22 Journal New York Entomological Society. [Vol. xxvii. 



the above plan, the first and last merely differing from the others in 

 their larger size. 



B. Tracheal System. 



It is necessary to make a large number of dissections in order to 

 obtain an accurate knowledge of this complicated system, for it. is 

 impossible by a single dissection to show all the tracheae contained 

 within the body. 



In studying the tracheal system alcoholic specimens proved to be 

 of very little value, as tracheae filled with this fluid can be traced 

 only with great difficulty. An attempt was made to fill the tracheae 

 with india ink, melted wax, and other substances, by submerging the 

 insect in these materials and exhausting the air from the container, 

 in the hope that atmospheric pressure would force these substances 

 into the tracheae when the air was again admitted, but these proved 

 unsatisfactory. It was found that tracheae containing air were very 

 easily traced when dissected specimens were submerged in water, but 

 this necessitated fresh insects. 



After considerable experimentation the following method was 

 devised for the preservation of locusts with air in their tracheae. 



The Carolina locusts were caught and killed in a cyanide bottle 

 in the usual manner. At the close of a collecting trip these insects 

 were placed in a desiccator on a wire gauze, the bottom part of which 

 was partly filled with 8 per cent, formalin solution. This proved to 

 be very satisfactory in preserving the air in the tracheal system and 

 preventing the insects from hardening while the formalin gas lib- 

 erated prevented molds and bacteria from breaking down the insect 

 tissue. 

 I. Structure of the Tracheae. 



The tracheal system of insects originates in the embryo as tubular 

 invaginations of the ectodermal layer, and therefore the fundamental 

 structure is similar to that of the body wall. The tracheae are elastic 

 tubes lined by a chitinous layer corresponding to the chitinous exo- 

 skeleton of an insect, and are surrounded by an epithelial layer con- 

 tinuous with that of the hypodermis. The inner chitinous lining, 

 called the intima, is thickened at regular intervals to form spiral 

 threads, called taenidia, which are not continuous throughout the 

 tracheal tube but are frequently broken. The taenidia function in 



