HYMENOPTERA 923 



have been described in the preceding pages, and galls are also produced 

 by beetles and certain other insects; but the great majority of these 

 strange growths are made either by gall-midges, mites, plant-lice, or 

 true gall-flies (Cynipinae) . 



The galls made by mites and plant -lice have open mouths, from 

 which the young of the original dweller escape. But in the case of 

 the gall-flies the gall is closed, and a hole must be made by the insect 

 in order to emerge. Moreover, there is no reproduction of insects 

 within the galls of gall-flies, as there is within the galls of mites and 

 plant-lice. 



It is a remarkable fact that each species of gall-insects 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. 



Galls produced by the different species of cynipids differ greatly 

 in form and are found on all parts of plants. A most useful manual 

 for the identification of galls is the "Key to American Insect Galls" 

 by Dr. Felt ('i8) in which, are figures of the galls of many of our 

 gall-making insects, including those of all orders. 



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 com- 

 pound, containing 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 tan- 

 ning, and for dyeing. A simimary of the literature dealing with the 

 uses of insect-galls was published by Fagan ('i8). 



There exists in many species oi gall-flies an alternation of 

 generations; that is, the individuals of one generation do not re- 

 semble 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. 



