112 



gon as Isosoma is to Blymus, but this and other prairie grasses which 

 grow in bunches or stools evidently formed the optimum hibernating 

 cjuarters of these pests when they lived upon the original prairie 

 (Fitch, '56, p. 283; Marlatt, '94a; Schwarz, '05) and upon the sea- 

 shore. Osborn and Ball ('97a and '97b) have listed several grass- 

 feeding Jassidce from Andropogon and Sporobolus. Osborn and Sir- 

 rine ('94, p. 897) found a plant-louse on the roots of Andropogon, 

 and Patch ('12, p. 191) lists Sclihoneura corni Fabr. on A. fitrcatus. 



8. A Milkweed Community 



Bordering the gravelly ballast along the rails north of Charleston at 

 Station I (PI. II, fig. 2) may be seen a large-leaved plant, the common 

 milkweed (Asclepias syriaca). This plant flourishes along the track 

 in many places, and wherever it was found there tended to appear a 

 small but very well-defined animal community. To determine the com- 

 position of this social community, a few collections were made at vari- 

 ous points within Station I. That this milkweed is the hub of this 

 microcosm is clearly shown by the fact that no similar association was 

 found grouped around any other plant in the area, not even about the 

 other milkweeds, A. sidlivantii, or A. iiicarnata. The collections are 

 numbered as follows: Nos. 27-30, 33, 34, and 154. 



The terminal young and tender leaves of the plant are often densely 

 covered with the plant-louse Aphis asclepiadis Fitch (Nos. 28, 29), 

 and these lice are attended by the workers of the ant Formica fusca 

 subsericea Say (Nos. 30, 154). On another plant no plant-lice are 

 recorded, but upon it were found their common enemy, the nine- 

 spotted ladybird, Coccinella g-notata; two species of ants (Formica 

 pallide-fulva schaufussi inccrta, and Myrmica rubra scabrinodis sabu- 

 leti) ; besides, running about on the leaves, the pretty, metallic, long- 

 legged flies Psilopiis sipho (No. 27). They run with a singular rapid 

 glide, stop suddenly for a moment, and then continue their rapid pace. 

 Certain flies of this family are said to be predaceous, but I have never 

 seen Psilopus capture any small animal. On the same plant just men- 

 tioned a small bug, Harmostes reflexulns, was also taken ; and in the 

 flowers of this plant were hundreds of a small dark-colored empidid 

 fly, Bmpis clausa (No. 27). Two other animals were found on this 

 plant; Zonitis bilineata Say (No. 33), and a jumping spider (attid), 

 which had in its jaws what appeared to be the remains of the beetle 

 Diabrotica r^-piinctata (No. 34). Contrary to my usual experience, 

 these plants did not abound with milkweed beetles (Tetraopes) or with 

 the common milkweed bugs (Lygcciis kalmii and Oncopeltus fascia- 

 tns), which are usually numerous. The proximity of the fragrant 



