526 HYMENOPTERA 



by a great variety of Insects, as well as by mites, on many plants ; 

 and it must not be concluded that a gall has been formed by 

 Hymenoptera even when these Insects are reared from one. 

 Extremely curious galls are formed by scale-Insects of the sub- 

 family Brachyscelides on Eucalyptus trees in Australia ; they are 

 much inhabited by parasitic Hymenoptera, and Froggatt has 

 obtained 100 specimens of a small black Chalcid from a single dead 

 Brachyscelid. 1 The exact manner in which many of these galls 

 originate is not yet sufficiently ascertained ; but the subject of 

 the galls resulting from the actions of Cynipidae has received 

 special attention, and we are now able to form a conception 

 of their nature. They are produced by the meristematic or 

 dividing tissue of plants, and frequently in the cambium 

 zone, which is caused to develop to an unusual extent, and 

 in a more or less abnormal manner, by the presence of the 

 Insect. The exact way in which a Cynipid affects the plant is 

 perhaps not conclusively settled, and may be found to differ in 

 the cases of different Cynipidae, but the view advocated by Adler 

 and others, and recently stated by Riley, 2 seems satisfactory ; it 

 is to the effect that the activity of the larva probably affects 

 the meristem, by means of a secretion exuded by the larva. 

 The mere presence of the egg does not suffice to give rise to the 

 gall, for the egg may be deposited months before the gall begins 

 to form. It is for the same reason improbable that a fluid injected 

 by the parent fly determines the gall's growth. It is true that 1 

 the parent fly does exude a liquid during the act of oviposition, 

 but this is believed to be merely of a lubricant nature, and not to 

 influence the development. It is said that the gall begins to form 

 in some cases before the larva is actually hatched, but the eggs of 

 some Hymenoptera exhibit remarkable phenomena of growth, so 

 that the egg, even during development of the embryo in it, may 

 in these cases, exert an influence on the meristem. It is to 

 reactions between the physiological processes of the meristem 

 and the growing Insect that the gall and its form are due. 



The investigations of several recent naturalists lend support 

 to the view that only the meristematic cells of the plant can 

 give rise to a gall. Eiley says that the rate of growth of the 

 gall is dependent on the activity of the meristem, galls on cat- 



1 P. Linn. Soc. N. S. Wales (2), vii. 1892, p. 357. 

 2 Science (n.s.), i. 1895, p. 457. 



