﻿Aquatic 
  insects 
  in 
  the 
  adirondacks 
  531 
  

  

  known, 
  and 
  are 
  described 
  and 
  distinguished 
  below. 
  So 
  much 
  aHke 
  are 
  

   they 
  that 
  a 
  general 
  account 
  of 
  the 
  nymphal 
  characters 
  will 
  save 
  much 
  

   restatement. 
  

  

  The 
  known 
  nymphs 
  of 
  this 
  genus 
  agree 
  in 
  having 
  the 
  body 
  elongate, 
  

   tapering 
  to 
  the 
  pointed 
  apex 
  of 
  the 
  abdomen, 
  hairy, 
  the 
  hairs 
  serving 
  to 
  

   hold 
  an 
  ambuscade 
  of 
  silt 
  about 
  the 
  body. 
  Head 
  compact, 
  httle 
  wider 
  than 
  

   long, 
  with 
  the 
  eyes 
  capping 
  the 
  anterolateral 
  angles, 
  and 
  directed 
  ante- 
  

   riorly; 
  head 
  little 
  narrowed 
  behind 
  the 
  eyes; 
  labium 
  large, 
  reaching 
  

   posteriorly 
  between 
  the 
  bases 
  of 
  the 
  middle 
  legs 
  ; 
  median 
  lobe 
  with 
  its 
  

   front 
  border 
  not 
  crenate; 
  mental 
  setae 
  always 
  present, 
  variable 
  in 
  

   number; 
  lateral 
  setae 
  five 
  to 
  nine; 
  prothorax 
  with 
  a 
  flatish 
  dorsal 
  shield, 
  

   whose 
  margins 
  are 
  generally 
  fringed 
  with 
  coarse 
  hairs 
  ; 
  wing 
  cases 
  reach- 
  

   ing 
  the 
  base 
  of 
  the 
  sixth 
  abdomiaal 
  segment; 
  abdomen 
  triquetral, 
  

   its 
  lateral 
  margins 
  becoming 
  acute 
  posteriorly, 
  with 
  short 
  lateral 
  spines 
  

   on 
  segments 
  8 
  and 
  9 
  ; 
  a 
  variable 
  number 
  of 
  dorsal 
  hooks 
  beginning 
  

   on 
  the 
  third 
  or 
  fourth 
  segment, 
  sometimes 
  quite 
  rudimentary 
  ; 
  ninth 
  

   segment 
  two 
  to 
  three 
  times 
  as 
  long 
  as 
  the 
  loth 
  ; 
  lateral 
  appendages 
  half 
  

   as 
  long 
  as 
  the 
  others; 
  tarsi 
  with 
  the 
  second 
  and 
  third 
  joints 
  successively 
  

   each 
  a 
  very 
  little 
  longer 
  than 
  the 
  first. 
  

  

  The 
  images 
  discussed 
  below, 
  and 
  the 
  known 
  nymphs 
  of 
  the 
  same 
  

   species 
  may 
  be 
  separated 
  by 
  the 
  following 
  keys. 
  

  

  KEY 
  TO 
  SPECIES 
  OF 
  LIBELLULA 
  

  

  Imagos 
  

  

  a 
  Wings 
  with 
  no 
  spot 
  at 
  the 
  nodus 
  

  

  h 
  With 
  a 
  broad 
  basal 
  band 
  of 
  black 
  covering 
  the 
  basal 
  third 
  of 
  both 
  wings 
  

  

  bas 
  alls 
  

  

  bh 
  With 
  the 
  black 
  color 
  of 
  the 
  base 
  of 
  the 
  wings 
  confiued 
  to 
  a 
  narrow 
  streak 
  

  

  in 
  the 
  subcostal 
  space, 
  or 
  entirely 
  wanting 
  

  

  c 
  Stigma 
  bicolored 
  

  

  d 
  luner 
  half 
  of 
  stigma 
  white 
  or 
  yellow, 
  outer 
  half 
  dark 
  brown, 
  c 
  y 
  a 
  n 
  e 
  a 
  

  

  dd 
  Stigma 
  mainly 
  yellow, 
  but 
  distinctly 
  darker 
  at 
  the 
  outer 
  end 
  

  

  plumbea 
  

   CO 
  Stigma 
  not 
  differing 
  in 
  color 
  at 
  its 
  inner 
  and 
  outer 
  ends 
  

  

  d 
  Stigma 
  red 
  or 
  yellow; 
  wings 
  flavescent, 
  unspotted 
  auripennis 
  

  

  dd 
  Stigma 
  black 
  incesta 
  

  

  aa 
  Wings 
  with 
  a 
  small 
  nodal 
  spot 
  which 
  is 
  restricted 
  to 
  the 
  outer 
  (distal) 
  

  

  side 
  of 
  the 
  nodus 
  

  

  h 
  With 
  a 
  large 
  triangular 
  patch 
  of 
  black 
  extending 
  from 
  the 
  triangle 
  to 
  the 
  

  

  hind 
  margin 
  quadrimaculata 
  

  

  hh 
  Without 
  a 
  black 
  patch 
  between 
  the 
  triangle 
  and 
  the 
  hind 
  margin 
  in 
  the 
  

  

  hind 
  wing 
  vibrans 
  

  

  aaa 
  With 
  a 
  large 
  nodal 
  spot 
  which 
  completely 
  surrounds 
  the 
  nodus 
  

  

  J) 
  Nodal 
  and 
  apical 
  wing 
  spots 
  yellowish 
  or 
  reddish 
  serai 
  fa 
  sciata 
  

  

  hh 
  Nodal 
  and 
  apical 
  wing 
  spots 
  blackish 
  pnlehella 
  

  

  