CHARACTERS OF COMMUNITIES 251 



Field and shrub strata: The field and shrub strata of the cottonwood, 

 pine, and oak communities are less easily characterized. The cotton- 

 woods of the beach are far less commonly infested with aphid galls than 

 are trees of the same species growing in less exposed situations. Further- 

 more we have never found any of the lepidopterous larvae such as 

 Basilarchia archippus Cram, near the beach. Animals living exposed 

 upon the trees are few in number. The same general conditions obtain 

 on and among the pines but spiders are more numerous. On the black 

 oak the number of phytophagous animals is increased and the number of 

 galls appears to be greater than in the later stages; the inhabitants of 

 the herbaceous vegetation are chiefly those found in open situations such 

 as prairies and roadsides, where the physical conditions are similar. 

 Some animals of the same species which make up the black-oak com- 

 munity were taken from a roadside, and after being mixed with the 

 inhabitants of the shrubs of the beech forest were placed in a light gra- 

 dient. Soon the insects and spiders of the two communities separated 

 sharply from each other, the beech-inhabiting species going to the dark- 

 est end while the roadside species crowded to the light. 



b) Later communities. With the coming-in of red oak, true forest 

 with the mineral soil largely covered with humus and leaves is present 

 and very different mores obtain. The diurnal diggers are practically 

 absent. Snails, beetles, grasshoppers, spiders, and myriopods living 

 under bark, decaying wood, and leaves, avoiding strong light and 

 requiring moisture, are the chief types. The mores are typically forest 

 in character. The differences between these and the later stages are 

 those of detail and degree. In general with a lessening in the severity 

 of the conditions and an increase in the denseness of vegetation, there is 

 a proportional increase in the use of the vegetation as a place of abode. 



In the field and shrub strata, we note that the animals of the cotton- 

 wood, pine, and oak stages are characteristic of open dry situations, 

 requiring or tolerating strong light, while those animals of the red-oak, 

 hickory, and beech stage are negatively phototactic to light of the same 

 intensity, as shown by mixing the animals in a gradient. 



The animals of the tree strata frequent a limited number of kinds 

 of trees. Tree inhabitants are few and scattered in the cotton wood ? 

 pine, and black-oak stage while animals inclosed in galls or cases are 

 common, if not dominant. In the red-oak, hickory, and beech stage 

 phytophagous animals are often gregarious and numerous. Groups such 

 as Orthoptera, beetles, bees, and wasps are represented more and more 

 by species which make use of the vegetation as forest development 

 goes on. 



