The Collaborative International Dictionary of English v.0.48:

Tanystomata \Tan`y*stom"a*ta\, n. pl. [NL., fr. Gr. ? to stretch
   + ?, ?, mouth.] (Zool.)
   A division of dipterous insects in which the proboscis is
   large and contains lancelike mandibles and maxillae. The
   horseflies and robber flies are examples.
   [1913 Webster]