18 TRANSFORMATIONS OF INSECTS. 
These pieces are intimately united, and have been called pre- 
scutum, scutum, scutellum, and postscutellum. The first is always 
very rudimentary, and is a little lamina placed vertically to the 
edge of the segment, and remains in a membranous condition 
in a great number of insects. The second, which supports the 
wings, is larger. The third, often very small, is placed between 
the wing cases of most beetles. The last piece, often reduced 
to a mere ridge behind the third piece, assists in the formation 
of the joints of the wings. 
The thorax sustains the locomotive organs. These are the 
true legs which belong to the lower arches of the segments, and 
the wings which are connected to the upper arches. 
The true legs often exist in the young insect, but are then 
very small and scale-like, and they do not attain a considerable 
size until adult age is reached. They are from the first in three 
LARVA OF THE SILKWORM. TRUE AND MEMBRANOUS LEGS. 
pairs. Those attached to the prothorax are the anterior legs; 
those of the mesothorax are the intermediate; and the posterior 
are in relation with the metathorax. 
The legs upon the segments of the abdomen are, as has already 
been noticed, not persistent; they exist in caterpillars especially, 
and are really membranous feet, being provided with hairs, spines, 
and hooks to hold on with, but they are lost during the metamor- 
phosis into the chrysalis condition. The front or true legs are the 
curious jointed structures so finely hairy and so well provided with 
suckers, claws, and brushes of hair, and are mainly developed during 
the period of rest which characterises the chrysalis state. 
The distinction .between the true legs and the sucker-like 
feet of the caterpillar is very observable in the engraving of the 
silkworm larva; the true or scale-like legs are in front, and are 
not adhering to anything, but the membranous legs support the 
insect. 
