118 



The prevailing condition is therefore one of dense vegetation. I know 

 of no detailed study of the amount of life which develops in this layer 

 of prairie vegetation. For this reason certain observations made in 

 meadows and pastures are of interest. McAtee ('07) examined a 

 grassy meadow and the surface of the soil for bird food, and a corre- 

 sponding area of four square feet of a forest floor. He concluded that 

 the population in a meadow is much more dense than that in a forest. 

 This conclusion, however, is not valid, as Banks ('07) has pointed out, 

 because the two areas are not strictly comparable ecologically. In the 

 meadow life is concentrated near the surface; in the forest it is 

 largely in the trees and not on the forest floor. Clearly the ecologically 

 comparable areas of the open and the forest are their subsurface soils, 

 the surface soil and the layer of vegetation, and the space above the 

 vegetational layer. As previously pointed out in this paper, the forest 

 should be looked upon as a very thick layer of vegetation. Another 

 estimate of the population of pasture vegetation has been made by 

 Osborn ('90, pp. 20-23). This is a rough estimate, but it shows that 

 there were about one million Jassidcr present per acre. He further 

 estimated that that the amount of vegetation per acre eaten by insects 

 amounted to about one half of that eaten by a cow. This example aids 

 one in understanding how it was possible for the insects of the origi- 

 nal prairie to influence the amount of food available for the buffalo, 

 particularly during dry seasons when there was limited grass growth, 

 and when grasshoppers throve in large numbers. In this layer of vege- 

 tation, in addition to the general feeders, eating almost any kind of 

 vegetation, there is a rather extensive population which has a restricted 

 diet, feeding upon a single food plant, or on only a few species. There 

 are a number of cases where, though an insect has several food plants, 

 all, or nearly all, belong to the same plant association, and often have 

 much the same geographic range. A good example of this among 

 prairie animals is the case of the plant-louse Macrosiphum rudbeckice 

 Fitch, which lives on a variety of prairie plants ; as Vcrnonia, Solidago, 

 Bidcns, Ambrosia, Cirsium, Silphium, and Lactuca (Cf. Hunter, '01, 

 p. 116). The beetle Chrysochus and the bugs Lygccns kalmii and On- 

 copeltus fasciahts are often found on Asdepias and Apocynum; Aphis 

 asclepiadis lives on Asclcpias and on Euphorbia, Though pollen- and 

 nectar-feeding insects often forage over many kinds of plants, some 

 of them have clearly defined preferences, almost amounting to limita- 

 tion to a single food plant. Thus the bee Mclissodcs obliqna seeks 

 pollen largelv from LcpacJiys pinna ta, and the Pennsylvania soldier- 

 beetle, though very abundant on flowers, is not numerous in corn 

 fields even when pollen is excessively abundant. 



