105 



XVIII, figures 5, 6, and 7) breed in root-bulbs of Scirpus, and that 

 these beetles eat the leaves of Phragmites. Webster ('90, pp 52-55) 

 observed these beetles feeding on the leaves of Scirpus and the larvse 

 feeding on its roots. I have found great numbers of these beetles cast 

 up on the beach of Lake Michigan. Evidently they breed in the 

 swamps about the lake, fall into it when on the wing, and are washed 

 ashore. 



2. The Cottomvood Community 



Ordinarily we are accustomed to think of the prairie as treeless, 

 and yet one large tree was relatively abundant upon the original prairie 

 of Illinois, particularly upon wet prairie, or, when pools were present, 

 even upon the uplands. This was the cottonwood, Popiilus deltoides. 

 These trees were often important landmarks when isolated ; and today 

 the large trees or their stumps are important guides in determining the 

 former extent of the prairie. In the region studied there were no large 

 mature cottonwoods, although saplings were present, but north of 

 Charleston in the adjacent fields mature trees were found. They grow 

 normally at the margins of wet places, as about prairie ponds and 

 swamps, or along the small ill-defined moist sags and small prairie 

 brooks. This tree is usually solitary or in irregular scattered rows 

 when along streams, and does not, as a rule, form clumps or groves. 

 This relatively isolated habit may be a factor in the comparatively 

 small number of invertebrates which are associated with it, or at least 

 in the amount of serious injury which they do to these trees upon the 

 prairie. Many of the larger trees are mutilated, or even destroyed by 

 lightning (Cf. Plummer, '12), and such injury favors entrance of in- 

 sects on account of the rupturing of the thick bark. 



The galls on the leaves and twigs of the trees often attract atten- 

 tion. ^ A large irregular gall on the ends of the twigs becomes conspic- 

 uous in winter. This is formed by the vagabond gall-louse. Pemphigus 

 oestlundi Ckll. (PI. XIX, fig. i) {vagahundiis Walsh, Ent. News, 

 Vol. 17, p. 34. 1906). I have found these galls abundant upon the 

 prairie at Bloomington, 111. At this same locality I found a large 

 bullet-like gall at the junction of the petiole and the leaf — that of Pem- 

 phigus popidicaidis Fitch (PI. XIX, fig. 2), and at Urbana, 111., on 

 other large prairie cottonwoods, a somewhat similar gall, on the side 

 of the petioles, caused by P. populi-transversus Riley (PI. XIX, fig. 3). 

 I have also taken large caterpillars of the genus Apatela on leaves of 

 cottonwood, and September 3, at Urbana, upon its cultivated form, the 

 Carolina poplar, A. popiili Riley (PI. XX, fig. 6). These caterpillars 

 have bodies covered by yellow hair penciled with black. At dusk 

 swarnjs of May-beetles (Lachnostcrna) can be seen and heard feeding 



