40 LEAF-MINING INSECTS 



Hence there is no better clue to follow in identifying leaf- 

 miners than the name of the plant in whose leaves they are 

 found. 



Some of them live in plants that are to us poisonous or 

 otherwise offensive, but their dwelling place is the soft, 

 nourishing parenchyma well apart from noxious products. 

 The aromatics of the mints and the irritants of the poison 

 ivy are produced in epidermal hairs, far over their heads, and 

 the latex of milkweed is in special vessels in the veins be- 

 neath their feet. However, if we open a number of mines of 

 the little fly, Agromyza pusilla, in milkweed leaves, we are 

 likely to find that now and then some careless maggot has cut 

 into a latex vessel, and it has leaked into the mine and 

 formed a white clot there; and occasionally we may find a 

 larva dead and buried in a large clot, having paid the penalty 

 for his careless operations. 



