HYMENOPTERA. 617 
are maggot-like, and without a caudal opening to the ali¬ 
mentary canal. 
It is a remarkable fact that each species of gall-insect 
infests a special part of one or more particular species of 
plants, and the gall produced by each species of insect is of 
a definite form. Hence when an entomologist who has 
studied these insects sees a familiar gall, he knows at once 
what species of insect produced it. 
Naturalists have speculated much as to the way galls are 
made to grow. It has been supposed that at the time the egg 
is laid there is deposited in the tissue of the plant with it a 
drop of poison, which causes the abnormal growth. By this 
theory the differences between the galls of different insects 
was explained by supposing that the fluid produced by each 
species of insect had peculiar properties. There are certain 
kinds of galls which may be produced in this way. Thus it 
is said that the wound made by a certain saw-fly in the leaves 
of willow causes an abundant formation of plant-cells, and 
the gall thus formed attains its full growth at the end of a 
few days, and before the larva has escaped from the egg. 
But with the gall-flies the gall does not begin to grow until 
the larva is hatched ; but as soon as the larva begins to feed, 
the abnormal growth of the plant commences. In this case, 
therefore, if the gall is produced by a poison, this poison 
must be excreted by the larva. 
There exists in many species of gall-flies an alternation of 
generations ; that is, the individuals of one generation do not 
resemble their parents, but are like their grandparents. In 
many cases the two succeeding generations of a species 
differ so greatly that they have been considered not merely 
as distinct species, but have been placed in different genera. 
Thus it has been found by Mr. Adler, of Schleswig, that 
that while a certain species of Neuroterus (Neu-rot'e-rus) is 
developed in one kind of gall on the leaves of oak, the 
larvae that hatch from eggs laid by it produce a different 
kind of gall, and develop into gall-flies which do not present 
