ON ROCK 



217 



of the segments striped with yellow, is one of the most characteristic 

 of moist woods, while others {Geophilus rubens and Lysiopetalum lac- 

 tarium) are not uncommon. Ground beetles (Calathus gregarius Say) 

 and bugs (Reduviolus subcoleoptratus) occur. In logs of fallen basswood 

 we found the larvae of Tenebrionidae and Cerambycidae and of horntails, 

 the burrowing hymenoptera, and the Mycetophilidae larvae (Sciara) 

 (Fig. 174) (165). 



c) Field stratum and shrub stratum. — The field stratum has been but 

 little studied. We have taken a few Scudderia nymphs, some spiders, 

 and bugs, but no adequate study has been carried on. 



d) Tree stratum. — This has likewise been but little studied, but in 

 these young forests, while the ground stratum is like that in the older 

 forest, the tree stratum is poorly de- 

 veloped because the trees are short 

 saplings. As time goes on, however, 

 the forest becomes more dense. Such 

 a forest may be seen on the bluff at 

 Lake Bluff, 111. 



5. OTHER BARE CLAY FORESTS 



Other bare clay young forests may 

 be seen along the dumps of the drainage 

 and Chicago-Michigan canals at Summit. 

 Here we find practically the same stages 

 as at Glencoe on the lake bluff. There 

 are the steep clay bluffs with no perma- 

 nent residents, the semi-stable bluffs, or 

 weed-occupied areas. These are like the 

 semi-stable bluffs at Glencoe but the tiger-beetle is another species and 

 selects more nearly level places; otherwise it is very similar in habits. 



The shrub stage occurs but is without the snails, since the ground- 

 water level is lower and the moisture in the soil of the lake bluff is wanting 

 here. This causes the development of the ground stratum to lag behind, 

 while it is in advance in the bluff forests. Accordingly we find a sapling 

 forest made up largely of cottonwoods. This has not been studied. 



Fig. 174. — One of the fungus 

 gnats {Sciara sp.) the larvae of 

 which are commonly found under 

 the bark of trees, feeding on fungus. 



III. Forest Communities on Rock 

 (Station 55) 

 The rock exposures near Chicago are not numerous, and we have 

 studied only those at Stony Island. There the bare rock is inhabited 



