26 
BRITISH GALLS 
“ plurilocular.” The presence of inquilinesmay convert a truly 
monothalamous gall into an apparently polythalamous one. 
It should be noted that these galls may contain, in addition to 
the true gall-flies or Psenides, insects whose parents effected 
a burglarious entrance, and deposited their eggs within the 
young gall. These are known as guest-flies and parasites. 
The guest-flies are of two kinds: 
1. Inquilines .—These are more or less nearly related to 
the rightful occupants of the gall. The larvae live in the 
gall substance, and usually secure the maximum food- 
supply by killing off the owners. 
2. Commensals .—These feed on the gall substance, thus 
depriving the legitimate occupants of a certain amount of 
food-supply, but do not, as a rule, commit murder. 
The parasites devour the larvae and pupae not only of 
the freeholders, but also of both classes of burglars. They 
belong chiefly to the family Chalcididae, small hymenoptera 
with gorgeous raiment, brilliant metallic blues and greens 
being the predominating colours. 
It is obvious that the investigation of the contents of a 
cynipidous gall is one of considerable difficulty. Dr. Sharp 
remarks :* “ It is clear that, as we cannot ascertain what is 
inside a gall without opening it, and thereby killing the 
tenants, it is a most difficult matter to identify the larvae. 
The only safe method is that of observation of the act of 
oviposition; this may be supplemented by rearing the flies 
from galls, so as to ascertain what variety of flies are 
associated with each kind of gall. The last point has been 
well attended to; but the number of cases in which oviposition 
of inquiline gall-flies in the galls formed by the Psenides 
has been ascertained by direct observation is still very 
small; they are, however, sufficient to show that the 
inquilines deposit their eggs only after the galls are formed.” 
A surprising number of insects may be bred from the 
larger Oak galls ; it is on record that thirty different species, 
* “Insects,” in “Cambridge Natural History,” vol. i., p. 532. 
