FOSSORIAL HYMENOPTERA 
457 
Apparently the cell was then finished, for she did not return 
for half an hour, when I went out. She had made eighteen 
journeys, and the time taken by each complete cycle of arriving 
with a pellet at the nest, building up the material, flying away, 
collecting another pellet and arriving with it, averaged only 
3*6 minutes, varying from two to seven. There are, I think, 
few bees that could compare with that. The completed 
cell is stuffed with spiders ; but the amount of food provided 
varies very greatly — some larvae have a superabundance, 
while others scarcely seem to have sufficient. I have counted 
the number in twenty-nine cells — it varied from one to nineteen. 
The single one was only a very little larger than others of which 
several may be found in one cell. The average number was 
9*3. In one case five cells were aggregated together, and that 
single Sceliphron had accounted for sixty-eight spiders — even 
if, which is unlikely, those were the only cells she would make, 
or had made, during her life. 
The spiders are of many species — often each one in the 
cell may be a distinct species. They are usually of the web- 
spinning type, with rotund, fat bodies. 
Sceliphron has her enemies, and, curiously enough, the most 
conspicuous is a large ichneumon of the same black and 
canary-yellow colouration, and quite easily mistaken for 
Sceliphron, though its legs are longer. This is almost certainly 
a case of Mullerian, or synaposematic, resemblance — one form 
of mimicry. One would like to say a little more on this 
subject, but this paper is quite long enough. It may just 
be mentioned, however, that synaposematic resemblance is 
beautifully exemplified among Eossors by both Sphegidce and 
Pompilidce, which resemble in colouring the well-known and 
conspicuous black-and-orange Lycid beetles which have been 
proved to be highly distasteful to insect-eating animals. Before 
leaving Sceliphron, I should like again to allude to its extremely 
curious song while working. It must be produced by rapid 
vibration of some parts of the wing, but it is extremely 
difficult to ascertain. Certain flies — Hover- flies or Syrphidce 
— which one often finds on window-panes, produce a high- 
pitched, gradually rising, note by very rapid, but limited, 
vibrations of their wings while they are apparently closed 
