60 



usually bifid (an archaic feature) ; in Aeschninae there is no median 

 cleft but a line shewing the suture, and sometimes tubercles are 

 present on each side of it ; C(»dide(^aster has a set of supplementary 

 teeth to the end hook (perhaps carried over from the naiad stage) ; 

 in Libelliiliilae, the end hook has gone, the moveable hook is small 

 or represented only by a chitinous spot, lateral lobes are greatly 

 enlarged, median lobe is reduced to a small triangle above which 

 the palpi meet in the middle line, making a T-shaped closed mouth. 



But this evening I want especially to call attention to the highly 

 remarkable and quite unique labium of the dragonfly in the naiad* 

 stage. 



Very aptly the naiad labium is called a " mask," because it com- 

 pletely (or else very nearly) covers the other mouth parts. The 

 view in PL VII., fig. D, gives a very good idea of the shape and general 

 appearance of the mask in a di'agonfly of the Aeschuine type. It is 

 in four parts — submentum, mentum with ligula, two palpi, and two 

 moveable hooks. Consequently there are four hinges. These are 

 worked by suitable muscles, except the moveable hooks which have 

 none. The mask is extremely variable in shape, especially as 

 regards the lateral lobes (palpi). 



In the embryo before hatching the labium consists of a pair of 

 three-jointed appendages, separate except at their extreme base. As 

 the naiad grows up, it alters considerably. After hatching the 

 tendencies are towards : — 



(1) More complete fusion of parts. 



(2) Repression of galea and lacinia. 



(B) Growth and strengthening of moveable hook. 



(4) Increase in the number of setae. 



(5) Deeper cleft in Calopteryx. 



(6) In Anax setae which are present at first are afterwards lost. 

 In the last stage before becoming an imago, the masks of the 



various families and sub-families differ greatly, though not much 

 within the sub-families. One belonging to each will now be examined, 

 notice being taken of signs that point to an archaic form. Masks 

 may roughly be divided into (1) flat, and (2) spoon-shaped, the 

 former being the more archaic. 



1. Gomphinae. Gomp/iKs fidgatissimus, Linn. (PI. VIII., fig. 1). 



(1) Short, squarish, flat luentuin. 



(2) Setae none. 



* Following Comstock in his " Introduction to Entomology " (Ithaca, 1920), 

 I am restricting the term " nymph " to the immature instars of insects with 

 gradual metamorphosis such as grasshoppers ; adopting the term " naiad " for 

 those with incomvlete metamorphosis, such as dragontlies ; and reserving " larva 

 and pupa " for those with complete, metamorphosis, such as butterflies and lace- 

 wings. 



