400 . ENTOMOLOGY 



Western Xerophytic Evergreen Forest Formation. A dwarf and 

 open form of semi-forest that characterizes the edges of the preceding 

 formation. Pinon Association. The locust, Trimerotropis cyanea, is 

 especially at home here. The hemipteron, Peribalus limbolarius, is 

 very abundant on blossoms of Yucca. The skipper butterfly Epargyreus 

 tityrus seems to belong here. Cedar Association. The tarantula killer, 

 Pepsis formosa, is particularly abundant here. On milkweed, which is 

 more abundant here than elsewhere, is the cerambycid beetle, Tetraopes 

 iemoratus, and the hemipteron, Lygaus turcicus. 



Desert-grassland Transition Formation. Intermediate between 

 the Grasslands to the east and the Desert regions to the west. Opuntia 

 arborescens Society. Several insect species are quite characteristic of 

 this society. The nitidulid beetle, Carpophilus pallipennis, eats the 

 pollen and petals of the Opuntia and every blossom commonly shows 

 from a dozen to a hundred or more individuals. The peculiar ceram- 

 bycid beetle, Presmis pocularis, and the next insect are chiefly respon- 

 sible for weakening and killing the cactus plants. The beetle apparently 

 never leaves the plant, and its wings are degenerate. The female is 

 usually seen carrying her much smaller mate. The larvae bore in the tis- 

 sues of the plant. The cicada, Cacama valvata, is very abundant on the 

 tree cactus, but, unlike the last, also occurs on the prickly pears. The 

 loud calls of the males are heard on every hand when the sun is shining. 

 But let a cloud obscure the sky for a moment and all is hushed. The 

 larvae feed on the roots of the cactus. Shortgrass Association. The mesa 

 grasshopper, Trimerotropis vinculata is extremely numerous, ascending 

 even into the yellow pine association. It is very variable in color and 

 the variations have a very definite relation to that of the ground around 

 them, being very light on sandy soil, mottled on pebbly hills and darker 

 among the pines where there is more vegetation. The species migrates 

 in large numbers when the rains cease and the grasses on which it feeds 

 dry out. Many species of insects inhabit the mesa. Here lives the 

 harvesting ant, Pogonomyrmex occidentalis (see page 297). It is well 

 known that these ants bring their stores of grain out to air occasionally. 

 Professor Watson relates that "One somewhat windy day in September, 

 a hill was visited in which part of the ants were busily engaged in 

 bringing out the grain to air and others were as busily engaged in 

 carrying it back again. One ant would drop a grain and at once start 

 back without a load into the hill for another, whereupon the grain 

 would be at once seized by another ant and carried back into the gran- 

 ary. It is possible that this treatment is what the grain needed but it 



