530 ALTERATION OF FOKM BY GALL-PRODUCING INSECTS. 



myia Poce. The hair-shaped cells are epidermal, and spring from the stem above 

 the nodes; they break through the leaf -sheath which proceeds from the adjacent 

 node, and are arranged in two groups, which grow in opposite directions, so as to 

 wrap round the stem from the two sides. The whole hairy mass looks as if it had 

 been parted into two. At first the hairs are white; later they become light brown, 

 and when the gall is fully developed they have the form of brown felted strands, 

 wound round the stems and firmly inclosing the larva of the gnat in question. 



A large number of simple galls are grouped together under the name of Mantle- 

 galls. The insects which give rise to them spend their lives on the surface of the 

 leaves, where they multiply and attach their eggs to the epidermis. A growth is 

 excited in certain layers of the cell-tissue by the stimulus which the animals 

 exercise on their place of settlement. Cavities are thus formed which serve as 

 dwellings for the animals and their brood, and which surround them like a pro- 

 tecting mantle. Mantle-galls may be divided according to their structure into 

 scroll-, pocket-, and covering-galls. Scroll-galls are caused by gall-mites, leaf-lice, 

 tree-hoppers, and flies, and usually occur on the blades, rarely on the petioles of the 

 leaves. The surface inhabited by these animals, which, in the ordinary course 

 of things would have spread out flatly, grows more luxuriantly on one side than 

 on the other, and the result is the formation of a scroll, i.e. of a chamber in which 

 the animals are hidden. It is always the side on which the animals live which 

 becomes concave, and the leaf is usually curled up lengthwise. In the Alpine 

 Rose (Rhododendron), Crane's-bill (Geranium sanguineum), and Orache (Atriplex 

 hastata, oUongifolia, &c.), it is the upper side of the leaf which is tenanted by the 

 insects, and is therefore the one to roll up; it is the lower side, however, in the 

 Buckthorn (Rhamnus cathartica) and the non-climbing species of Honeysuckle 

 {Lonicera alpigena, &c.). In many instances the whole leaf-lamina is rolled up, 

 but more frequently the alteration is restricted to the edge of the leaf when the 

 margin appears to be bordered with a swollen hollow cushion often corrugated or 

 undulating. In the Alpine Rose (Rhododendron ferrugineum and hirsutum) both 

 halves of the leaf -blade are rolled round (see figs. 860 ^ and 360 ^), but usually the 

 rolling is so slight that the gall has the form of a boat or hollow trough. Some- 

 times an alteration in the shape of the leaf accompanies the rolling. For example, 

 the foliage of the Abele (Populus alba) on which Pachypappa vesicalis establishes 

 itself when the leaves are very young, exhibits in addition to the rolling a deep 

 hollowing of the blade. Instead of the short blunt lobes, long pointed segments 

 are formed, which stand side by side when they are rolled up, and cross over one 

 another in many ways so that the mantle-gall on the hollow side is shut in by a 

 veritable latticcrwork. The parts of the tissue brought into contact by the rolling 

 do not fuse together, and therefore the cavity in which the gall-producing insects 

 live is always in open communication with the exterior. In most cases the tissues 

 concerned are thickened, brittle, more or less devoid of chlorophyll, and yellow m 

 colour. Not infrequently a red pigment is formed in them, so that the outside of 

 the gall has a yellowish-red colour. The scroll-gall produced by the hemipterous 



