How to distinguish insect larvae
Caterpillars can be distinguished from larvae of beetles,
sawflies, and true flies by the number and arrangement of
their appendages. Caterpillars and larvae of beetles and
sawflies have three pairs of true legs, one pair on each
thoracic segment. Most sawfly larvae also have fleshy protuberances
or leglike appendages (called prolegs) on all of their abdominal
segments. Caterpillars have prolegs on some abdominal segments,
but never on the first two abdominal segments. Beetle larvae
have true legs, but no prolegs. Fly larvae (such as predatory
syrphids) have no true legs. Fly larvae can have either smooth
bodies or fleshy protuberances, which can be just on their
abdomen or on both their abdomen and thorax. | 
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