198 WET FOREST COMMUNITIES 



from the rest of the forest, but animals of the forest collect in the 

 ravines in dry seasons and apparently leave the ravines in the wet 

 seasons. 



We have noted that the animal species living at the headwaters of a 

 stream may move inland as the headwaters move inland. This is true 

 of aquatic species. In the case before us none of the species of the young 

 stream are at the headwaters of the older streams because the headwaters 

 of the older streams are in the forest of the upland w r hile the young 

 streams are in the unforested and exposed bluff of the lake. 



c) Flood-plain communities. — In streams not more than a mile long 

 we get suggestions of a small flood-plain near the mouth. Here we find 

 ragweeds and other pioneer plants with their full quota of animals, such 

 as the plant-bug (Lygus pratensis) and other common insects of rank 

 pioneer vegetation; willows w T ith their quota of cecropia caterpillars, 

 viceroy larvae, willow-beetles, etc., are found here as elsewhere. The 

 flood-plains of such small streams are hardly typical because the streams 

 are cutting downward so rapidly. They doubtless possess many special 

 features of interest which are subjects for detailed and special 

 investigation. 



Flood-plain forest is best developed among such streams as the 

 DesPlaines River and Hickory Creek. As the stream meanders from 

 side to side of its valley, it presents points of deposition and erosion. 

 The points of deposition are best for the study of the development of 

 flood-plain forest. 



Girdle of bare sand or gravel (Station 66) : On the wet portions of the 

 sandy margins one finds the ground beetles (Bembidium) (156), some- 

 times toadbugs (p. 180), and more rarely the mole cricket. On the 

 higher and drier portions we have taken the Carolina locust (Dissosteira 

 Carolina) (40) and the two-lined locust (Melanoplus bivittatus) (40) 

 hopping over the ground. 



Girdle of ragweed and helianthus (sub-formation) (Stations 66, 710; 

 Table XL VII) : Here (in September) we found several species of spiders, 

 the meadow grasshopper, long-legged flies, the leaf-hoppers, and the 

 common plant-bug. This girdle is later displaced by willows. 



Willow girdle (sub-formation) (Stations 66, 71a; Table XL VII): 

 When herbaceous plants have grown for a few years they become mixed 

 with willows which are inhabited by animals common in low forest mar- 

 gins. Here (in September) continues the same meadow grasshopper, the 

 same plant-bug of the earlier stage. Two different spiders are recorded 

 (Pisaurina and Epeira). From willows along other streams we have 



