

Chap. XXIII. CONDITIONS OF LIFE. 283 



namely, the epidermic, sub-epidermic, spongy, intermediate, 

 and the hard protective layer formed of curiously thickened 

 woody cells, and, lastly, the central mass abounding with starch- 

 granules on which the larva? feed. 



Galls are produced by insects of various orders, but the 

 greater number by species of Cynips. It is impossible to read 

 M. Lucaze-Duthier's discussion and doubt that the poisonous 

 secretion of the insect causes the growth of the gall, and every 

 one knows how virulent is the poison secreted by wasps and 

 bees, which belong to the same order with Cynips. Galls grow 

 with extraordinary rapidity, and it is said that they attain their 

 full size in a few days ; 49 it is certain that they are almost com- 

 pletely developed before the larva? are hatched. Considering 

 that many gall-insects are extremely small, the drop of secreted 

 poison must be excessively minute ; it probably acts on one 

 or two cells alone, which, being abnormally stimulated, rapidly 

 increase by a process of self-division. Galls, as Mr. Walsh 50 

 remarks, afford good, constant, and definite characters, each kind 

 keeping as true to form as does any independent organic being. 

 This fact becomes still more remarkable when we hear that, for 

 instance, seven out of the ten different kinds of galls produced 

 on Salix humilis are formed by gall-gnats (Cecidomyidce) which, 

 " though essentially distinct species, yet resemble one another 

 " so closely that in almost all cases it is difficult, and in some 

 " cases impossible, to distinguish the full-grown insects one from 

 " the other." 51 For in accordance with a wide-spread analogy 

 we may safely infer that the poison secreted by insects so closely 

 allied would not differ much in nature ; yet this slight difference 

 is sufficient to induce widely different results. In some few 

 cases the same species of gall-gnat produces on distinct species 

 of willows galls which cannot be distinguished; the Cynips 

 fecundatrix, also, has been known to produce on the Turkish 

 oak, to which it is not properly attached, exactly the same kind 

 of gall as on the European oak. 52 These latter facts apparently 

 prove that the nature of the poison is a much more powerful 



49 Kirby and Spence's < Entomology,' si Mr< B D Walgh; idenij p . 633 . and 



1818, vol. l. p. 450 ; Lucaze-Duthiers, Dec. 1866, p. 275. 

 idem, p. 284. 52 Mr> ' B d. Walsh, idem, 1864, p. 



^ 'Proc. Entomolog. Soc. Phila- 5-15, 411, 495; and Dec. 1866, p. 278. 

 delphia,' 1S64, p. 558. See alg0 Lucaze-Duthiers. 



