The Collaborative International Dictionary of English v.0.48:

Inchworm \Inch"worm`\, n. (Zool.)
   The larva of any geometrid moth. It progresses forward by
   first bringing the rear end of the body forward, forming a
   loop, then moving the front part of the body; called also
   measuring worm, measuringworm, spanner, and looper.
   See Geometrid.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dropworm \Drop"worm`\ (dr[o^]p"w[^u]rm`), n. (Zool.)
   The larva of any geometrid moth, which drops from trees by
   means of a thread of silk, as the cankerworm or inchworm.
   See inchworm and geometrid.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inchworm
    n 1: small hairless caterpillar having legs on only its front
         and rear segments; mostly larvae of moths of the family
         Geometridae [syn: measuring worm, inchworm, looper]