538 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



young bud-leaves are pierced and then the upper portion of the leaf becomes 

 atrophied. More rarely is the egg laid in the epidermis of one of the leaflets, 

 in which case the leaves attain their normal size and only this particular leaflet 

 is decorated with little bedeguars, as shown in fig. 361 ^ When the petioles of 

 three young leaf-rudiments are pierced simultaneously, as often happens, three 

 single galls are produced close together on a shortened axis, and the whole structure 

 may then attain the size of a pine-cone. 



The portion of meristematic tissue which is pierced by the insect when it 

 deposits its eggs sometimes remains an open passage; but more often a corky 

 tissue is formed at the wounded spot which quite closes the chamber wherein the 

 larva dwells. Under these circumstances the insect when it emerges must itself 

 make an exit-passage from the gall, and this it does by biting a hole through 

 it with its mandibles (see fig. 364 ^). The gall- wasps (Cynipedes) invariably leave 

 the chamber which has hitherto served them both as a safe habitation and as 

 an inexhaustible storehouse in this way. This does not occur, however, in some 

 of those solid galls which owe their origin to gall-gnats of the genera Hormomyia, 

 Diplosis, and Cecidomyia, for example, in those on the leaf-blade and petiole of 

 the Aspen (Populus tremula) produced by Diplosis tremulce and on the leaves 

 of Willows (Salix Caprea, cinerea, grandifolia) by Hormomyia Caprece. Here 

 the exit-passage is formed during the development of the pith. The gall consists, 

 as in most other solid galls, of a pith, a hard layer, and an epidermis, but the 

 enormously developed pith and the hard layer do not quite entirely surround the 

 small larval chamber, they leave a small aperture on the part of the gall which 

 is most arched. As long as the epidermis stretches over this place the mouth 

 of the passage is of course not evident, but when the time comes for the insect 

 to quit the chamber a gaping slit is spontaneously formed in the tense epidermis. 

 In many instances the insect or the pupa as it pushes forward may break through 

 the thin skin. A peculiar closure which might be compared to a lid is formed 

 in the common solid galls which are produced so abundantly on Beech leaves 

 by Hormomyia fagi and which have been already alluded to. Just as the pupa 

 of many Lepidoptera projects out of the hole in the cocoon which the caterpillar 

 has spun for it far enough to allow the insect to fly away uninjured when it 

 emerges, so that of Hormomyia fagi presses through the lid-like closure at the base 

 of the gall, and the winged insect comes out leaving the chrysalis-case behind it. 



The opening of some solid galls, which resemble operculate capsules, and which 

 may be termed capsule-galls, is especially remarkable and requires a more de- 

 tailed description. As long as the larva or grub can remain and obtain food in 

 the larval chamber the gall is completely closed, but when the time approaches 

 for it to move its quarters and to enter the pupal stage in the ground a circular 

 line of separation is formed in the tissue, and the part of the wall within the 

 circle comes away as a lid. The process is seen very prettily in the gall produced 

 on the leaves of the Turkey Oak {Quercus Cerris) by the gall-gnat Cecidomyia 

 cerris (see fig. 362 ' ). In its closed condition the gall is a firm rounded chamber 



