NON-PROTOZOAN FAUNA OF SOIL 359 



secta, Arachnida and other Arthropoda, Annelida and Gastropoda in an 

 acre of forest soil. 



A detailed study of the subterranean aphids has been made by Cut- 

 right. 49 



MOLLUSCA 



The molluscs possess a soft body encased in a hard shell consisting 

 either of one part (Gastropoda snails) or two parts (Lamellibranchiata) . 

 The soil molluscs include the snails and the slugs, which frequently 

 leave the soil which they inhabit for feeding purposes in the presence 

 of sufficient moisture. Most of these forms usually consume vegetable 

 matter, while some (Testacella) are carnivorous. France* found in the 

 soil the genera Carychium and Helix. 50 



Influence of environmental conditions on the invertebrate fauna of the 

 soil. Larvae of soil insects are very sensitive to evaporation of mois- 

 ture, especially at 20°C. or over. 51 They do not occur in dry exposed 

 soils, but rather in most soils where the humidity is not far below 

 saturation and the temperature seldom goes above 20° to 23°C. Below 

 8°C., most soil insects become inactive. Hibernating soil insects pos- 

 sess a great capacity of resisting freezing temperatures. They migrate 

 vertically, according to season, especially in cold climates. 52 The 

 invertebrate fauna of the soil is generally much more abundant in 

 heavy than in light sandy soils because of the moisture conditions. 53 

 According to Morris, the depth to which insects penetrate into the soil 

 is due to depth of food, aeration, moisture and soil temperature; in- 

 sects are found, therefore, at greater depths in arable soil than in pas- 

 ture land. 



The addition of organic matter to the soil increases the moisture 

 holding capacity of the soil and offers food material for many species. 

 Different soil insects respond differently to varying degrees of humidity, 



49 Cutright, C. R. Subterranean aphids of Ohio. Ohio Agr. Exp. Sta. Bui. 

 387: 175-238. 1925. 



60 The influence of soil reaction upon snail development is discussed by Atkins, 

 W. R. G., and Lebour, M. V. Soil reaction, water snails, and river flukes. Na- 

 ture, 11: 83. 1923. 



61 Hamilton, C. C. The behavior of some soil insects in gradients of evaporat- 

 ing power of air, carbon dioxide and ammonia. Biol. Bull. 32: 159-182. 1917. 



62 Griddle, N. The habits and control of white grubs in Manitoba. Agr. 

 Gaz. Canada, 5: No. 5. 1918. 



63 Ramann, E. Regenwurmer und Kleintiere im deutschen Waldboden. Int. 

 Mitt. Bodenk., 1: 138-164. 1911. 



