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 larvae 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 Microbembex 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 (Erax) (Fig. 186) (165) (Promachus vertebratus) (Fig. 

 187) are common; their larvae live in the sand as parasites on other 



