44 
DR ALFRED E. CAMERON. 
temperature, soil, wind, precipitation, pressure, altitude, exposure, slope, surface 
(cover), and animals. To these are added gravity and polarity, which are practically 
uniform for all habitats. In their total effect they represent the controlling forces 
of the environmental complex, and it is often difficult to attribute to each one its 
proper rank and value in determining the facies of the plant or animal association, 
on the latter of which they also have a more or less direct influence. 
The water-content of any soil is in inverse ratio to its fineness of texture, and is 
also for any particular area dependent upon the degree of humidity which prevails 
and upon the amount of rainfall at any given time. The question of soil-moisture 
has been already discussed in a previous paper # with regard to its relationship to 
soil-inhabiting insects. The author indicated its importance as bearing upon their 
welfare. In the same connection the questions of temperature and soil ventilation 
were also amplified. 
It is very doubtful whether such factors as altitude and pressure in any given 
region will be found to exert a measurable stimulus upon insect behaviour. In the 
first place, it is very difficult to analyse the real effects of altitude. To some extent 
it is associated with rainfall, but only in an indefinite degree. Clements, who has 
just been quoted, says (p. 18 ) that its mfluence, so far as the plant is concerned, is 
really pressure, and that, in consequence, its effect is exerted upon the climatic and 
not the edaphic factors of the habitat. Therefore so far as the present study is con- 
cerned, the consideration of altitude and pressure may be neglected. Daily records 
have, however, been kept of the barometric pressures. These have been averaged 
(cf. Table XII) for the various months during which the investigation lasted, in the 
hope that they might prove useful for comparison with the pressures prevailing in 
regions of lower or higher altitude. Thus, perhaps, an approximation to the proper 
value of pressure in influencing the composition of animal associations may be 
derived. It is a well-known fact that many insects inhabit only localities of high 
altitude, but as to whether this phenomenon can be directly assigned to the decreased 
density of the atmosphere is a matter of conjecture. 
From the standpoint of the animal association the surface covering of the soil, 
its nature and density, is recognised to be of such importance that it is customary to 
speak of insects of forest, of grassland, of corn and other cultivated crops. Some- 
times the mere fact that a particular type of vegetation offers a favourable means 
of shelter and protection from enemies will explain the presence of many insect 
species. Or it may be that predatory habits will induce them to frequent a 
particular locality in their search for food, other than the one in which they breed. 
This fact was boldly confronted in the present investigation. 
Wind as a distributive agent is perhaps more important to the plant than to the 
animal association, and, naturally, where soils are light and loose it may be quite 
effective in modifying the surface topography. In the Holmes Chapel locality its 
* Came eon, A. E., “ General Survey of the Insect Fauna of the Soil,” Journ. Econ. Biol., vol. viii, p. 189. 
