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 while 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 with 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, 714; 
Table XLVII): 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 XLVII): 
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 
