24 ELEMENTARY LESSONS ON INSECTS 



A. Internal structure, externally visible* — Select the most 

 transparent specimen available, half an inch or less in 

 length. Mount it on a slide in water. Add a coverglass, 

 and see that the space beneath the cover is filled with water. 

 Study with low power of the microscope. You will at once 

 see that it has the same general external organization of 

 body and appendages as has the stonefly: internally the 

 two are sufficiently alike for our present needs. 



The most striking feature of internal structure will be 

 the thing that is most distinctive of the insect group — the 

 air tubes or tracheae, which run through the body every- 

 where. These reflect the air strongly and appear black or 

 silvery. There are main trunks extending lengthwise, and 

 principal branches extending to the legs and other organs, 

 and innumerable branchlets ramifying through every or- 

 gan. In insects air is not carried to the tissues by th,e blood 

 as in us, but is distributed through these tracheae directly 

 to every part. 



The relation between the other principal internal organs 

 may be seen also by looking more closely: (1) an alimentary 

 canal running straight through the center of the body from 

 end to end; it is dark colored where filled with food. (2) A 

 very delicate and transparent dorsal vessel extended longi- 

 tudinally above the alimentary canal and just beneath the 

 skin; (3) ganglia of the central nervous system, lying beneath 



* May be shown to a whole class at one time by means of a projection 

 microscope. 



