170 
The Apple Mussel Scale. 
three pairs of legs, and two blunt outgrowths at the end of the body. 
On each side of the larva are marginal spines. The head segment 
carries a beak lengthened into a proboscis or sucker, twice the length 
of the body, and coiled up in a sort of pouch which occupies almost 
the entire length of the middle line along the under surface of the 
body. This sucker is unrolled when the larva desires to fix itself. 
At this stage the creature bears a close resemblance to the young 
wingless Phylloxera 1 which infests the grape vine. 
As the hatching progresses the larvae crawl over the surface of 
the tree, showing preference for the sunny side. According to the 
observations made they fix themselves not later than the end of 
three weeks, and henceforth cease their wanderings. They now 
begin to envelop themselves, behind and at the sides, with a lace- 
work of delicate white filaments, secreted from the blunt outgrowths 
at the rear end of the body : these threads are much more delicate 
than the fine ones which have been mentioned as passing amongst 
the eggs. Soon after the formation of this incomplete silky shroud 
the creature undergoes the first moult, due to its increasing size. 
The skin splits along the middle line below, thus setting free the 
sucker which, being uncoiled, is forced into the tissues of the bark 
of the tree, its extreme slenderness facilitating its penetration. As 
to the cast skin, it becomes adherent to the waxy secretion and forms, 
in conjunction with the latter, the first covering (the incipient scale) 
— at the outset very thin — of the animal. A further quantity of 
waxy material is secreted, to which a subsequent moult adds a fresh 
cast skin. By a continuation of this process the protective scale, 
which gradually thickens, is thus built up of a series of skins succes- 
sively rejected by the animal, and kept together by the glutinous 
secretion which is incessantly poured out. 
As the moults follow, one after another, the creature undergoes 
a retrogression ; first its eyes disappear, then its legs become atro- 
phied and are no more seen, and finally the antenna; vanish also. 
The preceding description applies only to the female insect. The 
males of Mytilaspis pomorum, like those of some other scale insects, 
have not yet been observed, and nothing is known as to the fecun- 
dation of the female. 
The number of scales to be met with, even upon one tree, is 
enormous. Generally, they arrange themselves in groups or clusters, 
but in some cases all the branches and also the trunk, especially of 
young trees, are entirely covered. As, beneath each scale, the insect 
buries its sucker in the tissues of the bark in order to extract there- 
from the juices of the tree, it is easy to understand how myriads of 
them, all working simultaneously, at length exhaust the tree and 
finally cause its death. 
Whilst these pests are mostly constrained, on account of the 
delicate structure of their suckers, to attack only the younger shoots, 
yet, as the apple trees grow, the bark exfoliates, and on the newly - 
1 An illustration of Phylloxera vastatrix is given in the Journal, Yol. III. 
3rd Series, Part II., 1892, p. 418. 
