GALLS 179 



Hair-galls and Alantle-galls are not sharply marked off 

 from one another, many forms combining the characters of 

 both groups. They agree in the gall-makers living on the 

 outer surface of the plants, not really penetrating the inner 

 tissues. 



C. The third class of Simple Galls differs from both the 

 former in the gall-maker living within the tissues of the host, 

 or piercing the tissues and depositing an egg within them, 

 around which the gall grows. These galls differ widely in 

 complexity, ranging from the blister-like galls of some mites 

 (as on Pear and Mountain Ash leaves) or the irregular 

 swellings caused by many fungi and nematoid worms, to 

 the most highly developed galls known to us, those namely 

 of the true Gall-flies or Cynipidce. In the simpler forms the 

 increase in size is almost wholly due to multiplication and 

 enlargement of the cells, which are otherwise little altered in 

 aspect. In them the parasites, if animals, live in irregular 

 spaces between the loosely massed cells, or, if fungi, bore 

 between and into the cells. In either case they draw their 

 food from the contents of the cells. Galls of this class 

 produced by mites almost always have the passage through 

 which the animal penetrated- to the interior kept open as an 

 exit. In almost all others of the class the passage caused 

 by the parasite when entering the tissues, or by the parent 

 insect in depositing an egg within them, is soon blocked ; 

 and it can be recognised only with great difficulty, if at all. 

 The latter are well distinguished as Closed Galls. Many of 

 them remain closed throughout their existence, the parasites 

 or their offspring being set free by the decay of the galls, as 

 occurs with the " finger and toe " of turnips and other 

 Crucifers, and the root-tubercles caused by some Nematoids. 

 The more highly developed closed galls are formed mostly 

 by Gall-flies and some Saw-flies for the protection and 

 nourishment of the larvae ; some of them being formed 

 around the egg, others only after the larvae have emerged 

 and begun to feed. The Gall-flies mostly undergo the full 

 metamorphosis within the galls, the perfect insects escaping 

 by holes eaten through the walls. The Saw-fly larvae when 

 full fed eat their way out, and, dropping to the earth, 

 become pupae, usually under ground. The galls of the 



