/.vyA'c)/ -IOLOGY. 



1C. The t"<>nn <>t" the body varies greatly, some 

 being quite slender, while others are nearly as broad as lo 

 Tlu ver, easily recogni/ed. The general app 



the body is quite characteristic in spite of the variations in 

 proportions. There are also certain \vell-marked structural char- 

 . hich distinguish the nymphs of dragon-flies from all other 

 The most available of these characters is the form of the 

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

 : maxilla^ all of which are armed with strong teeth, But 

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

 .tly 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 

 itly 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 si 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 org.r pira- 

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

 tracheal gills ''Figs. 68 and 70; in some of the C'alop- 

 ginae we find lateral abdominal tracheal gills: and ir 

 tain forms there are even closing stigmata on the thorax 

 and abdomen. 

 ; IC *} The rectal tracheal gill of immature dnu 



H of locomotion, as well as of respiration. By draw- 

 ing water into the rectum gradually, and expelling it 

 :bly. the insect is able to dart through the water with o 

 able rapidity. 



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



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



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



