85 
[ 345 ] 
more opaque, with the segmental divisions less sharply defined, 
and about four times as large; that is, about twice as long and 
twice as broad, indicating the corresponding growth of the insect. 
This is the medial scale. It is, in reality, the second larval skin, 
and though in the course of a very short time, not exceeding a 
day or two, the insect beneath becomes detached Irom it, as it did 
from the first envelope, yet there is a short period when it is evi¬ 
dently a part of the insect itself, and cannot be detached from it 
without violence. 
As soon as the medial scale is formed, there begins to appear 
from under its posterior edge, a white membraneous border,which 
is the commencement of the anal sack. This increases rapidly 
day by day, so that in from two to three weeks from the time the 
insect hatched from the egg, the growth of the whole scale is com. 
pleted. The anal sack, when fully formed, is more than four 
times "as large as both the former scales combined, by which I 
mean, as in the former case, not four times as long, but mere 
than twice as long, and considerably more than twice as broad. 
It is of a pure milk-white color, beautifully contrasting with the 
amber colored larval and medial scales, and rendering this a really 
elegant little insect, notwithstanding its pernicious habits and its 
opprobrious name. 
If we raise the scale at any time during the growth of the anal 
sack, we find the soft, wriukled, memberless body ot the insect 
itself, apparently wholly detached from the scales above, and at 
once suggesting the question, in what manner and from what 
source is the growth of the anal sack accomplished. Upon care¬ 
fully examining the insect, however, with a strong magnifier, a 
number of fine silken threads can be detected projecting Irom its 
sides and posterior extremity, which were ruptured in the act of 
raising the scale, and which formed the connecting tissue between 
the insect and the scale above. It must be by means ot these 
filaments that the anal sack is constructed. What strikes us as 
remarkable is, that so comparatively large and rapid a growth 
can take place through such sparse and attenuated media. We 
see, from this account, that the anal sack is very different in its 
nature from the two preceding envelopes, and never, like them, 
strictly constitutes a part of the insect itselt. 
