108 LEAF-MINING INSECTS 



again and feed in the following instar in an entirely similar 

 way. 



When the larvae are full-fed they spin pupating cocoons. 

 These are sometimes placed on the leaves themselves, but 

 are more often to be found on the twigs or upon nearby 

 herbage. Save in a few species that feed on compositae, 

 they are elongate structures of a workmanship so delicate 

 and finished that they have been the wonder and admiration 

 of naturalists of many generations. Because they are so 

 remarkable in themselves as well as being so characteristic 

 of the genus, we use the name, the ribbed-cocoon maker (first 

 suggested by Slingerland for the common species on apple) 

 to apply to any member of the group. A careful and ap- 

 preciative account of the making of such a cocoon was 

 written by Charles De Geer before the middle of the eight- 

 eenth century. It will be found in the first volume of his 

 Memoir es under the heading " Little, green, naked caterpillar 

 with sixteen legs which gnaws the lower surface of the leaves 

 of the frangula and is nourished of them." Though the 

 account is long it seems to me to warrant extensive quota- 

 tion. 1 He says 



When they [i.e., the larvae] have attained their full size .... 

 they spin upon the leaves extremely pretty little elongate cocoons, 

 that are noteworthy because of their unusual form. . . . 



They are of about the same length as the caterpillars. Their 

 color is greenish white . . . They are marked longitudinally 

 with folds. . . . which show on the cocoon as rather deep 

 furrows. One may count seven, eight or nine of them on each 

 cocoon and they extend from one end to the other. . . . The 

 form [i.e., of the cocoon] is like the half of a very elongate spheroid 

 that has been cut in two along its greatest diameter; for the lower 

 side of the cocoon, the surface which is applied to the leaf, is flat 

 and smooth and the other or superior surface is convex. 



The ridges and the furrows of the cocoon indicate much industry 



1 The translation is by Mrs. Tothill. 



