INSECT ECOLOGY 349 



ian one of large particles. Water evaporates more rapidly from 

 >arse soils than from fine ones, but in loose soils the more rapid evapora- 

 tion from the surface forms an aerated " mulch" which retards further 

 evaporation. The different capacities of different soils for absorbing 

 >r retaining moisture affects insects indirectly by its effects on vegetation, 

 >r may affect them directly. The compactness of loose soils varies with 

 le amount of water present, which is of importance to burrowing insects, 

 good example of this is seen in the sandy beach of a lake, when the 

 id wet by waves becomes firm*; the water evaporates rapidly, however, 

 itil the sand is dry again, in proportion to its nearness to the surface, 

 such sand, with frequent alternations from wet to dry, insects do not 

 ive; though some forms, as tiger beetles and beetles of the genus 

 lembidion, burrow in the sand a little farther back from the shore, 

 rhere the fluctuations in the water content are not so great. 



In a loose soil white grubs or wireworms go easily and rapidly from 

 me plant to another. Tiger beetles and ants need soil of a consistence 

 rhich will maintain the burrows after they are made. Caterpillars, 

 ibs, etc., about to pupate, can not burrow in soil that is too hard, and 

 requently avoid also soil that is too loose. The bollworm if unable to 

 lig into hard baked soil will enter cracks in the soil. Some grasshoppers, 

 m the other hand, prefer hard-packed soil in which to lay their eggs. 

 In making their pupal cells in the ground, larvae press the surrounding 

 soil into a compact wall, often adding a cementing fluid which is fre- 

 quently waterproof. The bollworm, or corn ear worm, lines its burrow 

 with silk. 



A soil of loose texture facilitates the emergence of adult insects. 

 If the soil is too hard they may not be able to emerge until it has been 

 softened by rain. Plowing and rolling the soil of a stubble field in 

 summer is known to prevent the exit of Hessian flies. 



The depth to which insects burrow in the ground depends upon the 

 physical nature of the soil, and temperature and moisture as well. 



Chemical Conditions. In addition to oxygen, carbon dioxide 

 and nitrogen, the soil contains other gases and various chemical com- 

 pounds, some of which are essential to plant life and therefore indirectly 

 to the welfare of animals. The character of the vegetation as determined 

 by the acidity or alkalinity of the soil affects the character of the insect 

 fauna. The acid water of bogs is directly unfavorable to insect life, 

 but is favorable to the growth of peculiar plants which are selected as 

 food by certain insects. In New Jersey, Mr. H. Bird, by acidulating 

 soil with an artificial bog water made with the extract of hemlock used 



