MOSQUITOES, SILKIVORMS, AND DRAGON-FLIES 15 



inside the body, and these various side-tubes lead into a 

 main longitudinal tube running along each side from 

 head to tail (fig. 5). From these main longitudinal trunks 

 branches and sub-branches go 

 to all parts of the body, the air 

 being distributed in the insect's 

 body by a distinct system of air- 

 tubes called trachecB, and not 

 entering a pair of lungs (fig. 6). 



Silkworms devour an enor- 

 mous amount of leaves during 

 the last few days of larval life, 

 much more indeed than they 

 need at that time. Later we 

 shall see the reason for this over- 

 eating. They have thick, heavy 

 bodies and reach a length of two 

 and a half inches. Make a draw- 

 ing of a full-grown larva from a 

 lateral view, showing all the legs 

 of one side, nine breathing holes 

 (tiny openings each surrounded Fi(~'- &■ — Diatjram of tracheal 



1 111 11- \ 1,1 1 system in boily of beetle; 



by a black ellipse), and the dor- ^p, spiracles; tr. trachea:. 



sal spine on the posterior end. (After Kolbe.) 



The cocoon and pupa. — About a week or nine days 

 after the fourth moult the silkworms stop feeding and 

 prepare for the fifth and final one. (Occasionally a silk- 

 worm moults six times.) But unusual preparations are 

 made this time ; each larva crawls alongside the edge of 

 the tray, or approaches some object in it, and begins to spin 

 silken thread from its mouth (fig. 5.) All of this spinning 

 should be watched closely. At first the thread is attached 

 irregularly and apparently aimlessly to the objects near 

 by, but when a sort of loose and irregular net or web of 

 silk has been made, the spinning becomes more regular, 



