BEHAVIOR OF SOME SOIL INSECTS. 179 



This concentration for different kinds of soil is shown by the 

 following table by Bassungault and Lewy (Van Rise, '04). 



COs in 10,000 

 Character of Soil Air. Parts by Weight. 



1. Sandy subsoil of forest 38 



2. Loamy subsoil of forest 124 



3- Surface soil of forest 130 



4. Surface soil of vineyard 146 



5. Pasture soil 270 



6. Rich in humus 543 



From this we see that the amount may reach as high as 5 or 6 

 per cent, in some situations. 



Little is known concerning the occurence of ammonia in soil 

 air. It usually occurs in very small quantities but may occa- 

 sionally be noticeable in places where there is much decaying 

 organic matter. The small amounts of ammonia which may 

 occur in the soil are usually held in electrolytic combination with 

 the soil itself and do not occur free in the air. 



The above is a very brief survey of some of the physical con- 

 ditions which must be encountered by soil inhabiting insects. If 

 we return to the experiments we will notice that the larvae were 

 very sensitive to evaporation, and especially so if the temperature 

 was about 20 C. or above. This is what could be expected, since 

 in their natural habitat the relative humidity of the air, in moist 

 or wet soil, is not far below saturation and the temperature of 

 the soil probably rarely goes above 20 to 23 degrees C., and then 

 only in exposed, dry, hard soil in which one would not find the 

 larvae. It will also be noticed that of those larvae which had been 

 outside in freezing and thawing weather, the one experimented 

 with at the lowest temperature behaved more normally and 

 showed a greater tendency to select air probably suitable to 

 its physiological condition. An increase in the rate of air flow, 

 other conditions remaining the same, did not seem to affect the 

 larvae as much as an increase in temperature or a decrease in the 

 relative humidity. They were probably affected more by the 

 breathing in of the dry air and an increase in the temperature 

 than by evaporation from the body surface. Their almost in- 

 stantaneous detection of dry air and a high temperature would 

 seem to point to this conclusion. 



