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. 
Most 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. Adler 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 little 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 
