10 University of California FuUications. [Extomology 



the skin in spite of the moisture when the insect retains, as 

 have the existing Thysanura, the old habitat. Another modi- 

 fication — one, indeed, that seems to be simpler and easier of 

 accomplishment — is the abandonment of the breathing of free 

 air and dependence entirely on the water for the air needed. 

 How efficient the skin is as a breathing organ in water in 

 the case of small insects may be judged from the fact that at 

 the present day certain small insects, such as mayflies and 

 dragonflies in their first stages, appear to have no need of 

 gills, being in some cases quite devoid of them. 



As soon as there is an increase in size, there comes to hQ the 

 necessity for a hardening of the l)ody wall for muscle attach- 

 ment, and at the same time the demand for oxygen increases 

 much more rapidly than does the surface of the body, 

 requiring that the breathing function be localized and per- 

 fected l)y the special modification of some part of the body 

 wall. This becomes absolutely essential to the life of the 

 insect whenever the other demands upon the integument have 

 reduced the unmodified oxygen-al)sorbing area to a point where 

 it is entirely inadequate to the demand. In the case of small 

 insects, as for instance the young of dragonflies and may- 

 flies, as stated above, this point has not been reached, but will 

 be in both these insects as soon as they become larger. 



Any part of the body may be specialized for this purpose 

 either l)y the adaptation of an organ already existing or the 

 development of an entirely new structure. A gill in an insect 

 is simply any outgrowth of the body wall that retains the 

 soft texture originally possessed by the whole body surface. 

 When of this simple form they have been called blood gills, 

 because the distribution of the air absorbed is dependent upon 

 the general circulation of the blood. This organ becomes 

 much more efficient as soon as it is invaded by a tracheal twig, 

 for it then permits the more rapid transfer of the carbonic 

 acid to the water outside it, and the al:)Sorption of the oxygen 

 from the water through the thin layer of blood and the walls 

 of the tracheal twig and the gill itself. The exchange of 

 gases that results tends to approach a condition of equilibrium. 

 Such traclieal gills appear to be very efficient organs of respi- 

 ration. 



The position of a tracheal gill probably depends upon the 

 condition of the insect at the time that the gills are specialized. 



