434 INTRODUCTION TO ZOOLOGY chap, xxvn 



{sderenchyma), the rest of the gall being of softer thin-walled 

 cells. 



Comparatively few galls are without the hard, inner 

 shell, which doubtless serves to protect the larva within from 

 the attacks of parasites, and from small birds that might 

 try to peck open the gall to get at the grub. The 

 tannin in some, such as the marble gall, renders them still 

 more distasteful to birds. Within the gall the larva grows 

 to its full size and pupates. The perfect insect finally bites 

 its way out and flies off'. In Plate IV. a Gall Wasp is shown 

 just about to emerge from the marble gall — she is just looking 

 out of the hole she has made. Another is drawn below. 



Pig. 326. — Trausverse Section through an Oak leaf bearing a Spangle Gall. 



ujj.s, Upper surface of leaf ; c, cavity in wliich the gall grub lives ; %, nutritive tissue ; 

 s, hard protective cells (sderenchyma) ; p, starch-containing cells ; n, one of the 

 hairs on the surface of the gall. 



enlarged, showing the characteristic, rather clumsy, fat body 

 of the Marble Gall Wasp. 

 Cause of When the egg is laid, a liquid is exuded with 

 Gall- it into the tissues, and it used to be thought 

 formation. ^T^gj^ ^yg ^^g ^\^q irritant that caused the growth 

 of the gall. Since, however, gall-formation rarely, if ever, 

 begins until the larva is hatched, and ceases if the larva is 

 killed, it seems probable that the explanation of the exciting 

 cause is to be found in the activity of the larva as ex- 

 plained above, rather than in the liquid inserted with the 



egg- 

 There are many different kinds of Gall Wasps, and each 

 produces a characteristic gall. It is strange that, of all 

 plants, the oak is the most affected by them, over forty 

 kinds of galls being found on it, 



