14 POEMS OF GALLS. 



How are we to account for the various forms, 

 colour, and texture of galls ? I believe largely to the 

 action of natural selection in developing them, so that 

 their forms, texture, clothing, and colour may afford 

 protection, directly or indirectly, to the tenants. 

 These means of defence may be classified as follows. 



First, Size. — Bearing in mind that the larva in- 

 habits the centre of the gall only where it occupies a 

 cell of its own size, surrounded by a thickened wall, it 

 is obvious that the greater the distance between the 

 centre and the external layer, the better it is protected 

 from the attacks of its enemies. Hence largeness 

 of size is a powerful aid to the larva : first, from the 

 attacks of Ghalcididce and ichnemunos, very few of 

 which have sufficiently long ovipositors to reach the 

 centre of a gall like Gynips hungarica; and second, 

 from the inquilines [Synergi), which can live in the 

 galls without disturbing the legitimate tenant. I 

 have several times reared Synergi and the Gynips from 

 G. Kollari galls ; the inquiline in these instances being 

 in a cell near the edge of the gall, as shown in PI. II, 

 fig. 6, a. Further, those large galls are made chiefly 

 by species of the genus Gynips, which is a unisexual 

 genus ; the larva, pupa, or imago remaining in them 

 during the greater pirt of the year, and hence are 

 liable to attacks for a long period. The space be- 

 tween the larval chamber and the circumference, at first 

 spongy or fungoid-like in texture, becomes with age 

 converted into a mass of powdery substance, which is 

 certainly highly astringent from the presence of tannic 

 acid. Inexperienced birds, we know, peck into the 

 galls for the larva ; and in doing so get this bitter 

 dust into their mouths — not, it may be imagined, to 

 their relish ; while, again, the larvas themselves contain 

 some noxious compound. One of the best examples 

 of this class is the well-known " Dead Sea apples " 

 (Gynips insana), — 



" Which tempt the eye, 

 But turn to ashes on the lips." 



