INSECTS AND OTHER ORGANISMS 387 



wingless females lay their eggs in the buds of the oak-trees ; 

 the result is, that after the grubs are hatched, oak-apples 

 form on the developing shoots, whence, as already men- 

 tioned, male and female Teras-flies emerge next summer. 

 Thus the two quite dissimilar sets of insects, one sexual 

 and the other virgin, are alternating forms of the same 

 species, each generation beautifully adapted to the seasonal 

 cycle of the tree on which it lives. A similar alternation is 

 shown in the life-history of the cynipid which produces the 

 large yellow or reddish " cherry-galls," conspicuous on 

 oak-leaves from July onwards, becoming brown and " ripe " 

 in October. From the larvae feeding within these are 

 developed, for emergence in midsummer, virgin female 

 gall-flies (Dryophanta scutellaris), which pierce the resting 

 buds and lay their eggs within. As a result of the larval 

 stimulation, the affected buds, instead of developing into 

 shoots, become enlarged and swollen and violet in colour. 

 The larvae feeding within develop by spring or early summer 

 into the sexual brood of flies called Spathegaster taschen- 

 bergi, the females of which pierce the leaf-tissue of the oak 

 with their ovipositors, the presence of the grubs, when 

 hatched from their eggs, leading to the formation and 

 growth of the cherry galls in late summer and autumn. 



Galls made through the stimulation of cynipid grubs I 

 are often inhabited by larvae of other insects. Several / 

 genera of Cynipidae {Synergiis for instance) have the habits 

 of inquihnes or " cuckoo-parasites," laying their eggs in 

 galls already formed so that the larvae may feed within ; in 

 many cases there is sufficient provender for both *' host " 

 and " guest." Cosens has found that some of these 

 inquilines " possess the gall-producing power " in a 

 reduced degree. C. R. Osten-Sacken long ago (1863) drew 

 attention to the close resemblance frequently observable 

 between a true gall-fly and its inquiline. " The one is the 

 very counterpart of the other, hardly showing any difference 

 except the strictly generic characters." Then there are 

 the true parasites — a few belonging to the Cynipidae, but 

 most to the Chalcididae and other families of Hymenoptera — 



