THE AMERICAN L50TAXIST. 
{Potentilhi Canadensis) furnishes a curious liUle -all in 
the axils of some of its leaves. It is about the size of a 
large pea, green in summer and brown in winter. 
The gail flies seem to be partial to the different species 
of the oak, for not only the leaves but the twigs of these 
trees are made the rect- ' * f '•■ 
duce the cradle fo r t h 
grows on the red ' • 
sometimes filled wit : 
which is a hard k.jn 
verA- similar in app. 
let and red oaks. It; 
en appears to Ix; eni[':_ 
will l>e seen that the larvel cell is kept in its place in tlie 
middle by means of some stiff radiating threads or filments. 
It is known as the empty oak apple {AmphiboUps inanis). 
I once saw a large oak near Newburyport. Mass. on whicli 
were thousands of these galls with thousands more fallen 
to the ground beneath. A whole swarm of flies must have 
attacked this tree at once. 
The so-called bullet gall {Holcaspis globulus) is com- 
mon on the terminal twigs of the white oak. A round 
hard body sometimes three-fourths of an inch in diameter. 
It is yellow or reddish in summer but brown and hard in 
winter. On twigs of the swamp white oak(^«ercas plan- 
tanoides) there occurs another bullet gall {Holcaspis 
duricaria), which is very similar in appearance to the pre- 
ceeding. It has a sharp little point on the end and for this 
reason is called the poinrcd bullet gall. The oak petiole 
gall (Audricus petiolicohi) is common at the base of the 
midrib on leaves of the white chestnut, swamp while and 
post oaks. It is a hanl irregular woody growth about 
three-fourths of an inch long and contains several cells. 
The gall-gnats, a farailv of small tlies. are responsible 
for many galls some <>t which are Urand on willows. One 
the trgg gall {£«ura ovum) appears as an elongated nodule 
t)n twigs of willow growing in swampy places. Some- 
