270 Experimental Zoology 



can itself start the gall. Inquilines, or guests, also frequently 

 ^ occur in galls. They appear to have, in most cases, no effects 

 on the gall-growth so long as they do not injure the gall maker. 

 Alost of the species of gallflies show an alternation of sexual 

 and parthenogenetic generations, both generations producing 

 galls, generally on the same species of plant, but the gall may 

 differ somewhat in character. x\dler has studied especially the 

 galls of the oak. He collected the galls and kept them under 

 proper conditions until the gallflies emerged. He placed these 

 on the leaves of young twigs of httle oak trees grown in pots, 

 covering the branches with gauze to confine the flies. The buds 

 that were pierced were marked with threads and their history 

 followed. The life history of one of the species of gallflies will 

 serve to illustrate the details of gall formation. Neuroterus 

 lenticularis produces galls on the under surface of oak leaves, 

 sometimes forty to fifty on one leaf. The galls are 4 to 6 

 millimeters in diameter and of a yellowish red color. They 

 appear in June, and, maturing in September, fall from the 

 leaves to the ground about the beginning of October. At 

 this time the larva is still minute and requires much moisture 

 for its subsequent development. If the galls are laid on 

 damp soil, the larva will develop, at house temperature, in 

 about four weeks ; but out of doors, under natural conditions, 

 the flies do not emerge until April. Adler put the galls in 

 pots filled with earth which were then sunk in the soil. Each 

 pot was covered with gauze to confine the gallflies when they 

 emerged. He placed the flies on his saplings and saw them 

 pierce the buds. The ovipositor is pushed under one of the 

 bud scales as far as the base of the bud, which is then pene- 

 trated from without inward, and the egg is then deposited in 

 the bud. When the young leaves develop the gall is very small 

 and difficult to detect, but soon grows rapidly. Relatively few 

 of the buds pierced produce galls, because apparently of the 

 difficulty in placing the egg in exactly the right place. The 

 fly that emerges from the gall in June is known as Spathegaster 

 baccarum, and until its connection with Neuroterus lenticularis 



