46 Galls, Gall-makers, and Cuckoo Flies. [Sess. 



ines, but perhaps benefit them. Cetonia and other beetles are 

 present in the oak-apple apparently on some such footing of 

 social and friendly relationship. Various insects also take up 

 their abode in galls which have been vacated, either for shelter 

 or to feed on the gall substance. Commensals in galls offer 

 a striking analogy to those constantly observed in the nests 

 of some species of ant. Many kinds of beetles live unmolested 

 in the galleries of these ants ; some of them are supposed to 

 act as scavengers, others, like the blind Claviger, are fed by 

 the ants and apparently kept by them very much as we keep 

 our domestic pets. 



Facts like the foregoing explain why, when the attempt is 

 made to rear the insects from their galls, many different kinds 

 may emerge from the same gall. From a willow gall one 

 observer obtained sixteen different insects, preyed upon by 

 eight others, twenty-four in all, representing eight orders. 

 Another entomologist, from a quantity of oak-apples, succeeded 

 in raising no less than seventy-five different species, more than 

 half of them being parasitic Hymenoptera. The insects in- 

 habiting galls are thus of the most varied description. When 

 it is desired to rear them, a convenient method is to place the 

 leaf or shoot which bears the galls in a wide-mouthed bottle, 

 having damp sand in the bottom, and to cover it with muslin. 

 For this purpose the more advanced galls must be selected. 



The study of gall-making insects is still further compli- 

 cated by the circumstance that many of the Cynipidae are 

 dimorphic ; forms which were originally described as distinct 

 species turn out to be only successive phases in the life-cycle 

 of the same insect, a state of things exactly similar to that 

 prevailing among parasitic leaf-fungi. Most Cynipidse are 

 double-brooded ; the brood which issues from the galls formed 

 in spring consists of both males and females ; from the 

 autumnal galls females alone emerge. The fertilised eggs of 

 the sexual brood in the autumnal galls are hatched in spring 

 and yield females only ; the latter place their unfertilised 

 eggs in the spring galls, and from them the sexual brood is 

 developed. There is thus an alternation of generations, a 

 sexual alternating with an asexual one. Not only are the 

 insects composing the two broods perfectly distinct, but they 

 give rise to very dissimilar galls. Thus the currant galls are 



