PARASITIC ENEMIES OF TREES AND PLANTS 



487 



Few, very few, of the galls caused by insects on about fifteen other genera, those of the genus Cynips 



plants are of any use to us indeed, those occurring on 

 oak trees are practically the only kind that possess any 

 economic value, they being used commercially in enor- 

 mous quantities in the making of various inks, and in 

 the processes of dyeing and tanning. 



The study of these varied and curious excrescences 

 especially ap- 

 peals to forest- 

 ers, to zoolo- 

 gists, and to 

 botanists, and 

 for the very 

 good reason 

 that the field 

 contains so 

 much that has, 

 as yet, in no 

 way been work- 

 ed out; it ac- 

 tually bristles 

 with problems 

 unsolved. Then 

 too, we are 

 compelled t o 

 study them, in 

 that we may 

 derive the nec- 

 essary knowl- 

 edge to proper- 

 ly care for and 

 protect our use- 

 ful as well as 

 ornamental 

 plants. Again, 

 a knowledge of 

 them is essen- 

 tial to the stu- 

 dent of food 

 products in the 

 plant world and 

 their cultiva- 

 tion, as not a 

 few of them 

 are nothing 

 more than fun- 

 gus root-galls, 

 those of the 

 legumes being 

 of bacterial or- 

 igin. 



Galls are produced by insects belonging to very differ- 

 ent orders, in so far as their classification goes. Various 

 genera of mites produce some of them, as do beetles, 

 .scores of flies representing upwards of twenty genera, 

 not to mention the Hymenoptcra or Saw-flies, espec- 

 ially those of the family Cynipidae, in which we have 



BLACKBERRY GALLS GREATLY DISFIGURE THE PLANT 



Fig. 5 All parts of a blackberry bush beyond the site of the gall invariably die, while, 

 close to the gall healthy stems are to be seen. One of these, bearing buds and a blossom, 

 was present, as shown in this particular specimen. Insert cuts: A., Gall-gnat (female), a 

 species of Cecidomyia, about natural size; it makes galls on some of our annual plants. 

 B., The Misnamed Gall-moth {Paedisca saligncana). C, The. same with closed wings. 

 D., Natural size of B. E., Larva. This species makes galls on the goldenrods. 



having been most exhaustively studied. 



Dr. Frank E. Lutz, in his excellent little work, the 

 "Field Book of Insects," states that "if the galls are 

 inhabited, a clue to the makers may be gained by a 

 study of the inhabitants. Mites have four pairs of legs, 

 at least when full grown ; no wings ; and are very small. 



Aphids have 

 three pairs of 

 legs, aaJ they 

 sometimes have 

 no wings. Galls 

 made by both 

 of these groups 

 are usually 

 open. Saw-flies 

 have thoracic, 

 ind usually dis- 

 tinct abdominal 

 legs ; their galls 

 usually have a 

 large hollow on 

 the inside. Gall- 

 making lepi- 

 dopterous lar- 

 vae have thor- 

 acic but no ab- 

 dominal legs. It 

 is not so easy 

 to distinguish 

 Hy me no pt er- 

 ous and Dip- 

 terous larvae ; 

 and it should 

 always be re- 

 membered that 

 galls may be 

 inhabited b y 

 creatures which 

 did not make 

 them parasites 

 of the makers 

 and also inqui- 

 lines, 'guesis' 

 which avail 

 themselves of 

 the abundant 

 food, but do 

 not directly in- 

 jure the maker 

 of the gall. 

 Some galls are complicated communities. We speak of 

 creatures 'making' the galls; the plants really do this, 

 acting on some (not understood) stimulus furnished by, 

 the animals. It is exceedingly curious that insects which 

 are so similar that they may be distinguished only with 

 difficulty cause such different and distinctive galls. In 



