loo On the Formation of 



producing insects, there can be no question of any- 

 wound of the plant cells producing the gall, because 

 these minute flies have no piercing apparatus. They 

 are enabled to introduce their egg into an opening bud 

 by means of a long protruded ovipositor, and then 

 it is the emerging larva which first stimulates gall 

 formation. Among gall-flies the same thing takes 

 place, the emerging larva invariably determines gall 

 production, and this can easily be proved. In the 

 course of the experiments in breeding, it was invariably 

 observed that, whether the flies laid their eggs in buds 

 or leaves, the wound was not immediately succeeded 

 by reflex plant growth. If buds in which eggs had been 

 laid were opened, no change was found in the interior 

 of the bud until the larva emerged (except of course 

 the delicate canals pierced by the ovipositor). It is 

 simpler still to observe leaf-pricking gall-flies. If a leaf 

 has been pricked by Spathegasler haccarum, the spot 

 where the ovipositor penetrated is quite visible ; but 

 during the first fourteen days there is no further 

 change in the leaf surface, and this only begins on 

 the emergence of the larva. Undoubtedly at the time 

 of pricking some secretion from the poison gland 

 reaches the wound, but this merely serves to seal over 

 the cut made by the ovipositor in the leaf surface, and 

 does not stimulate cell activity. This is seen still 

 more strikingly in Trigonaspis crustatis. This fly 

 pricks the leaf in May, but months pass before any 

 trace of gall formation can be seen. It has a tolerably 

 strong ovipositor with w^hich it cuts into the veins of 

 the leaf, and in this way a distinct mark is left wherever 



