﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  7 
  

  

  at 
  the 
  inner 
  end 
  of 
  the 
  rhabdoni. 
  Jonas 
  (Zeit. 
  Wiss. 
  Zool. 
  

   1911) 
  seems 
  to 
  have 
  been 
  the 
  first 
  to 
  notice 
  this, 
  and 
  

   strangely 
  enough 
  observed 
  it 
  in 
  only 
  one 
  species 
  of 
  butterfly. 
  

   He 
  gives 
  a 
  small 
  text 
  figure 
  and 
  states 
  that 
  he 
  is 
  unable 
  

   to 
  offer 
  any 
  explanation 
  of 
  it. 
  I 
  have 
  observed 
  it 
  in 
  all 
  the 
  

   butterflies 
  I 
  have 
  examined, 
  and 
  have 
  studied 
  it 
  especially 
  

   in 
  Vanessa 
  urticae 
  and 
  Ganoris 
  brassicae. 
  It 
  is, 
  as 
  above 
  

   stated, 
  a 
  part 
  of 
  the 
  trachaeal 
  system 
  and 
  has 
  a 
  some- 
  

   what 
  remarkable 
  structure. 
  There 
  passes 
  through 
  the 
  

   basal 
  membrane 
  to 
  each 
  ommatidium 
  a 
  fine 
  trachaeal 
  tube 
  

   (see 
  PI. 
  11, 
  fig. 
  1, 
  t.) 
  which 
  immediately 
  increases 
  consider- 
  

   ably 
  in 
  diameter, 
  and 
  the 
  usual 
  spiral 
  thickening 
  of 
  its 
  

   walls 
  can 
  be 
  distinctly 
  seen. 
  This 
  widened 
  portion 
  of 
  the 
  

   tube 
  contains 
  the 
  refractive 
  body 
  already 
  referred 
  to. 
  

   Judging 
  by 
  the 
  manner 
  in 
  which 
  it 
  stains 
  it 
  is 
  strongly 
  

   chitinised. 
  In 
  longitudinal 
  section 
  it 
  is 
  cylindro-conical 
  

   (see 
  PI. 
  II, 
  fig. 
  1, 
  tf.), 
  but 
  in 
  transverse 
  section 
  it 
  is 
  

   cruciform. 
  Fig. 
  5, 
  PI. 
  II 
  shows 
  a 
  transverse 
  section 
  of 
  

   four 
  ommatidia, 
  passing 
  through 
  these 
  bodies 
  near 
  their 
  

   basal 
  or 
  outer 
  (distal) 
  ends. 
  It 
  will 
  be 
  seen 
  that 
  the 
  

   four 
  arms 
  of 
  the 
  cruciform 
  section 
  come 
  outwardly 
  into 
  

   contact 
  with 
  the 
  widened 
  portion 
  of 
  the 
  trachaeal 
  tube, 
  so 
  

   that 
  four 
  spaces 
  remain. 
  A 
  little 
  higher 
  up, 
  fig. 
  6, 
  each 
  

   of 
  these 
  four 
  spaces 
  is 
  divided 
  into 
  two, 
  so 
  that 
  eight 
  spaces 
  

   are 
  now 
  seen, 
  and 
  these 
  are 
  the 
  lumina 
  of 
  the 
  eight 
  trachaeal 
  

   tubes 
  which 
  shortly 
  afterwards 
  pass 
  outwards 
  to 
  the 
  sur- 
  

   face 
  of 
  the 
  ommatidium 
  and 
  extend 
  forwards 
  towards 
  the 
  

   corneal 
  layer, 
  ending 
  blindly 
  not 
  far 
  from 
  the 
  crystaUine 
  

   cones. 
  A 
  similar 
  arrangement 
  appears 
  in 
  V. 
  to 
  and 
  

   probably 
  in 
  other 
  Vanessidae. 
  In 
  G. 
  brassicae 
  the 
  structure 
  

   is 
  different. 
  Instead 
  of 
  the 
  conical 
  body 
  there 
  is 
  a 
  thick 
  

   chitinous 
  septum 
  which 
  divides 
  the 
  trachaeal 
  tube 
  into 
  two 
  

   divisions 
  only, 
  and 
  these 
  pass 
  forward 
  to 
  be 
  again 
  divided 
  

   into 
  two, 
  but 
  at 
  a 
  higher 
  level 
  than 
  in 
  V. 
  urticae, 
  ultimately 
  

   resulting 
  in 
  only 
  four 
  trachaeal 
  tubes. 
  We 
  m.a)^, 
  I 
  

   think, 
  conveniently 
  call 
  this 
  structure 
  the 
  " 
  trachaeal 
  

   distributor." 
  

  

  I 
  have 
  already 
  pointed 
  out 
  that 
  the 
  nerves 
  pass 
  through 
  

   the 
  basal 
  membrane 
  into 
  the 
  retinulae 
  or 
  visual 
  cells. 
  

   They 
  can 
  still 
  be 
  seen 
  in 
  transverse 
  sections, 
  as 
  eight 
  small 
  

   circles 
  lying 
  outside 
  the 
  eight 
  branches 
  of 
  the 
  trachaeal 
  

   system 
  (PL 
  II, 
  fig. 
  6, 
  ret.). 
  Negative 
  results 
  in 
  research 
  

   are 
  rarely 
  satisfactory, 
  and 
  a 
  recital 
  of 
  the 
  efforts 
  I 
  have 
  

   made 
  to 
  trace 
  the 
  course 
  of 
  these 
  nerve 
  fibres 
  would 
  make 
  

  

  