350 THE STUDY OF IXSECTS 



the wound made by a certain sawfly 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 

 fnun 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 ab- 

 normal 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. 

 Galls produced by the different species of cynipids differ greatly in 

 form and are found on all parts of plants. 



There are two terms that are frequently used in the descriptions of 

 galls; these are monothalamous , indicating that the gall contains a single 

 larval cell, and polythalamous indicating that the gall is compound, con- 

 taining more than one larval cell. 



Certain insect-galls have been found valuable for various purposes; 

 they have been used in medicine, in the manufacture of ink, for tanning, 

 and for dyeing. 



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 

 as distinct species until by careful studies of the life-cycle one has been 

 found to be the offspring of the other. In those species where an alterna- 

 tion of generations exists, one generation consists only of agamic females 

 while the other consists of both males and females, which reproduce sex- 

 ually. In some cases the galls produced by the two generations are quite 

 similar; but in others they are very different and are found on different 

 parts of the host-plant. 



The guest gall-flies or inquilines. — Some species of this subfamily do 

 not form galls but lay their eggs in the galls made by other species. The 

 larvae of these inquilines feed upon the galls produced by their hosts and in 

 some cases do not discommode the owners of the galls in the least. But 

 some guest gall-flies are parasites as well as guests and their larvae mine 

 from cavity to cavity of the gall and feed on the occupant of each in turn. 

 Among the more conspicuous of our cynipid galls 

 are the following. 



The oak hedgehog gall, Andricus erinacei. — A 



common gall on the leaves of white oak is one known 



as the oak hedgehog gall. This gall is rounded or 



oblong, with the surface finely netted with fissures, and 



more or less densely covered with spines (Fig. 593, 



a). It varies in length from f to f of an inch, and 



occurs on both sides of the leaves. The point of 



attachment is generally on the midrib, though it is 



often found on the lateral veins. When young the gall 



fig so? —The "oak * s yellowish-green, but in autumn it becomes yellowish- 



hedgehog gall: a, gall on brown. This gall is polythalamous, containing from 



two to eight larval cells (Fig. 593, b). 



Within the hedgehog galls is developed one generation, the agamic 



one, of Andricus erinacei. The alternating generation, the sexual one, is 



developed in very different galls made on the terminal growing points of 



buds and bud-scales. These are small, thin-walled, elongate, egg-shaped 



galls from T ^ to f of an inch in length, and are monothalamous. 



