506 



INVER TEBRA TE MORPHOLOG T. 



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

 sects. 



The larvae are aquatic and provided with tracheal bran- 

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

 Thysauura. 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 uusegmeuted 

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

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

 slender, though somewhat stouter in the genus Libellula. The 

 antenna} 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 larvaa are aquatic 

 and are characterized by 

 the remarkable develop- 

 ment of the labiuin, which 

 is very much enlarged, 

 terminating in two power- 

 FIG. M5.-Diplaz elisa (from PACKARD). ful j awg and prov ia e d 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 



