6 NEW YORK STATE MUSEUM 



separated groups and must have originated in a mutual reaction 

 between the insects and their host plants, which has reached its 

 climax in the many apparently inexplicable deformations of the 

 present day. All stages of the process may be observed among the 

 gall midges, some of which live among succulent fungous growths 

 and either feed a little upon the fungi or obtain nourishment by 

 absorption from the humid surfaces of the host. There are certain 

 predaceous maggots in this group which have the mouth-parts 

 greatly prolonged and apparently specially adapted to withdraw 

 by suction the body fluids of their hosts. It may be one or the 

 other or possibly a combination of the two methods which obtains 

 among the fungivorous species. It is only a step from this to absorp- 

 tion with apparently no mechanical injury as in the leaf spot gall 

 of the soft maple or the pod leaf galls mentioned below. The habit 

 once started, it is possible to understand how the process might 

 continue with infinite variations among a host of species, which is 

 just what has taken place. The adaptations have continued along 

 a nimiber of lines to such an extent that many insect galls give little 

 external evidence of their origin. Gall insects live at the expense 

 of their hosts and in some instances, at least in the case of certain 

 plant lice, the mere satisfying of the primitive instinct of hunger 

 seems to be all that is necessary, not only to preserve life but to 

 compel or cajole, as it were, the host plant, to grow or throw around 

 its enemy a defensive barrier of gall within which the aphid may 

 live in the presence of abundance, be comparatively safe and obtain 

 like conditions for its mrmerous progeny. This sheltered, luxurious 

 type of existence appears to be essential to many species and the 

 tendencies along these lines have developed to such an extent that 

 twenty-nine species of gall-making aphids, Phylloxera, are known to 

 live at the expense of our hickories and in a similar manner a number 

 of species of jumping plant lice, Pachypsylla, subsist on hackberry. 

 A very large proportion of insect galls begin growth in the bud 

 and therefore at a time when the plant tissues are plastic and more 

 easily modified. Not a few of the strange forms are to be explained 

 as the arrested development of buds, fruit or leaves as the case may 

 be, frequently accompanied by an excessive development or swelling 

 of parts immediately adjacent to the source of irritation. This 

 latter may be brought about by fluids in the egg or injected with it, 

 as in the case of the galls of certain sawflies which become full size 

 before the larvae hatch. Sometimes it is a reaction between the 

 contents of the egg or larva and adjacent tissues through excretion 

 or osmosis, a condition which must obtain in the familiar pod galls 



