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  PROCEEDINGS 
  OF 
  THE 
  FOURTH 
  ENTOMOLOGICAL 
  MEETING 
  

  

  whereas 
  if 
  it 
  remained 
  at 
  rest 
  it 
  could 
  not 
  be 
  distinguished 
  at 
  a 
  little 
  

   distance 
  from 
  many 
  small 
  Muscoids. 
  The 
  habit 
  seemed 
  to 
  me 
  so 
  defi- 
  

   nite, 
  although 
  so 
  apparently 
  purposeless, 
  that 
  it 
  was 
  hardly 
  to 
  be 
  ex- 
  

   plained 
  as 
  a 
  mere 
  exhibition 
  of 
  uncontrollable 
  restlessness. 
  

  

  A 
  fly 
  of 
  this 
  genus 
  (almost 
  certainly 
  PZa^7/;pe2;a 
  argyrogyna, 
  de 
  Meij.) 
  

   was 
  seen 
  sitting 
  on 
  the 
  broad 
  leaves 
  of 
  a 
  creeper 
  in 
  my 
  compound 
  on 
  

   the 
  12th 
  of 
  August 
  1913. 
  It 
  happened 
  to 
  catch 
  my 
  attention 
  while 
  

   I 
  was 
  four 
  or 
  five 
  yards 
  away, 
  and 
  it 
  was 
  then 
  quite 
  motionless. 
  I 
  

   approached 
  to 
  within 
  a 
  few 
  feet, 
  when 
  it 
  suddenly 
  began 
  its 
  darting 
  

   and 
  circling 
  motions. 
  On 
  my 
  retiring 
  again 
  to 
  a 
  distance 
  of 
  five 
  yards, 
  

   the 
  movements 
  ceased, 
  but 
  at 
  once 
  began 
  again 
  when 
  I 
  approached 
  

   to 
  within 
  a 
  few 
  feet. 
  This 
  advance 
  and 
  retirement 
  was 
  repeated 
  several 
  

   times, 
  and 
  always 
  with 
  exactly 
  the 
  same 
  result. 
  The 
  movements 
  began 
  

   whenever 
  I 
  got 
  within 
  a 
  certain 
  distance, 
  this 
  distance 
  being 
  smaller 
  

   the 
  more 
  cautious 
  and 
  gradual 
  the 
  advance. 
  

  

  It 
  seemed 
  evident 
  that 
  the 
  movements 
  were 
  executed 
  in 
  response 
  

   to 
  the 
  sight 
  of 
  objects 
  moving 
  in 
  the 
  neighbourhood, 
  and 
  moreover 
  

   that 
  they 
  followed 
  the 
  stimulus 
  with 
  remarkable 
  regularity. 
  The 
  idea 
  

   occurred 
  to 
  me 
  that 
  by 
  making 
  a 
  series 
  of 
  trials 
  at 
  varying 
  distances 
  

   we 
  might 
  obtain 
  some 
  idea 
  of 
  the 
  range 
  and 
  acuteness 
  of 
  vision 
  in 
  

   this 
  particular 
  fly. 
  I 
  procured 
  a 
  piece 
  of 
  white 
  paper 
  four 
  inches 
  

   square, 
  and 
  a 
  roughly 
  circular 
  palm-leaf 
  fan 
  12 
  inches 
  in 
  diameter 
  and 
  

   coloured 
  red 
  and 
  blue 
  with 
  large 
  gilt 
  spots 
  ; 
  for 
  the 
  longer 
  distance 
  

   I 
  employed 
  my 
  own 
  body 
  (in 
  white 
  clothes) 
  as 
  the 
  moving 
  object, 
  against 
  

   a 
  back 
  ground 
  of 
  grass 
  and 
  trees. 
  These 
  objects 
  were 
  shifted 
  for 
  varying 
  

   distances 
  across 
  the 
  line 
  of 
  sight 
  of 
  the 
  fly, 
  the 
  minimum 
  shift 
  required 
  

   to 
  make 
  the 
  fly 
  move 
  being 
  noted 
  for 
  each 
  distance. 
  Not 
  less 
  than 
  

   five 
  trials 
  were 
  made 
  at 
  each 
  distance, 
  and 
  in 
  the 
  case 
  of 
  Fly 
  A 
  it 
  is 
  not 
  

   unlikely 
  that 
  fatigue 
  or 
  a 
  growing 
  indifference 
  may 
  have 
  decreased 
  

   its 
  sensitiveness 
  in 
  the 
  latter 
  trials 
  ; 
  it 
  will 
  be 
  noticed 
  that 
  in 
  these 
  it 
  

   appears 
  to 
  require 
  a 
  stronger 
  stimulation 
  than 
  Fly 
  B, 
  which 
  was 
  tested 
  

   five 
  days 
  later, 
  the 
  short 
  distance 
  trials 
  being 
  in 
  this 
  case 
  omitted. 
  

   Another 
  possible 
  cause 
  of 
  the 
  greater 
  alertness 
  of 
  fly 
  B 
  is 
  that 
  it 
  was 
  

   tested 
  at 
  9-15 
  a.m., 
  whereas 
  fly 
  A's 
  tests 
  began 
  at 
  11 
  a.m. 
  and 
  continued 
  

   for 
  more 
  than 
  an 
  hour 
  in 
  the 
  heat 
  of 
  the 
  day, 
  a 
  time 
  when 
  many 
  insects 
  

   and 
  animals 
  are 
  more 
  or 
  less 
  inactive. 
  

  

  Apart 
  from 
  this 
  individual 
  difference 
  the 
  results 
  as 
  shown 
  on 
  the 
  

   curve 
  (Plate 
  XLVIII) 
  seem 
  on 
  the 
  whole 
  very 
  regular, 
  and 
  in 
  carrying 
  

   out 
  the 
  tests 
  this 
  impression 
  of 
  regularity 
  was 
  particularly 
  strong. 
  

   There 
  seemed 
  for 
  each 
  distance 
  and 
  each 
  object 
  to 
  be 
  a 
  definite 
  

   minimum 
  amount 
  of 
  movement 
  which 
  had 
  to 
  be 
  exceeded 
  in 
  order 
  to 
  

   make 
  the 
  fly 
  dodge. 
  

  

  