274 KANSAS UNIVERSITY SCIENCE BULLETIN. 



sixteenths to five-eighths of an inch. In two cases the cells 

 were lengthened and the ends rounded to such an extent that 

 they had the shape of ellipsoids. 



The cells and burrows of 0. arvensis were roughly exca- 

 vated. The walls were packed a little, but are not always 

 smooth as in the case of 0. papagorum. They never formed a 

 layer of earth around the nest distinct from the surrounding 

 earth. 



The tube which 0. arvensis built over its nest was an erect, 

 or bent, cylindrical, earthen chimney. In length it varied from 

 one to one and one-fourth inches. The interior diameter of 

 the tube at its base is about the same as that of the burrow, 

 one-fourth of an inch. As the wall decreases in thickness 

 toward the terminal end of the tube, these inside measure- 

 ments of the tube grow larger. The tube has a substantial ap- 

 pearance. At the base the walls are about one-eighth of an 

 inch in thickness. The tube wall is solid, not a network as the 

 tube of 0. papagorum. Its exterior has a granular appear- 

 ance. The shorter tubes are generally strai^fht, while the 

 larger ones are bent. 



In spite of its solid appearance, this tube is only a temporary 

 structure. A light rain will dissolve it. If it escapes the rain, 

 Mrs. Wasp tears it down when the nest is finished and stuffs 

 it into the burrow. Thus the burrow, which could easily be 

 betrayed by the large tube, is quite securely hidden. The tube 

 is probably a temporary defense against parasites while the 

 provisioning of the nest is in progress. 



The nest-building I observed on five occasions, and in its 

 very beginning twice. In a dry bed of an intermittent stream 

 in Russell county, I noted a black Odyncnis make a "locality 

 study" — a few irregular circles over an area about eighteen 

 inches in diameter — and then fly away. In less than two min- 

 utes she was digging in a spot over which she had been flying. 

 She had moistened the spot and was working with her fore 

 feet and mandibles. The excavated earth was used in tube- 

 building. She worked much as did O. papagorum, putting the 

 freshly excavated earth in place with her mandibles, then 

 working with her mandibles and fore feet inside the tube, 

 while the tip of her abdomen pressed against the outer wall. 

 After the earth was in place she would quickly run down the 

 burrow again for another load. Every three minutes, as regu- 

 larly as if she had timed herself by a clock, she went to the 



