156 LEAF-MINING INSECTS 



leaf, but differs in the fact that the mine of the latter is only 

 observable from the upper side of the leaf, while that of our insect 

 can be seen from both sides and is also of a darker color. 



The mine when completed is an irregular, frequently more or 

 less triangular, rather dark colored blotch, averaging 6 or 7 mm. 

 in its longest diameter and observable from both surfaces of the 

 leaf. Up to the time when the larva has attained full growth, the 

 mine is translucent, the only dark spots being the larva itself and 

 the excrement which is collected in an irregular cake of minute 

 pellets in the region where the mine was first begun. Soon, how- 

 ever, the translucency of the broader end of the mine begins to 

 be obscured in an oval spot, and if it could be opened the larva 

 would be found busily engaged in lining both surfaces of the leaf 

 with white silk, mapping out the size and shape of its future case. 

 After this lining has become sufficiently thick, the larva commences 

 to cut through both surfaces of the leaf at the edge of the oval 

 lining, and to draw them together and fasten them with silk as 

 it goes. When the circumference of the oval has been cut and 

 fastened, with the exception of a small portion at one end, the 

 larva at that point cuts through the upper surface alone, partly 

 issues from its case, and weaves a strong cord of silk from the 

 surface of the leaf on beyond the mine back to the mouth of the 

 case. Then, everything being securely fastened, it cuts the last 

 band of the lower membrane which still remains intact, and stands 

 upon the upper surface of the leaf with its completed case upon 

 its back. The next step is to cut the supporting cord, and the 

 larva is free to start upon its travels. 



In walking, the head and first three thoracic segments alone are 

 protruded from the case, the soft hinder parts being thus pro- 

 tected. The abdomen with the inclosing case is lifted erect in 

 the air, so that it does not drag upon the insect as it walks. After 

 progressing for an inch or so the larva usually drops from the 

 leaf, spinning a long silken thread as it falls. In this way it either 

 reaches the ground, or, what is much more common, falls upon or 

 is blown by the wind to a limb or the trunk. It travels a greater 

 or less distance further until it finds what seems to it to be a proper 

 place, and there, after attaching the case firmly to the bark by a 

 button of silk, it sooner or later transforms to a pupa. 



