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Some seven miles east of Boulder, nestled at the foot of the Colorado Rockies, there rise some sandstone cliffs of peculiar interest to the biologist and the geologist. These are termed the White Rocks in reference to their appearance when seen at a distance. Around them there stretch for miles and miles the fertile farming lands of Boulder County. Thus the White Rocks are isolated from other similar rock formations; and here there lives a bee that has been officially reported from nowhere else in the world. It is Perdita opuntio Ckll. One may find this remarkable bee in the early summer, in the months of May ad June, as it hovers and darts from flower to flower in its tireless search for pollen. And in this one notices a peculiar adaptation. Apparently it visits but one plant species, the bright yellow flowers of the cactus, Opuntia mesacantha Raf., from which it derives its name. How specially adapted this insect is! Not content with limitation to White Rocks, it must also depend on the flowers of but one kind of plant for its existence. 1The writer is indebted to Mr. S. A. Rohwer and Miss Grace Sandhouse. for the determination of specimens; to Dr. Edna Johnson for the identification of some plants; to Miss Helen Mowry and Miss Celia Light for assistance in the photography and for helpful suggestions; to Professor R. D. George for some geological material as well as interesting information concerning tunnelways in the rock formations of different localities; and to Mrs. B. O. Custer, Miss Arline Elftman and Mr. Russel Mann for assistance in the field work. 2Determined by Miss Grace Sandhouse. 3Determined by Dr. Edna Johnson. Synonyms are O. humifusus

In addition to lice, the author has found mites and, in the case of mammals, fleas entangled in the cotton The above method, with the cooperation of ornithologists and Entomologists, ought to stimulate and further our knowledge of this little known group of insects.

It is during the month of August that one can find the wild bee, Perdita zebrata, nesting in the sandy soil at White Rocks near Boulder, Colorado. The nests are usually found together in groups of eight to ten, one female to a nest. Those studied last summer were located in a sandy field sloping to the south at the base of the cliffs.

The nests of the bees of this genus ure generally hurd to find. The insects are so small, usually not more than quarter inch long, that they are difficult to follow in their flight. In fact, so fur as I cu.n tell, no information is available on their nesting habits. It was on fine sunny day in the ltter prt of August, 1927, while visiting the Country Club t Denver, Colordo, that my ttention ws called to some smll mounds of durk soil resembling nt nests. These were located long the side of