Oct., 1920 Bulletin of the Brooklyn Entomological Society 93 



vised by Krogh which is able to measure extremely minute 

 quantities of the gases. 



Since very little nitrogen is used up there is a constant nitro- 

 gen equilibrium, while a continuous flow of oxygen takes place. 

 The same is true for carbon dioxide, since there is already an 

 overbalance in the blood in the form of carbonic acid. 



The topic of gases entering in solution raises the question : how 

 do gases leave aquatic larvae? (See section 9 for expiration of 

 adults.) In aquatic insects there is only an afferent stream of 

 gas through the tracheae to the tissues. Oxygen flows to the cells 

 and is used up in oxidation, and carbon dioxide is formed. What 

 becomes of the carbon dioxide? We know that it is taken up by 

 the blood. But how does it leave the body ? The hypothesis was 

 suggested by Tillyard that it probably would go out through thin 

 places in the epidermis in the form of carbonic acid. Personally 

 I was able to show that this hypothesis might apply. In some 

 experiments on the permeability of chitin membranes I was able 

 to show that not only thin portions, but also thick portions of a 

 chitin membrane will' very readily permit the passage of car- 

 bonic acid in practically all parts of the body. What I did not 

 show was that this passage actually takes place in the living in- 

 sect. But if a stripped chitin membrane shows such a consider- 

 able degree of permeability to the flow of carbonic acid, it can be 

 assumed that it may show a similar degree of permeability in the 

 living aquatic insect. It would then once more be a question of 

 simple diffusion. 



In the Zygoptera we meet with a somewhat different type of 

 structure. There are three caudal gill plates, each supplied with 

 tracheal trunks, the middle one with four, the lateral one with 

 two each. As the tracheae are often pigmented and as their type 

 of branching is distinctive of the species they are used as taxo- 

 nomic characters. Histologically, a cross section shows the outer 

 cuticle, hypodermis, the two or four tracheal trunks lying in a 

 network of alveoli, and a dorsal and ventral blood channel. The 

 alveoli appear to be derived from the hypodermis, and form in- 

 growths filling the gills with a meshwork. Their function is ob- 

 scure. The tracheal trunks break up into smaller trunks, and 

 these into tracheoles, which then form an anastomosing network. 



