192 WET FOREST COMMUNITIES 
The insects feeding on the leaves are numerous on all the trees. The 
following are common to the three trees mentioned (137): the cater- 
pillars of the hickory tussock-moth, the American dagger-moth, the forest 
tent caterpillar, the white-marked tussock-moth; each has a preference 
for one of the trees. The larvae of several other common moths occur 
on two of the trees, a few are confined to one. Beetle and sawfly larvae 
also attack the leaves. Each tree has its characteristic gall insects and 
galls; for example, on the elm, the coxcomb gall (Colopha ulmicola Fitch), 
on the ash, the midrib gall (Cecidomyia verrucicola O.S.). These are 
believed to be confined to particular tree species. 
According to Wood (21) such forests are the chief haunts of the gray 
squirrel. The green heron is especially likely to nest on the low trees 
of such a forest if they are near water. 
2. OTHER TYPES OF SWAMP FOREST COMMUNITIES 
The swamp forest formation is well developed in the Skokie marsh 
area. We have visited these woods at a point west of Dempster Street, 
Evanston. This was originally characterized by trees very much larger 
than those at Wolf Lake. The soil at Wolf Lake is sand, while that at 
Evanston is clay, which is probably more favorable for trees. However, 
the most important cause of the greater luxuriance is greater age. 
The subterranean stratum has not been studied. 
The ground stratum: Here we find, in addition to those species 
of the temporary ponds at Wolf Lake, a snail (A plexa hypnorum Linn.) 
which is characteristic of very transient ponds (100). 
On November 27, 1903, the condition of the animals of this stratum 
was noteworthy. In the lower moister parts of the wood we found the 
mollusks, especially Pyramidula alternata, in groups under logs. One of 
these groups contained 12 individuals. Under another log was a group 
of about 50 ground beetles (Platynus sp.). Under one small piece of 
bark were found three ground beetles, three rove-beetles, one slug, and 
two snails. Under another, one tetrigid or grouse locust, several ground 
beetles, and a rove-beetle. Under the bark of a log on the above date 
we found the hibernating parasitic hymenoptera (Jchneumon extrematatus 
Cress., galenus Cress. and mendax), also a queen white-faced hornet 
(Vespa maculata), which with its colony builds a large spherical nest 
in a tree in summer. 
Most noticeable of all was a group of several hundred small blue 
chrysomelid beetles (Haltica ignita Illig.). They were under the leaves 
at the base of a tree down the sides of which individuals of the same 
