NO. 2 DRAGONFLY LARVA — SNODGRASS 7 



The occipital foramen has contracted to a small keyhole- shaped open- 

 ing in the center of the wide, vertical, posterior surface of the cranium, 

 so that the head is freely movable in all directions on a narrow neck. 

 In a transforming Anax larva the compound eyes of the adult may 

 be seen within the larval cuticle to have extended mesally behind the 

 frons. Finally in the imago (fig. 2D) they all but meet along the 

 median line, being separated by only a narrow infolding of the head 

 wall representing the remnant of the larval vertex. The growth of 

 the eyes during the larval period has been shown by Lew (1934) to 

 take place by the differentiation of new ommatidia in a "budding- 

 zone" of the epidermis along the inner margin of each eye, to which 

 they are added from instar to instar. The greatest increase takes place 

 at the transformation of the larva to the imago. Three ocelli are 

 present on the adult head, including a large median ocellus and two 

 very small lateral ocelli. The antennae have moved upward on the 

 face until they arise on the upper part of the frons at the sides of the 

 lateral ocelli. The f rontoclypeal region forms a large protruding quad- 

 rate plaque on the middle of the face closely embraced by the com- 

 pound eyes. 



The head of the adult dragonfly is as clearly adapted in its struc- 

 ture to the feeding habits of the adult as is the larval head to the feed- 

 ing of the larva. The imaginal head, however, with all its specializa- 

 tion, is still of a more generalized type of structure than is the larval 

 head. Adaptation for catching live prey in the water has involved a 

 greater degree of modification than adaptation for catching live prey 

 in the air. Neither type of head can be regarded as primitive, or even 

 generalized, but the larval head has departed farther from the gen- 

 eralized structure because of the more specialized feeding mechanism 

 of the larva. The metamorphosis of the dragonfly's head gives us a 

 striking example of how a major part of an insect can be structurally 

 modified in two different ways to serve the needs of the insect in two 

 different phases of its life. 



II. THE FEEDING APPARATUS 



The external feeding organs of the dragonfly larva include the usual 

 insect mouth parts, namely, the mandibles, the maxillae, the labium, 

 and the hypopharynx, and also the preoral cibarium. Of these parts 

 the labium is the only one with a distinctive structure, but its struc- 

 ture is so extraordinary that it sets the dragonfly larva apart from the 

 rest of the insects. No other insect or any other animal has developed 

 its under lip into a protractile organ for grasping living prey. 



Mandibles. — The mandibles of an Anax larva are strongly toothed 



