26 BRITISH GALLS 



" plurilocular." The presence of inquilines may convert atruly 

 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 eflFected 

 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 ofi" 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 difiiculty. 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 inquihne 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. 



