HYMENOPTEHA OAK GALLS. 



141 



Fly : colour black, shining, length about 2 to 2.5 mm. The thorax 

 is rather dull in appearance ; abdomen shining and almost round in 

 form when looked at from the side. The legs are of a lightish-brown 

 colour. The wings are iridescent and considerably longer than the 

 body; antennse brownish, 15-jointed, the seven apical joints being 

 nearly equal in size and much smaller than the basal joints (fig. 131). 

 The insects hatched out in Northumberland from March 23 to April 

 12, 1906, and only lived for about ten days within the glass case. 



Prior to understanding the subject, a curious fact had often sug- 

 gested itself to me as a problem difficult of solution viz., if the 

 insect appears in March, and lives only for a short time, where does 

 it find leaves to deposit its eggs on for the next generation, seeing 

 that leaves do not often appear until 

 the end of May or beginning of 

 June 1 ? The first thought which sug- 

 gested itself was that the eggs were 

 deposited in the buds, and when the 

 leaves unfolded the galls would de- 

 velop. Then the problem appeared 

 more difficult, inasmuch as the spangle 

 galls chiefly appear, not on the first 

 developed leaves from the winter 

 buds, but on leaves not appearing 



until July in other words, on the young shoots which grew after 

 the fly of the Neuroterus lenticularis must have been dead. 



Dr Adler, before commencing to work out the subject, must have 

 been likewise puzzled, and accordingly he commenced a series of ex- 

 periments in 1875. He grew a number of young oaks in flower-pots, 

 hatched out the spangle-gall flies in March, and induced them to 

 deposit eggs in the young oak buds, under glass, when the leaves 

 unfolded and developed, together with the galls resulting from the 

 previous deposition of Cynip eggs of the spangle-gall species. The 

 galls, instead of being spangle galls, were another type viz., Spatlie- 

 gaster baccarum or currant gall (figs. 132 and 133). The galls were 

 different, and the flies also, for they were sexual, males as well as 

 females being found, whereas in the previous species the insects were 

 all females. Adler then induced SpatJiegaster baccarum to deposit 

 eggs on the leaves, and obtained as a result the oak-spangle galls, 

 the galls on the leaves of the young shoots. Thus the oak-spangle 



Fig. 131. Neuroterus lenticularis. 

 (Drawn by J. Patten, jun.) 



