26 ELEMENTARY LESSONS ON INSECTS 



the tree-like distribution of tracheal branches into 

 the substance of the brain. 



II. Study the circulatory apparatus. This will call for 

 careful focusing of your microscope on parts that lie at 

 different levels. With the living specimen back uppermost 

 on the slide: 



1. Focus just beneath the skin of the rear part of the ab- 

 domen, and watch for the movements of the pulsating 

 walls of the very transparent dorsal vessel. When 

 found, move along its edge to find one of the valves 

 thru which a stream of minute, pale, white blood 

 corpuscles may be seen passing into it. Find the valves 

 at its rear end. 



2. Trace the blood current forward to the head. 



3. Observe the flow of the blood corpuscles out into the 

 bases of the legs and back again; out into the gills and 

 back again. 



Some of these things are difficult to make out and may 

 have to be shown in demonstration, using the most favorable 

 specimens. 



III. See what you can of the divisions of the alimentary 

 canal: it will be partly dark in color, because of contained 

 food. The slender Malpighian tubules will mark the posterior 

 end of the stomach and the beginning of the intestine. 



B. Internal Structure Seen In Preparations. 

 I. In dissections of the food tube, note the three principal 

 divisions: 



