6 ALTERNATION OP GENERATION. 



which remain (on the assumption that they were not 

 dimorphic) dormant till the following spring. In the 

 case of a species like D. foli, whose larval, pupal, and 

 imaginal existence is short, it followed that the egg 

 must remain undeveloped in the bud. The species 

 of the restricted genus Gynips stand on a different 

 footing, for we now undoubtedly know that they are 

 without exception unisexual ; but we are also aware 

 that it is the larva, not the egg, which remains 

 dormant. 



The subject remained in this state for many years 

 until 1861, when Baron Osten Sacken (Stett. Ent. 

 Zeit., 1861) emitted the view that the males did 

 exist, but that they lived in separate galls — an 

 opinion on which there was no proof available. Three 

 years later the well-known American naturalist, B. D. 

 Walsh, produced evidence to show that Gynips spongi- 

 fex, 0. S., caused a large round swelling on the 

 leaves of Quercus tinctoria, and that the insect issuing 

 from this gave origin to another gall, known as Gynips 

 aciculata— the one appearing in the spring in a bi- 

 sexual form, the other in the autumn being agamic. 

 Walsh looked upon this as a case of dimorphism. 



Bassett, another American entomologist, placed the 

 matter on a clearer footing by showing how some 

 bisexual spring forms, living in a distinct gall, gave 

 origin to a unisexual brood living in a differently 

 constructed gall in the autumn. 



Riley, the American State Entomologist, also eluci- 

 dated the subject by showing that G. operator, 0. S., 

 which comes out of a " large woolly gall, the de- 

 formation of a bud which grows on our black oaks 

 in spring, produces in summer a common gall-fly 

 (G. q. operator, 0. S.), which is bisexual. The female 

 oviposits between the acorn and cupule of the pre- 

 vious year's setting, aud the result is a pip -like gall 

 (G. operatola, KLy) embedded in that position, and 

 generally half exposed. These fall with the acorn to 

 the ground, and the second spring generation sue- 



