^OTEMBNT AND KEOEGAKIZATION OF CELLS. 27 



"WTiere different species of insects, so far as our knowl- 

 edge enables ns to determine, produce galls out of ident- 

 ically the same materials, they are invariably of a differ- 

 ent structure and form, showing that however slight the 

 variation in the chemical or mechanical nature of the 

 cause, the results may be widely variable. For instance, 

 there are several species of gall-gnats that breed in the 

 Grape- vine, but each produces a distinct form of gaU. 

 One species that attacks the young succulent canes 

 makes a gall somewhat resembling a small apple, and it 

 is known as the " Apple Gall " of the vine, while the 

 habitation of another species resembles a cluster of fil- 



Fig. 9. — TRUMPBT OAU. ON GEAPE-LEAP. 



berts. The same rule holds good with galls produced on 

 the leaves of the Grape-vine. The galls produced by the 

 minute parasite formerly known as Pemphigus vitifoHa, 

 Fitch, but now as Phylloxera vastatrix, Planchon, appear 

 on the underside of the leaf, and are merely small green 

 fleshy swellings, more or less wrinkled, and with a slight 

 depression on the upper side, forming a cup with a kind 

 of hairy or pubescent margin. But in another leaf-gall, 

 known as the "Trumpet Grape-gall," produced by a 



