350 ENTOMOLOGY 



for tanning leather, succeeded in growing the pitcher plant, Sarracenia 

 purpurea, and in raising thereon a species of rare moth, Papaipema 

 appassionato,, known for thirty years only by a unique type in the 

 British Museum. 



Plants of alkaline desert soils have their characteristic insect fauna. 



Air. The oxygen content of subterranean air is important directly 

 for respiration; indirectly for its effects on vegetation. Aeration of the 

 soil is essential to subterranean life. In too compact a soil insects 

 suffer from lack of oxygen and excess of carbon dioxide, and may 

 experience also the effects of excessive evaporation and mechanical 

 difficulties in burrowing. 



Water. For their welfare, soil insects must have neither too much 

 nor too little water. They may be drowned by gravitational water, 

 which acts also by filling the air spaces around soil particles. Sub- 

 mersion was used effectively in France against the destructive Phyl- 

 loxera of the grape. Capillary water is, on the contrary, favorable to 

 the insect life until it evaporates to excess. As with terrestrial insects, 

 the vital effects of water and temperature are produced through 

 evaporation, in relation to which soil forms exhibit various adaptations. 

 As the soil dries, ants dig deeper. The depth of pupal chambers, 

 their compact waterproofed walls, and the air space around the pupa, 

 and the closing of the entrance to the burrow, all tend to protect the 

 pupa from undue loss of bodily moisture. 



With the tiger beetles, the amount of moisture determines whether 

 eggs are to be laid, and their number if laid; eggs being absent in dry 

 soil (Shelf ord). 



Temperature. There are great differences in the temperatures of 

 different soils, from dry sands to moist shaded humus. Temperature 

 and moisture determine largely the character of the flora and of the 

 accompanying fauna. The greater the depth of soil the lower the tem- 

 perature as a rule. Ants and other insects will dig deeper to avoid heat 

 as well as dryness. Ants often find suitable conditions of temperature 

 and moisture under stones or logs. 



In the case of insects that are said to be killed by heat, the mortality 

 is due primarily to evaporation and secondarily to the coagulation of the 

 protoplasm; perhaps also to ultra-violet rays. 



Physiographic Conditions. As environmental factors there must 

 be considered also the nature of the surface of the soil as regards exposure 

 or cover, the slope of the soil, and the altitude at which it is found. 

 All these things affect the fauna. Angle of slope has an effect in 



