INSECT ECOLOGY 367 



crease the growth of the plant and lead to the production of a larger 

 number of squares which may become infested. Driving rains knock 

 off infested squares, and by softening and moistening the food hasten 

 the development of the larvae within. Squares which are already upon 

 the ground are protected during rainy weather from sunshine and dry- 

 ing. Rain hinders the enemies of the weevil far more than it does the 

 development of the weevils themselves. On the other hand, it seems 

 probable that as many of the hibernating weevils perish from frequent 

 wetting as from exposure to the cold. (Hunter and Hinds.) 



Frequent heavy rains in spring reduce greatly the numbers of 

 immature chinch bugs. 



Moisture increases the mortality of insects indirectly by favoring 

 the growth of parasitic fungi or bacteria. Thus, in moist weather 

 chinch bugs may be almost exterminated by the fungus Sporotrichum, 

 as described with other examples in a preceding chapter. (See 

 page 218.) 



This chinch bug fungus will not grow in a relative humidity of 90 

 per cent, or less, but will remain dormant in the spore stage for an indefi- 

 nite period (more than eighteen months, in dryness.) The fungus can 

 hardly have too much moisture in a state of nature; dashing and wash- 

 ing rains serving merely to distribute it. (Headlee and McColloch.) 



COMPOSITION 



The fact that animals require oxygen for respiration, and give off 

 carbon dioxide, while plants utilize carbon dioxide and set free oxygen, 

 need only be alluded to. 



The chief constituents of air are oxygen, nitrogen, carbon dioxide, 

 water-vapor in varying amounts, with small quantities of gaseous 

 ammonia and hydrogen dioxide, and extremely small amounts of argon. 

 The organic matters present, as bacteria, spores of fungi, pollen grains, 

 etc., are highly important biologically. 



In atmospheric air there are very nearly three parts of nitrogen to 

 one part of oxygen, whether by volume or by weight; with slight varia- 

 tions in these proportions. 



The carbon dioxide is present in relatively small quantity, about 

 three parts in ten thousand, the proportion varying according to the 

 locality and season; being greater in cities than in the country; in sum- 

 mer than in winter; in warm climates than in cold; in lower altitudes than 

 in higher; and " greatest near the ground where decomposition is taking 

 place." 



