ARTICULATA. 135 
When the caterpillar incloses itself in a cocoon to take the helpless con- 
dition of a pupa, various internal changes take place, tending towards the 
organization of the future insect; and even previous to this, the internal 
organization of the larva had been gradually undergoing various changes 
in the nervous and alimentary systems, unaccompanied by any external 
change except that of size. Some pupe are protected by a cocoon, and 
some are not thus protected. Of the latter, some are suspended by the 
posterior extremity, and others, as Papilio, attach themselves with the head 
above, and a thread around the body to maintain it in its position. Some 
suspended pupz are finely marked with bright colors and golden spots, 
whence the name of Chrysalis, which is seldom used, and aurelia, which is 
obsolete. 
Agassiz has pointed out several curious analogies among the classes of 
Articulata, composing the larve of Lepidoptera (particularly those with 
bristly tufts) to the Annelida; and the pupe in which the abdomen alone is 
capable of motion (the head and thorax being united under a kind of cara- 
pace) to the decapodous Crustacea, which are, on account of this affinity 
with one of the conditions of insects, placed at the head of their class. He 
places the Insects above the Crustacea, because the former leave the con- 
dition in which they are covered by a carapace, and advance a step further. 
Finally, this philosophical author places the Lepidoptera at the head of the 
insects (as Swainson had done upon different grounds), because the larva 
is mandibulate, and the adult insect perfectly haustellate, so that it advances 
further from the larva condition than any of the orders. See Lectures on 
Embryology ; and Proceed. Am. Assoc., Charleston, 1850. 
Imago. The perfect insect or imago appears when the case of the pupa 
is split, and in the winged species, the wings, which were closely folded, 
begin to expand and take their final shape. The imago differs from the 
larva in having the body divided into the three principal divisions of head, 
thorax, and abdomen. 
The integument of insects contains a peculiar principle, named chiztine 
by Odier, which constitutes a third or fourth of its bulk. It resembles bone 
somewhat in its composition, as it contains phosphate of lime, and a trace 
‘of other compounds found in bone. The chitine forms part of the dermis, 
which is covered by a thin epidermis. Coccine is another chemical con- 
stitnent, found particularly in the genus Coccus. 
The head of insects is usually regarded as a single piece, but as the cor- 
-responding part in the Crustacea is considered to be made up of minor parts, 
it has been proposed to extend the same theory to the class under con- 
sideration, either allowing as many theoretical segments as there are kinds 
of appendages, or allowing two kinds of appendages to some of them. Of 
these two modes of viewing the question, the former is preferred in studying 
the Crustacea,.and should therefore have the preference in this class. 
The organs concerned in manducation (enumerating them from above) 
are, the upper lip or labrum, the mandibula, or upper jaws, the maxille or * 
lower jaws, the tongue or dégula, and the lower lip or labium. Brullé 
divides the ligula into an upper organ or epipharynx, and a lower one or 
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