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 Biplosis 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, 



Galls. 



.eoTion of one the.e galls. V c!ZrJn' onTellortZlt^eiZm n' '''°''^'" **™'^- ' ^""^''^Imal 



s one Of these galls out through with the optr ulut t^^^ C«<*""»^*« «-*• 



away ; x 3. The remaining figures natural size. attached, and the same with the operculum falling 



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

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

 gluUnosa), where the galls are produced by gall-gnats {Gecidomyi^ vHic<^, alni), 

 and on the mxdnb of Elm leaves (Ulmus canupestris; see fig. 361«), where the galls 

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



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



