The Collaborative International Dictionary of English v.0.48:

Proleg \Pro"leg\, n. [Pref. pro- for, in place of + leg.]
   (Zool.)
   One of the fleshy legs found on the abdominal segments of the
   larv[ae] of Lepidoptera, sawflies, and some other insects.
   Those of Lepidoptera have a circle of hooks. Called also
   proped, propleg, and falseleg.
   [1913 Webster]