20 IOWA STUDIES IN NATURAL HISTORY 
conditions of drouth either by the development of hairs, thick- 
ened epidermal layer, reduction of the number of stomata, re- 
duction in the number or size of leaves, or a general dwarfing, 
provide fresh living tissue for innumerable leaf and plant-juice 
feeders such as the larvae of Lepidoptera and Coleoptera and 
the Hemiptera. All these in turn permit the existence of a great 
variety of parasitic and predaceous forms. The relatively ten- 
der plants of summer with their seeds for winter consumption, 
together with the fact that the loose soil made easy the construc- 
tion of burrows, summoned the gopher, Franklin’s ground squir- 
rel, and the 13-lined spermophile into the already complex animal 
association. The abundant insect life makes a paradise for such 
birds as the meadow lark and the dickcissel. 
Allwval prairie. The alluvial prairie is represented mainly 
by the short stretch just south of Iowa City and it differs from 
the rolling prairie chiefly in topography and soil. The topog- 
raphy is inclined to be level rather than undulating and the soil 
is usually a very sandy loam. The spermophile prefers the 
higher slopes of the rolling prairie, but the dry land tortoise is 
here met much more frequently. 
The prairie openings. The prairie openings or ‘‘oak open- 
ings,’’ as has been stated, are no longer found in any degree 
sufficiently lke their original condition to warrant a detailed 
treatment in an account of contemporaneous habitats. 
The sand dune prairies. Attention has already been ealled to 
the fact that these sand dunes possess certain characters which 
serve to differentiate them from the surrounding territory and 
thus create conditions favorable for a different set of animal 
associations. 
These dunes have been built of shifting sands, and in one or 
two instanees this shifting is still going on. Dunes which are 
under cultivation have not been considered, and only those were 
mapped which are practically barren at their summits. The 
barrenness is due, of course, to their extreme aridity. This is 
accounted for by their inability to retain moisture through their 
physical nature and also to the fact that they are thoroughly 
exposed to the evaporating forces of extremity of temperature, 
and particularly that of the hot southwest winds of late summer. 
