SURROUNDINGS OF GROWING INSECTS 211 



nearly-related species of gall-fly, and even those of alternate 

 generations of the same species produce galls of widely different 

 appearance and texture. 1 The "cherry-galls' of Dryo- 

 phanta, for example, beneath oak leaves, are inhabited by 

 larvae hatched from fertilized eggs laid by females in the 

 leaves about midsummer. These larvae pupate within the 

 galls, whence emerge in midwinter when the galls are no 

 longer succulent, but dry and hard larger virgin females 

 which lay eggs in the buds of the tree. The buds are arrested 

 in their growth and form small violet galls, visible on the 

 branches in springtime, from which the summer sexual brood 

 of flies emerge to lay their eggs in the leaves, whereon the 

 cherry-galls are produced as a reaction to the presence of the 

 larvae. The gall-flies are all of small size, and the larva, a leg- 

 less grub with small head and soft cuticle, like the great majority 

 of hymenopterous larvae, lives passively within the chamber 

 of the gall, its body bent so as to fit the sub-spherical cavity. 



From cynipid galls there frequently emerge insects not of 

 the gall-producer's species, though of the same family. These 

 are inquilines or " cuckoo-parasites ", the female fly having 

 laid her eggs in a gall already formed in response to the irrita- 

 tion set up by its proper larva. From this egg is hatched the 

 inquiline larva which feeds on the gall-tissues rightly belonging 

 to the latter. A few cynipid larvae live as parasites ; hatched 

 from eggs inserted into a gall, they devour its rightful occupier, 

 and then, completing their transformations, appear as an 

 unexpected type of fly. 



Returning now to the willows, we may find several examples 

 of galls due to the presence of insects of another order the 

 CecidomyidcB or gall-midges, a family of the Diptera. In 

 summer it may be seen that some of the buds on willows and 

 osiers have been arrested in their growth ; instead of lengthen- 

 ing to form normal shoots, they become changed into spreading 

 rosettes of small reduced leaves which are covered by a dense 

 hairy coating. Within the shelter afforded by this simple 

 type of gall, the little yellow larvae of a gall-midge, Rhabdo- 

 phaga heterobia, may be found living six or eight together. 

 These grubs (Fig. 107 b c) have a very small head, a series of 



1 H. Adler and C. R. Straton : " Alternating Generations ". Oxford, 

 1894. 



