FORMS OF MANTLE-GALLS. 



533 



and Wayfaring Tree (Viburnum Lantana) ciliated, whilst in the inflated galls of 

 the Elm caused by the white woolly leaf-louse (Schizoneura lanuginosa), it is 

 covered with fine hairs like velvet. The capitate galls on the foliage of Maples, 

 Alders, and Limes, of the Guelder-rose and Strawberry, are scattered abundantly 

 over the whole lamina; in the Sloe they stand out chiefly from the margin of the 

 leaf, and in Elms they occur singly or in groups on its central portions. The size of 

 these galls depends upon their distribution. Those which rise in hundreds from the 



Fig. 361.— Galls. 



>-* Solid galls on a Rose-leaf ; i of Mhodites Boste, 2 of Ekodites jEglanterice, s of Rhodites sjnnosissimce. * Wrinkled galls on 

 an Elm-leaf (JTlmus caynpestris) produced by Schizoneura Ulmi. s Purse galls on the same leaf, produced by Tetranevra 

 Ulmi. « Covering gall on the same leaf, produced by Tetratieura alba, f Solid galls on the leaf of the Purple Willow 

 {Salia purpurea), produced by Nematus gallarum. 8 Solid galls on the leaves of the same Willow, produced by Nematus 

 vematffr. 



same lamina have a diameter of 1-3 mm., while those which occur singly or in 

 small groups, often attain a diameter of 2-3 cm. 



Contrasting with these embossed or pocket-galls are the covering galls, forming 

 a third type of mantle-gall. In these, as in the embossed forms, the insects pro- 

 ducing the galls live in their cavities, but the course of development is quite 

 different in the two cases. The tissue round the place where an animal has settled 

 or where an egg has been fastened to the epidermis in this type begins to grow, 

 rising up in the form of a fleshy mound or wall which continues to grow until the 

 animal is wholly roofed in. The cavity in this case does not arise from an excava- 

 tion (as in the embossed or "pocket" type), but from an overarching of the tissue. The 



