104 LEAF-MINING insist- 



Group II. Phyllocni8ttB 



The young larvae of Phyllocnistis are so flat they look as 

 if fchey bad been run over by a steam roller. They are 

 exclusively sap feeders, having Bat triangular heads, thin 

 eell-shearing mandibles (see fig. 22), and a scries of lateral 

 expansions of body segments into paired marginal lobes 

 giving a moniliform outline. They are quite destitute of 

 legs. 



The mines are very long, narrow and tortuous. Their 

 shape perhaps has soi ii( 4 relation to the complete repression 

 of the usual motile organs and the use of the very elongated 

 lateral mamillations in progression. Since the larvae do 

 not ingest any of the solid matter of the leaf, the mines arc 

 but slightly transparent, even in thin leaves, and in the 

 thicker ones do not show at all by transmitted light. By 

 reflected light the mines show as white tracings on the leaves. 

 In some cases there is a central frass line of a dark color, for 

 instance in the mine of P. populiella; in others one cannot 

 distinguish any sign of excrement as is in the mine of P. viti- 

 genella in grape. The course looks like a tiny tracing of 

 mucous substance on the surface — a '-snail's track" it has 

 been called. 



In the fourth instar the larva assumes a strange form. It 

 is more nearly cylindrical than in the earlier instars but of 

 the mouth parts only the spinneret is functional. The 

 mandibles are rudimentary. Eyes are lacking entirely. 

 At the end of the third instar the mine is slightly widened 

 and in the pocket thus formed the fourth instar begins. 

 The larvae employ the newly developed spinneret in spin- 

 ning silk over the entire inner surface of this expansion. The 

 thin cuticle shrinks as the silk dries and, as the pocket is 

 usually formed at the very edge of the leaf, this causes the 

 margin of the leaf to be folded over the larvae in a little 

 pucker or knot. More silk is spun within to make the walls 

 firm and then the larvae transform to pupae. In less than 



