212 LEAF-MINING INSECTS 



With transmitted light and a good lens one may watch the 

 process of emergence from the egg into the leaf tissue. Most 

 sawflies emerge almost at once and feed on the surface of 

 the leaf but the true miners tunnel slowly upwards towards 

 the upper epidermis. Then, hanging upside down from the 

 upper cuticle, they tunnel through the parenchyma and the 

 finer divisions of the fibrovascular system. In feeding they 

 take rapid nibbling bites directly in line with the long plane 

 of their bodies. They turn the head and body from side 

 to side but in no case bend their heads alone to feed on the 

 exposed surface. They hold to the walls of the mine chiefly 

 by the posterior parts of the body. In Metallus rubi the 

 anal legs are somewhat fused and ridged at the base with 

 chitin. This gives them an anal sucker effectively employed 

 in holding and in moving about. 



Most of the leaf content is consumed. On the upper side 

 remains the cuticle with a faint tracery of the fibrovascular 

 pattern adhering to it. This tracery of filbrils distinguishes 

 at once a sawfly mine from the blotch mines of the sap-feed- 

 ing Lepidoptera, in which the cuticle is left entirely free of 

 any other tissue. On the lower side there is besides the 

 cuticle, usually a greater share of fibrovascular tissue than 

 is on the upper side — especially in leaves in which the veins 

 are prominent below; and, in the thicker leaved host plants, 

 as alder, some residual parenchyma. Most sawfly mines 

 are very transparent. Scattered through the mines is an 

 abundance of excrement in very black cylindrical fragments 

 perhaps four times as long as broad. The larvae seem bigger 

 in proportion to the mine than most lepidopterous miners. 

 They are not greatly depressed and they cause the mines to 

 become somewhat bulged. Large veins partly determine 

 the limits of the mines. 



The larva. In form, the larvae are generalized rather than 

 specialized miners. They are considerably depressed and 

 somewhat moniliform; the somewhat triangular head-cap- 



