Evolution of larval head-fans in Simuliidae (Diptera) as inferred from the structure and biology of Crozetia crozetensis (Womersley) compared with other genera

The life stages of C. crozetensis whose larvae have raking head-fans are described. Functional anatomical study of the fans compared with the filtering fans of normal Simuliidae together with SEM study of first instar larvae of Gymnopais, Twinnia and Prosimulium enable the evolution of cephilic fans to be inferred as follows: a group of lateral labral bristles evolved in two directions:

• 1. 

  To complete loss in Gymnopais and Twinnia whose ancestors therefore never possessed fully developed fans.
• 2. 

  Elaboration of the group of bristles first into raking organs, as in Crozetia , and later into even more complex filtering organs.

It is shown that unflexing of Simuliid fans depends on elastic energy stored in resilin-containing components deformed during flexing.
Adults of Crozetia mate on stones in streams on lie de la Possession, Ties Crozet. The females are non-biting and rest near streams while their eggs mature, thus forming a sub-Antarctic parallel to certain sub-Arctic Simuliidae.