536 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



whether only one or several (cf. figs. 363 ^ and 363 '^). A great variety is met with 

 in the structure of the wall of the larval chamber. It always has a layer of juicy, 

 thin-walled cells immediately surrounding the egg, known as the medulla or pith 

 of the gall, and an outer layer which surrounds the inner like a skin or bark 

 (see fig. 360 ^*'). In most instances a third layer is inserted between them which 

 consists of very hard cells forming a protective layer. It should also be noted that 

 the layers of the wall of the gall separate in many instances, so that it is possible 

 to distinguish an " inner " and an " outer gall ". The gall-pith furnishes the larva 

 with food when it emerges from the egg, and for this purpose the cells are stored 

 with nourishing substances. The development of the pith takes place with great 

 rapidity, and begins as soon as the egg has been laid in the tissue. The larva when 

 hatched finds the inner wall of the chamber which has been fitted for its tempoi-ary 

 abode always provided with the necessary food, and it immediately attacks and 

 devours the juicy tissue with great avidity. The cells which are demolished, 

 wonderful to relate, are replaced almost at once. The cells of the gall-pith remain 

 capable of division as long as the larva in the chamber requires food, and the 

 surface cells which have been devoured in the gall-chamber are soon replaced by 

 new cells from below, just as grass which has been mown down or cropped by 

 cattle in a meadow sends up new stems and leaves. The spheroidal gall arising 

 on the leaves of Salix incana {cf. fig. 360^) has only one chamber, and here the 

 larva lives at the expense of the starch and other food-materials contained in the 

 extremely thin-walled cells which constitute the gall-pith (fig. 360^^). The larva 

 traverses the chamber in a circle, beginning the destruction of the cells at a certain 

 place and eating on as it continues its peregrination (fig. 360^°). New cells have 

 already been formed for its nourishment by the time it again reaches the place 

 from which it started. 



The hard and cortical layers are modified in very many ways as protective 

 measures against the drying up of the gall in summer on the one hand, and 

 against the attacks of birds and larger animals on the other. For the latter 

 purpose the cortical layer is often fashioned like the pericarps of fruits which 

 have to protect the seeds {cf. p. 442). This also explains the bitter substances, 

 hard skin, furry coat, bristling processes, and numerous other protective structures 

 which are developed in and on galls just as on pericarps, and which contribute 

 not a little to the remarkable similarity between galls and fruits. Many peculiar 

 developments on the surface of these fruit-like galls cannot indeed be explained 

 in this way, but, as in so many other cases, we conclude that they must aflbrd 

 some other advantage concerning which our understanding is still at fault. 



The external similarity between fruits and solid galls affords us useful points 

 for classifying the latter into groups, which we may name berry-like, plum-like, 

 apple-like, nut-like, capsule-like, &c. The currant gall produced by Spathegaster 

 baccarum on the male catkins of the Oak has not only the form and size of a Eed 

 Currant berry, but is also succulent and coloured red, and when several of these 

 galls are formed on the same inflorescence it looks at first sight just as if racemes 



