COTTONWOOD ASSOCIATION 223 
species. Some bee-flies (Exoprospa) (Fig. 188) lay their eggs at the 
entrances of the burrows of Microbembex. The roots of the beach grasses 
are probably attacked by the larvae of snout-beetles (Sphenophorus) 
(Fig. 189) (174) of which several species are very common in the vicinity. 
The white grasshopper (Trimerotropis maritima) (40) and the white tiger- 
beetle (Cicindela lepida) are most characteristic. The long-horned 
locust (Psinidia fenestralis) (Fig. 189) occurs commonly. 
b) Field stratum.—The field stratum is made up of animals that 
occupy the gresses, sagebrush, and a few other xerophytes. Animals 
DIGGER-WASPS OF THE COTTONWOOD OR WHITE TIGER-BEETLE ASSOCIATION 
Fic. 183.—Photograph of a number of the burrows of one of the digger-wasps 
(Microbembex monodonta) at Pine, Ind. 
Fic. 184.—A digger-wasp (Bembex spinolae); about twice natural size. 
Fic. 185.—A sectional drawing of a burrow of the digger-wasp (Bembex spinolae); 
reduced (after the Peckhams, Wis. Geol. and N. H. Surv.). 
are few. An occasional red-legged locust (Melanoplus femur-rubrum) 
occurs here. Midges, mosquitoes, and the flies which breed on the beach 
rest on the leeward side of the grasses (169). Various native sparrows 
are common in fall, feeding on grass and weed seeds. 
c) Shrub stratum.—On the young cottonwoods we find the crab- 
spider (Philodromus alaskensis), often with its appendages stretched out 
on the petiole or midrib of a leaf. The animals feeding on the cotton- 
wood here are few. In early spring the willow blossoms are frequented 
