The labrum and cephalic fans of larval Simuliidae (Diptera: Nematocera)

Abstract

First-instar larvae of Gymnopais sp. (near dichopticus Stone), Twinnia tibblesi Stone and Jamnback, Prosimulium magnum Dyar and Shannon, P. susanae Peterson, P. travisi Stone, and Crozetia crozetense (Womersley) possess simple cephalic fans. Simple cephalic fans are absent from first-instar G. dichopticus Stone and T. biclavata Stone and from all later instars. Later-instar Gymnopais retain vestigial messores and messorial apodemes which make contact with the tormal apodeme.Between first and later instars of Gymnopais and Twinnia, the tormal apodeme connection shifts from the torma to the anterior palatal bar, through differential sclerotization.Homologies are established for the labral and labropalatal sclerites, within the Simuliidae, and with those of other nematocerous larvae.The anterior palatal bar, present in first-instar Prosimulium larvae is not fully expressed again until later instars.First instars of the prosimuliine genera, Gymnopais, Prosimulium, Twinnia, and Crozetia, have apomorphically reduced cephalic fans and small labra, but retain many plesiomorphic characteristics.First-instar simuliine larvae of Cnephia dacotensis (Dyar and Shannon), Metacnephia sp. (near saileri Stone), and Simulium vittatum Zetterstedt have well-developed cephalic fans and broad labra. Larvae of S. oviceps Edwards possess apomorphically reduced cephalic fans and associated muscles.Factors involved in the evolution of the cephalic fan stem are discussed.