WHITE PINE LEAF-LOUSE. 



85 



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 from 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,whieh 

 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 more 

 than twice as loag, 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, wrinkled, memberless body of 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 from 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 of 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 difi'erent in its 

 nature from the two preceding envelopes, and never, like them, 

 strictly constitutes a part of the insect itself. 



