417 



the base of the labium are important in throwing the submentum away 

 from the head. In this, however, they are aided by the presence of 

 the obHque, chitinous rod and by the contraction of the dorso-sub- 

 mental muscles. The action of the ventral pair seems reasonably clear, 

 as it can, by contraction, swing the mentum back towards the sub- 

 mentum, and by action in conjunction with the chitinous rod, swing 

 the submentum back towards the head. The mentum is swung out 

 away from the submentum mainly by the action of the dorsal sub- 

 mental muscles which are attached to it above the hinge and in this 

 capacity are aided to some extent by the contraction of the muscles 

 within the mentum, especially those which operate 'the labial palpi. 



Glossae and Paraglossac. — The glossae and paraglossae are fused 

 and the suture is untraceable except in the embryo (Butler, '04). The 

 nymph of Agrion show^s a fold extending between the labial palpi 

 beneath the median cleft. This fold probably represents all that is 

 left of the suture between mentum and paraglossae. In other species 

 the course of the suture is marked by a row of setae, the mental setae. 

 The area occupied by the submentum is indefinite, but it is probable 

 tha,t the mental setae, even if secondary in origin and occurring only 

 in the more specialized forms, are near the location of the original 

 mento-paraglossal suture. The latter suture has been thought to 

 become approximated to the distal border of the median lobe, but this 

 seems doubtful after comparisons of nymphal and embryonic labia 

 (Butler, '04). 



Labial Palpi. — The palpi of the labium are represented by the lat- 

 eral lobes, (Figs. 8, 9; Ipi, Ipo). These lobes bear at their distal ends 

 a number of fixed hooks, which are simple in some cases, but may 

 become modified in others, for instance in the Coenagrionidae, in 

 which the middle hook is replaced by a blunt process with teeth at 

 the apex. That the fixed hooks have not the same origin as the mov- 

 able ones, is shown by the fact that the latter bear, in certain nymphs, 

 a number of long setae. The movable hook has been considered (But- 

 ler, '04) as a modified palpal segment, and this interpretation is un- 

 doubtedly the correct one. The proximal segment or body of the 

 lateral arms also bears in many species of nymphs a row of long setae, 

 the number of which is used extensively in the classification of the 

 group. In Agrion, however, this row of setae is wanting and the lat- 

 eral lobe bears only two small setae near the base of the movable hook. 

 The body of the mentum is also provided with rows of setae in all 

 genera except Argia, Agrion, and Hetaerina, the bristles being in two 

 divergent lines beginning near the meson slightly distad of the center 

 and reaching nearly to the proximal end of the labial palpi. 



