86 WHITE PINE LEAF-LOUSE. 



Perhaps a correct idea of the nature of the several envelopes 

 with which these anomalous insects are invested, may be obtained 

 by tracing the analogy which exists between them and the succes- 

 sive integuments of the higher insects, more especially the cater- 

 pillars or larvae of the nocturnal Lepidoptera. The larval and 

 medial scales may be considered as analagous to the tirst and sec- 

 ond skins of the caterpillar before and after the first moult, the 

 most important difference between the two successive envelopes 

 being the increase of size, in order to accommodate the insects' 

 growth. The anal sack exhibits a remarkable analogy to the 

 cocoon in which the caterpillar subsequently incloses itself. Nei- 

 ther is strictly a part of the insect, but is constructed by it solely 

 as a means of protection. Both are constructed from silken threads 

 secreted by the insect, with this difference : that the caterpillar 

 constructs its cocoon with a single thread, secreted through a spin- 

 aret near the mouth, whilst the Coccus forms the anal sack from a 

 number of threads produced from pores in the posterior part of its 

 body. The analogy seems to fail in that the caterpillar leaves its 

 cocoon upon arriving at maturity and before depositing the germs 

 of its I'utnre progeny, whilst the Coccus lays her eggs beneath, or 

 more strictly speaking, within the anal sack. But even here the 

 analogy is maintained by certain exceptional moths, one of 

 which, the Tussock-moth {Orgyia lencostigma)^ never wholly de- 

 serts her cocoon, but lays her eggs upon the outside of it ; and 

 another, the Basket-worm moth {Tkyridopteryx ephemercBformis , 

 preserves the analogy still more completely by actually depositing 

 her eggs within her cocoon. 



Such is the view which I have been led to adopt, after an al- 

 most daily examination of tlie development of these insects du- 

 ing the past summer, of the nature and formation of these scales 

 and their component parts. The whole subject is, at first sight, 

 abstruse and difficult, and entomologists have held a diversity of 

 opinions concerning it. Drs. Harris and Fitch, probably from not 

 having traced the insect through all its stages, thought that the 

 whole scale was the dried remains of the mother insect. Dr. 

 Shimer supposed that all the parts of the scale were the results of 

 successive moultings, and adopted the gratuitous notion that they 

 are cemented together by the animal's excrement; and Mr.Walsh, 



