506 INVBBTEBBATE MOBPHOLOQT. 



pores instead of uniting as they do in the majority of In- 

 sects. 



The larvae are aquatic and provided vrith tracheal bran- 

 chige (see Fig. 226), recalling, except for these structures, the 

 Thysanura. By a series of moults the adult stage is gradu- 

 ally acquired, the wings appearing in what is termed the sub- 

 imago stage, a final moult being necessary before maturity is 

 reached. The metamorphosis is thus incomplete. 



The genus Ephemera is of frequent occurrence in the 

 neighborhood of lakes and ponds, sometimes occurring in 

 enormous numbers. 



4. Order Odonata. 



The members of this order, the Dragon-flies, are elongated 

 forms with two pairs of nearly equal, abundantly-veined wings 

 of usually large size, all the forms being excellent fliers and 

 seeking their prey in the air. The head is united to the 

 thorax by a narrow stalk which permits extensive rotation of 

 the head, and the abdomen, terminating in two unsegmented 

 platelike cerci, is long, and in the large Dragon-flies, JEsclina 

 and Diplax (Fig. 235), and in the brightly-colored Agrion very 

 slender, though somewhat stouter in the genus Zibellida. The 

 antennae are very small and the mouth-parts adapted for biting, 

 while the legs are slender, the anterior pair being directed 

 somewhat forwards so as to serve for grasping the prey. 

 The lateral compound eyes are very large, meeting on the 



dorsum of the head, and 

 in front of them are situ- 

 ated a pair of small ocelli. 

 The larvae are aquatic 

 and are characterized by 

 the remarkable develop- 

 ment of the labium, which 

 is very much enlarged, 

 terminating in two power- 



FlG. 235.~Diplax elisa (from Packard). f^j j^^^ ^^^ provided with 



a hinge, so that it can be flexed so as to lie beneath the head 

 or suddenly thrust out to capture the unwary prey. This 



