49 



of Study, p. 75.) But here — as also in the case of the common Bar- 

 nacle, which begins life as a highly-organized locomotive crab, and 

 ends life by becoming permanently attached like a plant to a ship's 

 bottom, and by having many of its former organs either aborted or 

 degraded — we clearly find an exception to what is undoubtedly a gen- 

 eral, though not a universal rule. Nor is the exception confined to 

 this one species of Bark-lice. So far as I have ascertained, it pre- 

 vails universally throughout two of the commonest genera of the 

 great Family of Bark-lice (Aspidiotus and Lecanium.) 



After this most anomalous and wonderful transformation, the 

 body of the original insect grows scarcely at all, the total increase in 

 its length or breadth being only about one-sixth. But now com- 

 mences another most strange and anomalous process. From the tail 

 end of the limbless and apparently lifeless scale, which is all that re- 

 mains of the once highly-organized larva, there gradually in the 

 course of a few days protrudes backwards a thin membranous sack, 

 closely appressed to the bark like the original scale, and so far as out- 

 line goes forming an elongated continuation of it, but differing from 

 it very obviously in color and texture. In 14 days' time this elon- 

 gated sack has become in many specimens as long as the original 

 body; and it grows and increases backwards at a prodigious rate 

 thereafter, till by the middle of August the whole has assumed its. 

 permanent shape; and what remains now of the original body of the 

 larva forms merely a minute, yellowish-brown, oval plate, pressed 

 down obliquely upon the forward end of the so-called scale-insect, just 

 as one of these stylish modern trencher-caps is pressed down upon the 

 forehead of a fashionably dressed young lady in the year of our Lord 

 3 867. Examine the Oyster-shell Bark-louse when you will, from 

 the middle of August to the middle of the following May, and you 

 will find it is externally always the same. In front there is what is 

 left of the originally perfect, but now degraded and defunctionate 

 larva, being an oval scale, of a somewhat shining yellowish brown 

 color, and with one longitudinal ridge running from end to end, on 

 each side of which are several indistinct transverse grooves, being all 

 that remains to indicate that this was once a highly-organized animal, 

 divided by the usual transverse sutures into the normal 13 segments 

 found in the larva of almost every insect. Behind this yellowish- 

 brown scale — which I shall for convenience' sake call "the larval 

 scale" — may be seen a rather longer and wider one — which I shall 

 call "the medial scale" — without any ridges or grooves and of the 

 same opaque greenish-brown color as the bark, but often, especially 

 at its hind end, tinged more or less with yellowish; and 

 behind this again, and closely connected with it, the rest of the enor- 



