143 



it is simply separated from the cuticle ; whether it is the one or 

 the other is a necessary consequence of the structure of the 

 head and trophi. At whatever period of larval life the moult 

 takes place, at which the form in fig. 4 is assumed, the character 

 of the mine is changed at once ; and thereupon the larva, as in 

 the cylindrical group of Lithocolletis and in Gracilaria, proceeds 

 to eat out the parenchyma, or leaves the mine ; or, as in the flat 

 and ornatella groups of Lithocolletis, and in Phyllocnistis, at 

 once ceases to feed and spins its cocoon, to do which L. ornatella 

 leaves the mine. In the cylindrical group and in Gracilaria, the 

 larva, after that change, finds itself unable, from the position of 

 the head and the structure of the trophi, to continue to separate 

 the cuticle from the parenchyma, and must feed upon the latter; 

 and the body having assumed a more cylindrical form, the cuticle 

 presses uncomfortably on it ; this leads the larva if it continues 

 to mine, as in Lithocolletis, to meet its changed condition by 

 making its mine into a tentiform one, or, as in most Gracilaria, 

 to leave the mine and feed externally. Some of the latter also, 

 as Gr. erigeronella^ make the mine tentiform ; others, with more 

 flattened bodies, as 6r. rohiniella, make sufficient room by eating 

 out the parenchyma. Previous to this cliange there is no indi- 

 cation of any instinct to spin a web, probably because there is 

 no organ to elaborate the silk, or to spin it. It is at this change 

 that the silk glands and the spinneret first appear, just in time 

 to meet the wants of the larva, which could neither give its 

 mine the tentiform character, nor subsist externally, without the 

 ability to spin. The tentiform character of the mine is caused, 

 in part at least, by the shrinkage of the silken web ; and to 

 secure its hold on the leaf, as an external feeder, the larva must 

 spin a few threads upon the surface. Large larvas no doubt aid 

 in curling the leaf to make the mine, or to feed externally, by 

 drawing the silken threads ; but in small larvae this is accom- 

 plished mainly by the contraction of the silk itself. 



In the Nat. Hist. Tin., v. 2, Mr. Stainton enumerates 

 seventy-six species of Lithocolletis as known in 1857, and many 

 others have been discovered in Europe since that date. In my 

 Index in the Bull. Geol. Geog. Surv., v. 3, seventy American 

 species are enumerated. The total number now known is 



