ID.£. 



In the labia of adult Libellulidfe the average width of the com- 

 bined mentum and ligula is 1.87 times its length, while the ligula is 

 broadly rounded and without a median cleft. The lateral lobes are 

 uniform, the proportion of length to width being as 1 to 1, they are 

 deeply concave. The end of the lateral lobe is rounded and covered 

 with hairs externally, and the movable hook is reduced to a very 

 small and unarticulated rudiment. Micrathyria berenice (PI. VII, 

 Fig. 3) is typical of the adult Libellulid labium. In this group the 

 chief variations are in condition of the movable-hook and in the dis- 

 position of the hairs on the lateral lol)e. In Synthemis brevistyln 

 (PI. VII, Fig. Sd) the movable hook is represented by a chitinized 

 spot, while the hairs are sparse and are placed all around the border 

 of the lateral lobe. Much the same condition exists in Didymops 

 transversa (PI. VII, fig. 86), but the hairs appear on the surface of 

 the lateral lobe, and there is a marked depression in tlie l)order of 

 the lobe directly behind the chitinized spot, forming a small lobe in 



