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Life-history 0/Melittobia acasta 
XI. Osmia rufa Larvae and Pupae used as Hosts. 
I have already said something about the apparently indefinite variety of 
hosts which Melittobia will attack but I have had one very curious experience 
in this matter. 
In a large number of my earlier experiments the larvae and pupae of the 
red Osmia (0. rufa) were used because it was one of the commonest species 
at my garden fence and, although Melittobia eggs were laid in the normal way 
upon the resting larvae and pupae in the laboratory, the eggs often failed to 
hatch out and frequently the larvae died before they were full grown, in fact 
it was rather exceptional to rear a Melittobia from egg to adult using this 
species as the host. 
At the time that my garden fence was infested with the parasite I opened 
more than 130 cells of Osmia rufa which had been constructed in tubes placed 
upon shelves attached to the fence and in only one case did I find any sign 
of the Melittobia and in that one cell were two half-grown larvae. And yet 
in the laboratory the resting larvae and pupae of the Osmia were readily 
accepted by the parasite which fed and oviposited upon them—but the 
majority of the brood died. 
Another interesting point in the relationship of Osmia rufa to Melittobia 
is that in the number of cases in which I have allowed one or more of the 
parasites to enter the cell of the growing larva, the parasites have invariably 
perished when the Osmia larva began to spin its cocoon, being caught amongst 
the outer threads—and yet Melittobia , in similar relationship with any of the 
three species of Odynerus with which I have worked, always survived and 
destroyed its host. 
In a number of cases I placed Melittobia females in cells with Osmia rufa 
pupae enclosed in cocoons and in almost every case the parasite failed to 
destroy the bee. In these cases it was evidently because the Osmia cocoon was 
too tough for the parasite to penetrate, since pupae removed from their cocoons 
were invariably oviposited upon and in such cases the female Melittobia lived 
for many weeks, feeding upon the blood of the pupa, while those placed with 
Osmia cocoons died after three or four weeks, evidently from starvation. 
This is the only host which so far I have found, upon which Melittobia 
does not really flourish and it seems as if there is some quality in the blood 
which has a deleterious effect upon the parasite. 
That the toughness of the cocoon should be an additional safeguard is 
worthy of remark since Malyshev suggests that some species may escape the 
parasite because of the nature of the building materials used in the con¬ 
struction of the cells. Thus he mentions some of the Heriades which build 
the cell-walls of resin in which Melittobia gets caught and he suggests that 
Odynerus aljpestris possibly escapes to some extent for the same reason. 
These few facts serve to emphasise the peculiarities of the relationship 
between parasite and host and to give an idea of the apparent triviality of 
