540 ALTEKATION OF FORM BY GALL-PRODUCING INSECTS. 



blunt cone and from the lower as a hemispherical wart. The gall-chamber is 

 inhabited by the maggot of the gall-gnat. The top of the conical part loses its 

 colour in July and becomes yellow and brown, and a rim is formed around its 

 summit. On cutting a vertical section through the gall at this stage it is seen that 

 the tissue forming the wall of the chamber consists of two parts (see figs. 363 2 ). 

 The inner layer, which contains the maggot, is surrounded by an outer one which 

 gradually passes into the green substance of the leaf and extends up to the rim just 

 mentioned. The whole structure has separated into an "outer" and an "inner" 

 gall, the inner gall resembling an egg lying in an egg-cup (cf. fig. 363 '-'). During 

 the summer the inner gall separates completely from the outer and is actually 

 thrown off by it. For the accomplishment of this the tissue of the outer gall swells 

 up very much, so that it exercises a pressure on the inner gall which is shaped not 

 unlike a cone, somewhat narrower below than at the top (see fig. 363 3 ). The 

 extruded inner gall falls on the ground below the Lime-tree and assumes a dark- 

 brown colour; the outer gall remains as a little crater embedded in the leaf-blade 

 and ultimately shrivels up (cf. figs. 363 1 and 363 4 ). The detached inner gall is 

 smooth at the blunt and previously upper extremity, and striated at the other; it is 

 not unlike a detached composite-fruit. The gall-gnat within feeds for a little time 

 longer on the succulent lining, and then rests through the winter; in the spring it 

 makes its escape. To do this it bites a ring-shaped groove below the conical top of 

 the gall and presses against the roof, which, owing to the breaking of the tissues 

 around the ring, comes away like a lid (see fig. 363 5 ). A similar state of affairs 

 prevails in a gall formed on the foliage of a Brazilian species of Celastrus (see 

 figs. 363 6 ' "• 8 ), but here the inner gall (which comes away) has several chambers, and 

 the outer gall has the form of a cup set in the green blade. 



The place of origin of all these solid galls depends of course upon the insects 

 producing them. These are usually very fastidious about the place where they 

 will lay their eggs, and it is truly astonishing with what care they search out spots 

 difficult of access, and at once favourably situated as regards food supply and 

 likely to afford a safe habitation for their offspring during the larval stages. The 

 small gall-wasp Blastophaga grossorum lays its eggs in the ovaries of the "gall- 

 flowers" in the interior of the figs of Ficus Carica (see p. 160 and figs. 240 14 and 

 240 1S , p. 157). The gall-wasps Andricus amenti and Neuroterus Schlecht' ■mluli 

 deposit them in the stamens of the Turkey Oak; the gall-wasp Gynips caput- 

 medusce lays hers in the side of the bract-scales which surround the pistillate 

 flowers of the Oak (Quercus sessiliflora and pubescens), and so produces a gall with 

 innumerable stiff-pointed fringes entangled with one another which ward off the 

 attacks of other animals (see fig. 364 10 ). Countless gall-producing insects deposit 

 their eggs on the lower side of foliage leaves, some preferring the lamina, others 

 the veins. Andricus curvator prefers the margin of Oak leaves, Diplosis tremulce 

 the petiole of the Aspen at its junction with the blade. Several gall- wasps, as, for 

 example, Andricus i.rstimlis and Andricus grossidtrriir, seek out the floral recep- 

 tacle in the male catkins of the Turkey Oak for the deposition of their eggs, whilst 



