84 WILLIAM T. M. FORBES 



cells on either side. Some species mine different sides of the leaf at different 

 periods of liirval life. The mine may be a linear tract, gradually increasing in 

 brendth to its end, or it may at some point suddenly enlarge into a blotch. A 

 change in -the character of the mine usually indicates the beginning of a new 

 instar. There are four larval instars. The mine formed during the first instar is 

 very short, rarely exceeding a few millimeters in length. The large, conspicuous 

 part is made during the last larval instar, in the few days preceding the escape 

 of the larva from the mine. The larva leaves the mine by a semi-circular slit 

 in the upper or the lower epidermis and spins the characteristic cocoon, usually 

 brownish, Imt occasionally yellowish or whitish, in the surface soil or amongst 

 rubbish, often near the base of the tree; occasionally it spins on the twigs or 

 branches. Pupation does not take place immediately; in the summer generations 

 it occurs a few days before emergence, in the overwintering generation it may be 

 delayed 'until spring. 



A few species have but one generation a year; most species have two or three; 

 a few of the oak-feeding species may have as many as four generations. The length 

 of the life cycle is approximately six weeks, except in the single-generation 

 species, where several months may elapse between oviposition and the attain- 

 ment of full growth by the larva. The moths from the over-wintering generation 

 of larva- emerge in May and .June, a few species, however, appearing in April. 



Key, to tl\c species 



a. Fore wings with pale spots or fascire. 



b. A silvery or pale golden metallic spot at, or very near, the base of the 



wing, 

 c. A median fascia. 



d. A costal and a dorsal spot at three-fourths 3. pteliaeella. 



dd. No such spots at three-fourths. 



e. Thorax and extreme base of wing purple 1. argentifasciella. 



ee. Thorax and extreme base of wing golden 2. scintillans. 



cc. No median fascia 4. quadrinotata. 



bb. Without such a spot. 



c. Basal third of wing bull' 9. cerea. 



cc. Basal third of wing not bull. 



d. A pale costal spot at one-third 5. trinotata. 



dd. Without a pale costal spot at one-third, 

 e. Two pale fasciae. 



f. Thorax creamy buff : 8. rhamnicola. 



ff. Thorax dark. 



g. Head black 7. intermedia. 



gg. Head ochraceous 6. bifasciella. 



ee. One pale fascia. 



f. Fascia more or less interrupted. 



g. Male with a chitinous plate from base to near middle of costa 

 of hind wing. 



h. Fascia silvery 32. platanella. 



hh. Fascia not silvery; male with long hair-pencil from base 



of costa of hind wing. 34. similella. 



gg. No such chitinous plate in male 33. clemensella. 



ff. Fascia complete. 



g. Fascia before the middle; wing lusterless. 



h. Whitish costal and dorsal spots at three-fourths. 



35. thoracealbella. 

 hh. No such spots at three-fourths 40. latifasciella.. 



