124 LEAF-MINING INSECTS 



ment of this leaf-rolling period continues to be deposited in 

 rather dry rounded globules and is somewhat localized 

 within the shelter. When a large share of the soft tissue 

 has been removed from the inner walls of this larger enclosure 

 with perhaps a little accessory tissue reached from beyond 

 its borders, the larvae prepare for pupation. They spin 

 rather dense and tough elongate, whitish cocoons within 

 the feeding shelter and presently cast their last larval skins 

 revealing a slender motile pupa with backwardly pointing 

 spines and a serrated cutting plate upon the ventral surface 

 of the head. These structures assist later in forcing the 

 pupal shell through the cocoon and encircling leaf tissues 

 to allow the emergence of the adult. 



Acrocercops 



This is a small group of leaf-miners whose larvae have 

 two, flat, sap-feeding instars, that are followed, so far as 

 known by three that are tissue-feeding. The mines begin 

 variously, at first being irregularly serpentine, but sooner 

 or later they widen into a blotch. The pupa is formed either 

 within or outside the mine. 



The strigiftn miner, A. strigifinitella, is the species whose 

 habits are best known. It is especially interesting, though 

 perhaps a bit aberrant, in its gradual shift from the flat to 

 the cylindric form of larva. We quote as follows from Hein- 

 rich and DeGryse's (1915) excellent account of it: 



Chestnut appears to be the favorite food plant and during mid- 

 summer the work of the species is very common, few of the young 

 leaves escaping infestation, some bearing as many as four separate 

 mines. When the proper food supply is abundant, however, 

 there is rarely more than one or two to the leaf. There are a 

 number of generations with considerable overlapping so that 

 larvae are to be found any time from May till well on into October. 

 The first larval brood appears in spring as soon as the leaves are 

 formed. During July and August the dominant period in the 



