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 (Fraximis excelsior, see fig. 362 ^), where it is produced 

 by the gall-gnat Diplosis hotularia. 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. 



Galls. 



I Pine-apple gall on twigs of the Spruce Fir produced by the Spruce-gall Aphis {Chcnnes abietis). - Covering gall on the 

 petiole of the pyramidal Poplar {Populus pyramidalis) produced by Pemphigus hursarius. s Covering galls on an Ash leal 

 {Fraxinus excelsior) produced by Diplosis botxdaria. * Covering gall on Pistacia {Pistttcia Lentiscxis) produced hy Pem- 

 phigus comicularius. 6 Solid galls on the cortex of Vnvaua longijolia produced by Cecidoses Eremite, e Longitudinal 

 section of one these galls. " Capsule galls on the leaf of the Turkey Oak (Querciis Cerris) produced by Cecidumyia cerris, 

 8 One of these galls cut through with the operculum still firmly attached, and s the same with the operculum falliug 

 away ; x 3. The remaining figures natural size. 



leaving a gaping slit as shown in fig. 362 ^. 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 ai-e produced by gall-gnats {Cecidomyia urticcB, alni), 

 and on the midrib of Elm leaves (Ulmus campestris; see fig. 361^), whei'e the galls 

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



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



