474 



Saw-flies, Gall-flies, Ichneumons, 



Fig. 667. — Cynips quercus- 

 saltalrix, the gall - fly 

 which produces the 

 jumping galls. (Much 

 enlarged.) 



These set the whole ph\-sioIogical apparatus in motion, and secure the 

 insertion of eggs at the right time and in the right place. The number 

 of eggs placed is instinctively proportionate to 

 the space suitable for oviposition, to the size of 

 the fully grown galls, and to the food-supplies 

 available for their nutrition. Dryophanta sciitellaris 

 will only j)lace from one to si.x eggs on a leaf which 

 Neurolertis lentictilaris would probably prick a 

 hundred times," 



"Whatever form the gall takes, the poten- 

 tialities of the tissue-growth exhibited by it must 

 be present at the spot pricked by the fly." 



"The potentialities of growth being present, they 

 are called into activity by the larva, a result advan- 

 tageous to the larva and sometimes described as 

 disinterested and self-sacrificing on the part of the 

 plant. We have just seen that, so far as the larva 

 is concerned, the peculiar structures of the gall 

 owe their origin to their success in feeding and defending it; and, so far as 

 the plant is concerned, these structures have been evolved in consequence 

 of their value in enabling the plant to repair injuries in general, and the 

 injuries inflicted by larvae in particular. If John Doe raises a cane to strike 

 Richard Roe, and Richard throws up his arms intuitively to parr\- the stroke, 

 the action does not indicate a prophetic arrangement of molecules to frustrate 

 John in particular, but an inherited action of defence. The first act of an 

 injured plant is to throw out a blastem, and only those larvae survive to hand 

 down their art which emerge from an egg so cunningly placed as to excite the 

 growth of a nutritive blastem. It is not always possible to keep the besiegers 

 from using the waters of the moat, although there is no disinterested thought 

 of the besiegers' wants when the ditches are planned. So in the war-game 

 that goes on between insect and plant, natural selection directs the moves 

 of both players, but there is nothing generous or altruistic on either side." 



The exact character of the i:)lant's abnormal growth has been recently 

 studied by several investigators. Cook, an American student, concludes 

 from his studies that in the formation of all leaf-galls (except the Cecidomyid 

 or dipterous midge-galls) the normal cell-structure of the leaf is first modi- 

 fied by the formation of a large number of small, compact, irregular-shaped 

 cells. The mesophyll is subject to the greatest modification and many small 

 fibro-vascular bundles form in this modified mesophyll. Both Adler and 

 Sockeu consider that after the first stages of formation the gall becomes an 

 independent organism growing upon the host-plant. Cook believes this 

 to be true of the Cynipid galls. A surprising conclusion arrived at by Cook 



