144 INSECTS 



partially disintegrating the walls of the cell: a heavy 

 frost following heaves the surface, and the water in 

 the soil in freezing breaks up the cells completely, 

 bringing the soil into direct contact with the soft in- 

 sects, crushing or otherwise destroying them. 



A variable winter, therefore, is a hard one on insect 

 life, and during the summer following certain species 

 are likely to be conspicuous by their absence. It is 

 such factors as these that tend to limit a fauna; only 

 species capable of withstanding their variations being 

 capable of continued existence under them. The com- 

 mon and widely distributed species are those that 

 have become adapted to a wide range of tenipera- 

 ture and relative humidity; the others are more lim- 

 ited as to the conditions under which they can exist 

 and die off in proportionately large numbers when 

 conditions are adverse. 



While climatic conditions are important factors in 

 limiting both numbers and distribution, they are per- 

 haps more effective in limiting distribution, since the 

 occurrence of a species within a faunal region presup- 

 poses an adaptation to its normal ranges of temperature 

 and moisture. A more effective agent in limiting num- 

 bers is found in the diseases to which insects are subject, 

 and yet the effectiveness of diseases as a check is in 

 large part due to climatic conditions, most of them 

 developing best or only in moist hot weather. 



Insects suffer severely from epidemic diseases due 

 to micro-organisms — fungus and bacterial — and of 

 these diseases we know, as yet, comparatively little. Al- 

 most every observant individual has seen late in the 

 season, attached to a window pane, specimens of the 

 common house-fly with abdomen distended and a 

 little whitish powder surrounding the points of attach- 

 ment. Such flies have been killed by a disease that is 



