GENERAL 23 



Dipterous larvae, accustomed to burrowing in soft sub- 

 stances, may have come to the mining of green leaves 

 through the habit of boring in dead ones. At any rate, 

 there are species that still do both these things and that are 

 sometimes saprophytes, at other times leaf-miners, being 

 not wholly committed to either habit. 



But it seems probable that leaf -mining caterpillars and 

 sawfly larvae and leaf-beetle grubs are derived from fore- 

 bears that fed on leaves in the ordinary way. Perhaps the 

 earliest among them merely ate a hole in the leaf: later 

 ones put their heads farther in and kept on eating down 

 underneath the epidermis: finally some of them bodily 

 followed their heads down into the hole and dwelt there. 



INTERGRADATION WITH OTHER HABITS 



Leaf -mining as a mode of life intergrades with gall-making, "^ 



with stem-boring and with feeding from shelter. All these 

 shifts for a living are common enough in the insect world. 

 Each has its host of strict adherents, but there are border- 

 line forms that combine two or more of these ways of getting 

 on. 



Gall-making insects enter the leaf while its tissues are 

 still in a formative state, and stimulate it to overgrowth and 

 to remarkable secretory activity. They do not need to 

 mine the tissues, for they possess the power to stimulate 

 the plant to produce both food and shelter. From the walls 

 of the gall chamber they lap up the food that the tissues 

 prepare for them and bring to them. This is a wonderful 

 power, but not all gall-making insects possess it to the 

 degree that is sufficient to meet all the needs of their larval 

 life. A number of them that are true gall-makers at first, 

 have to resort to the use of their jaws in ordinary foraging 

 when of a larger growth. The tulip-tree blister-gall maker, 

 Thecodiplosis liriodendri, for example, at first makes a thick- 

 walled discoid gall within the leaf, and later eats most of it 



