COTTONWOOD ASSOCIATION 225 
d) Tree stratum.—The cottonwood is attacked by many borers. 
The most characteristic is Plectrodera scalator, which is not common. 
There are few leaf-feeders excepting two gall aphids; the petiole gall is 
due to the work of Pemphigus populicaulis, and the terminal gall to 
Pemphigus vagabundus (137). These occur on the cottonwoods along 
the lake rarely, being more abundant farther inland, where they are 
protected from the severity of winter. The osprey nests in trees, and the 
tree-swallow in the dead ones. 
We have noted that this association often arises through the burying 
of the preceding one. Deposition of 
sand is the chief cause of succession 
up to this point. When cottonwoods 
and grasses begin to grow and digger- 
wasps begin to burrow, organic mat- ¢ 
ter is continually added to the soil. 
The grasses die down from time to 
time, the roots and leaves of the 
shrubs and other plants add humus. 
The myriads of digger-wasps which 
go elsewhere (probably commonly to 
the beach) for the animals with which 
to store their nests add a large amount Hie, 18g The Yone*homed “lovtst 
of organic matter at a depth of afew — (psinidia fenestralis) (after Lugger). 
inches. The grasses bind the dune 
sand; the conditions become favorable for other plants. At such a 
stage the bunch-grass and seedlings of pines appear. 
4. TRANSITION BELT 
(Station 58; Table L) (Fig. 190) (115, 170) 
The stage of mixed pine seedlings, old cottonwoods, and the begin- 
ning of the bunch-grass constitutes a well-marked belt. Along the 
shore, from Indiana Harbor to Gary, there was formerly a ridge upon 
which the lakeward-facing side supported the typical community of the 
cottonwoods and the landward side the transitional belt. When one 
crosses to the landward side of such a ridge he notes a change in the 
animals. The white tiger-beetles and the maritime grasshopper are 
practically absent. Digger-wasps are abundant. The larvae of the 
large tiger-beetle (Cicindela formosa generosa) (Figs. 191-193) with their 
pits and-crooked holes are added, but they rarely invade the dense pine 
areas. Another grasshopper (Fig. 194) (Melanoplus atlanis) and an 
