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^), 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. 



♦ Pine-apple gall on twigs of the Spruce Fir produced by the Spruce-gall Aphis (Chermes abietis). 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. 5 Solid galls on the cortex of Diivaua longifolia produced by Cecidoses Eremite. « Longitudinal 

 section of one these galls. ? Capsule galls on the leaf of the Turkey Oak {Quercus Cerris) produced by Cecidomyia cerris. 

 8 One of these galls cut through with the operculum still firmly attached, and 9 the same with the operculum falling 

 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 are produced by gall-gnats (Gecidoirnyia urticce, alni), 

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

 are produced by a leaf -louse (Tetraneurct cdba). 



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



