356 PRINCIPLES OF SOIL MICROBIOLOGY 



INSECTA 



The term "soil insect" comprises all insects which, at one time or 

 another in the course of development from the egg to the imago, spend 

 some stage or stages of their life-histories either on or beneath the 

 surface of the soil. 43 A great many species of insects, found in most 

 orders, are associated with the soil in one or more stages of their de- 

 velopment. As a matter of fact, it has been stated that as many as 

 95 per cent of all insect species invade the soil at some stage of their 

 development. Millions of insects are found in every acre of arable 

 land. 



On the basis of their feeding habits, the soil insects can be divided 

 into six groups: 44 



1. Those feeding on subterraneous parts of plants, as the larvae of Melolontha, 

 Agriotes and Tipula. 



2. Those living saprophytically in the soil, as Collembolla and larvae of Dip- 

 tera and Coleoplera. 



3. Those living on other members of the soil fauna, or predaceous species, 

 as the Carabidae and many larvae of Diptera. 



4. Parasitic species, as the Hymenoptera and the Tachinidae, which pass their 

 larval stages on or within the bodies of other organisms. 



5. Insects which find their habitat in the soil, without seeking a food supply 

 there, as in the case of ants. 



G. Insects which only undergo pupation in the soil, as in the case of the Lepi- 

 doptera. 



A survey of the insect fauna in cultivated and pasture lands revealed 

 the fact that the distribution and numbers of the soil fauna are more stable 

 on grass than on arable land. 45 This is due to the fact that grassland 

 bears a vegetative covering all the time, which offers food for the 

 fauna. In grass land, hibernation can proceed normally. Cultiva- 

 tion of land brings the fauna to the surface exposing it to harsh climatic 

 conditions and to bird attack. As vegetative growth increases, there 

 is a corresponding increase in the fauna in both arable and grass land. 

 Conditions in winter and early spring are detrimental to the soil fauna. 



The fauna of arable land consists of species which have passed the 

 winter in the soil and those which have migrated or are introduced 



43 Cameron, A. E. Soil insects. Science Progress, No. 77: 92-108. 1925. 



44 Imms, A. D. The invertebrate fauna of the soil (other than protozoa). 

 In book by Sir E. John Russell et al. The microorganisms of the soil. Long- 

 mans, Green & Co. 1923. 



46 Buckle, P. A preliminary survey of the soil fauna of agricultural land. 

 Ann. Appl. Biol., 8: 135-145. 1921. 



