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86 
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 first 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 future 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 encostigma), never wholly de¬ 
serts her cocoon, but lays her eggs upon the outside of it; and 
another, the Basket-worm moth (Thyridopteryx ephemerceformis , 
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 the 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 mouJtings, and adopted the gratuitous notion that they 
are cemented together by the animal’s excrement; and Mr.Walsh, 
