Formation of the Cells of Bees and Wasps. 121 
the cell, which angles disappear towards the base of the cell, as 
in the other, much more common, form of half-constructed cells. 
The other cells of the external series are Jess advanced and show 
no trace of angles externally, the outline of the part not in contact 
with other cells being, in a horizontal section, in the form of a 
segment of a circle. ‘These latter cells have not attained their 
fuil diameter. 
I have now to refer to some other specimens which I exhibit; 
and, first, to two cells, which were constructed by a bee—Osmia 
leucomelana. They are formed of mud; and each cell is built 
separately. The female bee having deposited a small pellet of 
mud in a sheltered spot, between some tufts of grass, imme- 
diately commences to excavate a small cavity in its upper sur- 
face, scraping the mud away from the centre towards the margin 
by means of her jaws. A small shallow mud-cup is thus pro- 
duced. It is rough and uneven on the outer surface, but beau- 
tifully smooth on the inner. On witnessing thus much of the 
work performed, I was struck with three points: Ist, the rapi- 
dity with which the insect worked; 2ndly, the tenacity with 
which she kept her original position whilst excavating; and, 
3rdly, her constantly going over work which had apparently been 
completed ; at every fresh addition of material brought up to 
the margin of the cell, the bee worked back again in the cavity 
which was already rendered smooth. ‘To continue the cell, more 
material has to be sought for. It is added to the margin of 
the little cup first scooped out, and is wrought, by scraping, 
on the inner side; and here, again, it is certain that the bee 
does not entirely confine the work of her jaws to the additional 
material, but again passes them over the old work, and hence 
the part of the work already completed has a certain influence 
upon that which follows. The curves first formed have an in- 
fluence upon those that follow, and, if continued, they would 
return again into themselves; that is to say, the cell would be- 
come a hollow sphere, but the tendency to this form is partially 
overcome by the fact that the cell has to be constructed by exca- 
vation, and by an insect which, when at work, is on the outside of 
it. The effect is, that it gradually passes from the spherical 
towards the cylindrical form, but never quite attains the latter, for 
the sides gradually close in, and ultimately the aperture becomes 
too contracted for the bee to insert her head and to carry on the 
work as before. Still fresh mud is added to the margin of the 
cell, but only to the inner edge, and this is continued horizontally, 
