The early stages of nymphal development of Ephoron leukon Williamson (Ephemeroptera: Polymitarcyidae) are described with special attention paid to the appearance and differentiation of the mouthparts and abdominal gills. Dramatic changes in the mouthparts of E. leukon include outgrowth of the mandibular tusks from the mandible proper, differentiation of the galealacinia and palps of the maxillae, and splitting of the labium into multiple lobes plus the adornment of various regions of the mouthparts with dense brushes of setae functioning in filter feeding. The abdominal gills of E. leukon originate as short threads, and then they progressively elongate and develop a fringe of long thin extensions around their margins, branch basally to form two separate lamellae and ultimately acquire a suite of structural adornments. In E. leukon, nymphs develop mature mouthparts and abdominal gills during early nymphal ontogeny, a developmental pattern that may facilitate key habitat transitions.
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Vol. 102 • No. 1