72 AN INTRODUCTION TO ENTOMOLOGY. 



as they can overcome. The form of the body varies greatly, some 

 species being quite slender, while others are nearly as broad as long. 

 These insects are, however, easily recognized. The general appear- 

 ance of the body is quite characteristic in spite of the variations in 

 its proportions. There are also certain well-marked structural char- 

 acters which distinguish the nymphs of dragon-flies from all other 

 insects. The most available of these characters is the form of the 

 mouth-parts. The mouth is furnished with well-developed mandi- 

 bles and maxillae, all of which are armed with strong teeth. But 

 none of these is visible when the insect is at rest. The lower lip is 

 greatly enlarged, and so formed that it closes over the jaws conceal- 

 ing them. For this reason it has been termed the mask. But it is 

 much more than a mask ; it is a powerful weapon of offence. It is 

 greatly elongated ; and is jointed in such a way that it can be thrust 

 out forward in front of the head. It is armed at its extremity 

 with a pair of sharp hooks, adapted for seizing and retaining its 

 prey. 



An equally remarkable peculiarity of these insects in their 

 nymph stage is the form of the organs of respiration. The caudal 

 part of the alimentary canal, the rectum, is modified so as to con- 

 stitute a tracheal gill. It is somewhat enlarged ; and its walls are 

 abundantly supplied with tracheae. Water is alternately taken in 

 and forced out through the anal opening; by this process the air in 

 the tracheae, with which the walls of the rectum are sup- 

 plied, is purified in the same manner as in an ordinary 

 tracheal gill. 



In addition to the rectal tracheal gill, certain dragon- 

 flies possess in their nymph stages other organs of respira- 

 tion. Thus, in the Agrioninae there are leaf-like caudal 

 tracheal gills (Figs. 68 and 70) ; in some of the Caloptery- 

 ginae we find lateral abdominal tracheal gills; and in cer- 

 tain forms there are even closing stigmata on the thorax 

 and abdomen. 



Tracheal T\\q rectal tracheal gill of immature dragon-flies is an 



r/ragon- organ of locomotion, as well as of respiration. By draw- 

 ing water into the rectum gradually, and expelling it 

 forcibly, the insect is able to dart through the water with consider- 

 able rapidity. 



When the nymph of a dragon-fly is fully grown it leaves the 

 water to transform. The skin of the nymph splits open on the back 

 of the thorax and head, and the adult emerges, leaving the empty 



