subimago

noun plural -imagoes or -imagines (-ɪˈmædʒəˌniːz)
  1. the first winged stage of the mayfly, with dull opaque wings, known to anglers as a dun, before it metamorphoses into the shiny gauzy imago or spinnerSee also dun 2 (def. 3), spinner (def. 6)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

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Historical Examples of subimago