kellogg] GALLS AND GALL-FLLES 109 



GALLS AND GALL-FLIES 



BY VERNON L. KELLOGG 

 Professor of Entomology in Stanford University 



Familiar to all children who have an opportunity to see trees and 

 plants out-of-doors, and familiar to all teachers interested in nature- 

 study, are the deforming galls which occur on oaks, willows and 

 roses, and on some herbaceous plants. These galls and the insects 

 which live in them can be readily got acquainted with by some little work 

 and observation, and their study offers an excellent chance for some 

 interesting nature-study lessons. The following account, which may 

 serve as a sort of guiding introduction to these galls and gall-flies, 

 is based upon my recently-issued " American Insects," which is pro- 

 bably not accessible to most readers of this magazine. 



Indications of the work of certain hymenopterous insects are fami- 

 liar to even the most casual observers in the variously shaped " galls ' 

 that occur on many kinds of trees and smaller plants, especially 

 abundant on oaks and rose-bushes. Not all galls on plants are 

 produced by insects, certain kinds of fungi giving rise to gall-like 

 malformations on plants, nor are all the insect-galls produced by 

 members of that family of small hymenopterous insects called the 

 Cynipidae, or gall-flies. But most of the closed plant-galls and par- 

 ticularly those conspicuous, variously shaped, and most familiar ones 

 found abundantly on oak-trees and rose-bushes, are abnormal growths 

 due to the irritation of the plant-tissue by the minute larvae of the 

 Cynipid gall-flies. These flies are all very small, the largest species 

 not being more than one-third inch long ; they are short-bodied and 

 have in most cases four clear wings with few veins. The females — 

 and in numerous species there seem to be no males — have a long, 

 slender, and flexible but strong, sharp-pointed ovipositor (see illus- 

 tration), composed of several needle — or awl-like pieces, which is 

 used to prick (pierce) the soft tissue of leaf or tender twig so that an 

 egg may be deposited in this succulent growing plant-tissue. 



Each female thus inserts into leaves or twigs many eggs, per- 

 haps but two or three in one leaf or stem if the galls are going to 

 be large ones, or perhaps a score or so if the galls will be so small as 

 to draw but little on the plant-stores and be capable of crowding. 

 In two or three weeks the egg gives birth to a tiny footless maggot-like 

 white larva which feeds without doubt largely through the skin, on 

 the sap abundantly flowing to the growing tissue in which it lies. 



