﻿4 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  In 
  the 
  Invertebrata 
  we 
  find 
  many 
  types 
  of 
  eyes, 
  i. 
  e. 
  

   organs 
  whose 
  function 
  is 
  to 
  perceive 
  light. 
  In 
  lower 
  forms 
  

   such 
  organs 
  are 
  of 
  so 
  simple 
  a 
  character 
  as 
  to 
  preclude 
  the 
  

   possibility 
  of 
  their 
  doing 
  more 
  than 
  detecting 
  the 
  differ- 
  

   ence 
  between 
  light 
  and 
  darkness. 
  All 
  stages 
  of 
  comphca- 
  

   tion 
  and 
  multiplication 
  are 
  found. 
  Some 
  molluscs 
  have 
  

   hundreds 
  of 
  such 
  organs, 
  many 
  of 
  them 
  capable 
  of 
  pro- 
  

   ducing 
  images, 
  though 
  in 
  many 
  cases 
  the 
  existence 
  of 
  a 
  

   nervous 
  mechanism 
  capable 
  of 
  perceiving 
  such 
  images 
  

   seems 
  doubtful, 
  and 
  the 
  value 
  of 
  so 
  many 
  eyes 
  to 
  creatures 
  

   of 
  such 
  sedentary 
  habit 
  is 
  rather 
  obscure. 
  

  

  In 
  other 
  invertebrate 
  creatures, 
  and 
  notably 
  in 
  spiders 
  

   and 
  insects, 
  we 
  find 
  what 
  are 
  known 
  as 
  simple 
  eyes 
  or 
  

   ocelh 
  each 
  having 
  a 
  single 
  lens 
  producing 
  an 
  inverted 
  image, 
  

   though 
  to 
  what 
  extent 
  such 
  image 
  is 
  perceived 
  by 
  the 
  

   animal's 
  consciousness 
  it 
  is 
  difficult 
  clearly 
  to 
  define. 
  

  

  The 
  most 
  remarkable 
  form 
  of 
  invertebrate 
  eye 
  is, 
  however, 
  

   the 
  compound 
  or 
  facetted 
  eye 
  of 
  insects 
  and 
  Crustacea. 
  

  

  Before 
  summarising 
  the 
  various 
  views 
  and 
  theories 
  which 
  

   have 
  been 
  advanced 
  on 
  the 
  subject 
  of 
  the 
  vision 
  of 
  the 
  

   compound 
  eye, 
  it 
  will 
  be 
  well 
  to 
  give 
  some 
  description 
  of 
  

   its 
  structure, 
  so 
  that 
  the 
  references 
  of 
  former 
  workers 
  to 
  

   its 
  various 
  parts 
  may 
  be 
  more 
  easily 
  understood. 
  

  

  Plate 
  I 
  shows 
  a 
  section 
  through 
  the 
  eye 
  of 
  an 
  ordinary 
  

   Small 
  Tortoise-shell 
  butterfly 
  (Vanessa 
  urticae) 
  which 
  may 
  

   conveniently 
  be 
  taken 
  as 
  a 
  type. 
  The 
  outer 
  part 
  of 
  the 
  

   eye 
  presents 
  the 
  surface 
  of 
  rather 
  more 
  than 
  a 
  quarter 
  of 
  

   a 
  sphere, 
  and 
  faces 
  in 
  all 
  directions 
  except 
  inwards 
  and 
  

   backwards. 
  It 
  is 
  separated 
  from 
  the 
  corresponding 
  eye 
  

   of 
  the 
  other 
  side 
  by 
  a 
  hard 
  chitinous 
  framework, 
  very 
  

   narrow 
  above 
  and 
  in 
  front, 
  but 
  wider 
  beneath, 
  where 
  a 
  

   somewhat 
  larger 
  space 
  is 
  required 
  for 
  the 
  palpi 
  and 
  the 
  

   proboscis. 
  The 
  external 
  layer 
  of 
  the 
  eye 
  is 
  of 
  fairly 
  hard 
  

   chitin 
  divided 
  into 
  a 
  great 
  number 
  of 
  hexagonal 
  elements, 
  

   the 
  facets, 
  in 
  V. 
  urticae 
  about 
  5000-6000 
  on 
  each 
  side. 
  A 
  

   facet 
  consists 
  of 
  a 
  minute 
  lens, 
  generally 
  if 
  not 
  invariably 
  

   colourless 
  in 
  the 
  centre, 
  but 
  often 
  yellow 
  or 
  brownish 
  round 
  

   the 
  edges. 
  This 
  layer 
  is 
  called 
  the 
  corneal 
  layer. 
  In 
  

   F. 
  urticae 
  and 
  many 
  other 
  butterflies 
  there 
  are 
  hairs 
  pro- 
  

   jecting 
  from 
  sockets 
  between 
  the 
  facets. 
  As 
  these 
  are 
  all 
  

   radially 
  arranged 
  they 
  do 
  not 
  interfere 
  with 
  the 
  vision 
  any 
  

   more 
  than 
  do 
  the 
  eyelashes 
  of 
  vertebrates. 
  In 
  transverse 
  

   section 
  the 
  lenses 
  are 
  more 
  or 
  less 
  biconvex, 
  and 
  frequently, 
  

   especially 
  in 
  butterflies, 
  there 
  lies 
  beneath 
  the 
  lenses 
  a 
  

  

  