The eyes are of so simple form that they are supposed only
to distinguish light from darkness. The complicated mouth is at
the lower part of the head. It carries a heavy pair of cutters
with which the caterpillar bites off large pieces of leaf, a first
pair of grinders with which it macerates the food, and a second
pair that join in forming the under lip. There is also the tube that
connects with the silk glands and ends in the spinneret. Through
this tube a fluid is forced that by movements of the head the
caterpillar attaches where it will and draws into fine threads that
at once harden in silk. This organism is sufficiently developed
for use in a newly emerged caterpillar, for it can spin threads by
which to drop from leaf to leaf or to guide it back to a starting
point.
The thorax is covered by the first three rings behind the head,
and on it are six legs, two on each segment. The remainder of the
caterpillar is abdominal and carries small pro-legs with which to
help it cling to twigs and leaves, and the heavy anal props that
support the vent.
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