GENERAL 17 



to the light and watching them at work by looking through 

 the transparent epidermis. However dark colored the 

 body of the mine, the outer margin when freshly excavated 

 is apt to be clear enough to permit the movement of the 

 larvae to be seen. By one means or another — legs, prolegs, 

 ambulatory processes or setae, anal sucker: any or all of 

 these — the body is held rigid while the jaws are moved up 

 against fresh mesophyll. But there is a great difference in 

 jaws and consequently in manner of attacking the mesophyll. 

 If one watch the larva of a beetle like Zeugophora or of a 

 sawfly like Fenusa, the head will be seen to be thrust for- 

 ward and drawn backward, in and out of the prothorax at 

 every bite. The opened mandibles are driven forward into 

 the new tissue, closed and withdrawn with each movement 

 of the whole head forward and backward. The prolonga- 

 tions of the chitinous shell of the head to rearward have 

 muscles attached that control this in and out movement. 



If one watch a highly specialized sap-feeding Lepidopter- 

 ous larva in its mine the thin, flat, saw-edged, wall-shearing 

 mandibles will be seen to be moved steadily along, oscillat- 

 ing as they go, cutting a thin slit through one cell-layer only, 

 while the freed cell contents as steadily flow into the mouth 

 of the caterpillar. Generally in these mandibulate larvae 

 the ventral side of the body lies flat against the epidermis. 



In contrast with this, the Dipterous maggot lies on its 

 side in the mine, as already stated, and swings its mouth 

 hooks vertically; i.e., in the median plane of its body, 

 parallel to the leaf surface. The body remains stationary 

 except at the anterior end, which swings in the arc of a 

 circle as the shearing of the mouth hooks proceeds. On 

 certain plants, like the windflower, Anemone pennsylvanica, 

 the shorn cellulose walls lie in the mine in swaths, like 

 leaning stubble cut with a dull scythe, and show exactly 

 the course the larva has taken from side to side within the 

 interspaces that are bounded by the stronger veins. The 



