300 Insect and other Invertebrate Fauna 



Effects of a similar nature due to the ploughing were observed in 

 some other groups, while with others, such as the Acarina, the effect 

 was very little marked, as they appeared to regain the upper layers 

 after being buried by the plough. 



Although the percentage at the different depths varied somewhat 

 between the two plots, the general distribution of the insects, etc., was 

 very little different in one plot from that in the other. 



No seasonal variation in the distribution in depth of the soil fauna 

 was observed. ' 



12, Comparison with Soil Fauna of Pasture Land. 



It is not possible to compare very fully the soil fauna found in the 

 present investigation with that previously found in the examination of 

 permanent pasture (lO) owing to the considerable difference in the con- 

 ditions under which it was existing. The localities in which the work was 

 carried out are widely separated, being in Hertfordshire and Cheshire 

 respectively, and the soil and weather conditions differ considerably. 



In pasture land few insects were found at a greater depth in the soil 

 than two inches, and none at a greater depth than six inches. The depth 

 to which insects penetrated into the soil was considered to be chiefly 

 influenced by four factors — depth to which their particular food occurs ; 

 aeration; moisture; and temperature of the soil. It was shown that in 

 permanent pasture these four factors all tended to restrict the insects 

 to the superficial layers of soil. 



In the present instance these four factors influence the fauna differ- 

 ently, owing to the field being under cultivation. The periodical turning 

 over and stirring of the soil makes it fairly certain that the soil, to the 

 depth to which the implements of cultivation penetrate, will be fairly 

 uniform in composition, and the aeration and drainage of the soil will 

 be more favourable owing to its greater looseness. 



In arable soil the conditions are thus much more favourable to deeper 

 penetration by the insects. The number of insects in the control plot is 

 less than was found in the pasture (3,586,088 per acre), but the number 

 in the manured plot is considerably greater. 



13. Relation of Soil Fauna to Soil Nitrogen. 



In order to determine the importance of the soil fauna as a reserve 

 and source of nitrogen, the nitrogen content of several groups of insects, 

 etc., was estimated, and from these figures it is possible to obtain an 

 estimate of the amount of nitrogen in the whole fauna. 



