2 BRITISH GALL-INSECTS. 



great quantities of Acarus domesticus. (Zoological Record, 

 vol. iv. p. 192.) 



Vegetable galls are excrescences or deformities of living 

 plants, caused by animal influence and serving for the pro- 

 tection and sustenance of animal brood. Their formation 

 always takes place while the plant is still growing, but their 

 functions do not always cease with the decay of the plant, 

 the juices of which have helped to form them. It happens 

 in many instances, that the ripe gall becomes detached and 

 continues to afford shelter and food, or, at least, the needful 

 moisture, to its inhabitant or inhabitants. This is particu- 

 larly the case with galls originally formed on organs of de- 

 ciduous trees or low plants of annual growth. 



Very few families of plants are altogether free from these 

 parasitic grovrths caused by insect-agency, but, so far as my 

 knowledge eroes, none have as yet been detected on funo-i 

 and mosses, although there is no apparent reason why these 

 kinds of organisms should enjoy immunity. 



Galls occur on all vegetable organs; none escape their 

 deleterious presence: root, stem, branch, bud, leaf, blossom, 

 fruit — all have to pay their tax in kind, — a tax of the most 

 injurious sort, consisting of their own sap. The outward 

 appearance and internal structure of vegetable galls are so 

 diversified, that I cannot even glance at these subjects 

 here; it Avould positively take dozens of pages to treat them 

 even in the most superficial way, and piecemeal information 

 is worse than useless. For the purpose of identification 

 I have, however, used in some instances the terms " mono- 

 thalamous" (single-celled) and " polythalamous" (many- 

 celled). In Mr. Francis Walker's '^ Insecta Britannica — 

 Diptera/' vol. iii (under the heading '^ Cecidomyia"), and in 

 the ^' Entomologist's Monthly Magazine," vol. v., observers 

 will find lists of plants, known or suspected to bear galls in 



