534 



ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



external appearance of these galls is very varied. One of the simplest forms occurs 

 on the leaves of the Ash (Fraxinus excelsior, see fig. 362 3 ), where it is produced 

 by the gall-gnat Diplosis botularia. The insect having laid its eggs in the chan- 

 nelled depressions above the leaf- veins, fleshy cushions arise on either side of the 

 groove which meet above and roof them over. The cushions of tissue forming the 

 roof do not fuse; their succulent edges merely meet, and when the time comes for 

 the gall-gnats to leave their temporary abode the tissue dries up and shrivels, 



Fig. 362. Galls. 



i Pine-apple gall on twigs of the Spruce Fir produced by the Spruce-gall Aphis (Chermes dbietis). 2 Covering gall on the 

 petiole of the pyramidal Poplar (Populus pyramidalis) produced by Pemphigus bursarius. s Covering galls on an Ash leaf 

 (Fraxinus excelsior) produced by Diplosis botularia. * Covering gall on Pistacia (Pistacia Lentiscus) produced by Pem- 

 phigus cornicularius. Solid galls on the cortex of Duvaua longifolia produced by Cecidoses Eremite. Longitudinal 

 section of one these galls, t Capsule galls on the leaf of the Turkey Oak (Quercus Cerris) produced by Cecidomyia cerris. 

 3 One of these galls cut through with the operculum still firmly attached, and the same with the operculum falling 

 away ; x 3. The remaining figures natural size. 



leaving a gaping slit as shown in fig. 362 3 . The same thing happens on the leaves 

 or rather leaf -veins of the Stinging-nettle (Urtica dioica) and of the Alder (Alnus 

 glutinosa), where the galls are produced by gall-gnats (Cecidomyia urticce, alni), 

 and on the midrib of Elm leaves (Ulmus campestris; see fig. 361 6 ), where the galls 

 are produced by a leaf -louse (Tetraneura alba). 



The so-called turpentine gall-apples (Carobe di Giude\ see fig. 362 4 ), which 



