50 



mous elongated sack protruded in the space of about two months from 

 the tail end of the larva, which is always of the same greenish-brown 

 color as the bark. This posterior sack, which I shall call the "anal 

 sack/' is in its widest part about twice as broad as the "larval scalfe" 

 is long, and, together with the ''modial scale," is from 4 to 10 times 

 as long as the "larval scale," but most commonly about 8 or 10 times 

 as long. If the whole scale-insect is lifted up by the point of a pen- 

 knife about the middle of August, the white eggs previously referred 

 to may be found underneath it, the delicate part of the protruded 

 sack that adheres to tlie bark being usually more or less torn open by 

 the operation; and the eggs remain under the scale, without further 

 development, all through the winter and until the middle of the fol- 

 lowing May. In the course of the winter they doubtless freeze and 

 thaw, and thaw and freeze, scores of times; but, as with almost all 

 insects when they are hibernating, this produces not the slightest 

 effect upon their vitality. 



Authors, who have never traced a Bark-louse day after day 

 through all these astonishing transformations, have erroneously 

 hinted that the "larval scale" represents the head, that the "medial 

 scale" represents the thorax, and the large "anal sack" behind the 

 whole represents the abdomen of a normal insect. (See Fitch, New 

 York Reports, I. p. 257.) But no such thing can possibly be; for 

 there is externally no perceptible change in the "larval scale," except 

 a very slight one in size, from the days when the first rudiments of 

 the '"medial scale" and of the "anal sack" are protruded from behind 

 it, to the day when both are fally developed. Consequently, as the 

 "larval scale" represented originally both head, thorax and abdomen 

 of a normal insect, and as it ever afterwards remains unchanged, it 

 cannot afterwards represent the head alone of a normal insect. 



What may be the precise nature of this singular "medial scale" 

 and "anal sack," is hard to tell with absolute certainty. They are 

 not, however, peculiar to the Oyster-shell Bark-louse, but are charac- 

 teristic of the whole genus {Aspidiotus,) to which both this species 

 and Harris's Bark-louse belong. In a very elongated and narrow 

 species of the same genus, the Pine-loaf Scale-insect, {Aspidiotus pini- 

 folice. Fitch,) found on the lea\es of the White Pine (Pinus stro- 

 bus) — which, by the way, I have ascertained to contain in Novem- 

 ber eggs of the same blood-red color, as those of Harris's Bark-louse 

 — the "anal sack" is of a pure milk-white color, and the "larval and 

 medial scales" are very distinct from each other and from the "anal 

 sack," and are both of them of a yellowish-brown color. In an un- 

 described species, which may be called the "Black-willow Bark-louse"' 



