222 DRY AND MESOPHYTIC FOREST COMMUNITIES 
feed on decaying wood (Fig. 178) and make their way to the under side 
of wood lying on the beach (Fig. 179). The bank swallow often nests 
in the sides of vertical sandbanks. Under the driftwood we find the 
scavengers and predatory species of the preceding belt. They spend 
their time here when the beach is not well covered with food. The 
sand-colored spider (Trochosa cinerea) (138) is a regular resident. The 
common toad finds shelter beneath the driftwood during the day, going 
forth in search of food at night. After sleeping near the beach one night 
we found the sand about where we had lain crossed and recrossed by the 
tracks of the toads and other smaller animals, such as beetles, spiders, 
etc. The toad finds food abundant near the shore. The white-footed 
mouse occasionally nests here under the largest driftwood. The spotted 
sandpiper and piping plover nest here occasionally. 
b) Field stratum.—There are occasionally very young seedling 
cottonwoods. Sea rockets and some other plants grow in this belt. 
Occasionally we find the jarvae of a cabbage butterfly (Pieris protodice 
Bdv.) (171) on the sea rocket (Figs. 176, 177). There is no shrub or 
tree stratum. 
3. THE WHITE TIGER-BEETLE OR COTTONWOOD ASSOCIATION 
(Stations 57, 58, 59; Tables L, LVI, LVII) 
(Fig. 180) (115) 
This begins with the line of young cottonwoods which we see in 
Fig. 175. The beach belt sometimes overlaps it because the large 
driftwood is sometimes mixed with the cottonwoods. The cottonwood 
belt is underlaid by the two preceding, and has succeeded them. 
a) Subterranean-ground stratum.—Here the white tiger-beetles (Figs. 
181, 182) reach their maximum abundance and the openings of their 
cylindrical burrows are numerous; the termites continue wherever there 
is wood for them to feed upon; the burrowing spider is commoner 
here than in the preceding zone (172). This is pre-eminently the zone 
of digger-wasps (173). Here the holes of Mucrobembex monodonta 
(Fig. 183) are numerous. This species is somewhat gregarious, the bur- 
‘rows usually being in groups. They probably store their nests with flies 
secured often from the beach. Another larger bembex (Figs. 184, 185) 
(B. spinolae) also stores its nest with flies. Anoplius divisus, the 
black digger, stores its nest with spiders. The velvet ant (Mutilla 
ornativentris) is present. Dielis plumipes appears in May and lays its 
eggs in the sand. 
The robber-flies (Zrax) (Fig. 186) (165) (Promachus vertebratus) (Fig. 
187) are common; their larvae live in the sand as parasites on other 
