﻿37.— 
  PROTECTIVE 
  MOVEMENTS 
  AND 
  RANGE 
  OF 
  VISION 
  IN 
  

   PLATYPEZID 
  FLIES. 
  

  

  (Plate 
  XLVIII). 
  

  

  By 
  {the 
  late) 
  F. 
  M. 
  Howlett, 
  B.A., 
  F.E.S., 
  Imjierial 
  Pathological 
  Ento- 
  

   mologist. 
  

  

  In 
  his 
  great 
  work 
  on 
  '' 
  British 
  Flies," 
  Verrall 
  has 
  the 
  following 
  note 
  

   on 
  Platypezidw 
  (Vol. 
  Syrphidse, 
  p. 
  677, 
  1901). 
  " 
  Colonel 
  Yerbury 
  has 
  

   informed 
  me 
  that 
  the 
  males 
  of 
  the 
  genus 
  Platypeza 
  hover 
  in 
  dull 
  weather, 
  

   but 
  sit 
  (or 
  to 
  some 
  extent 
  run) 
  about 
  on 
  leaves 
  during 
  sunshine, 
  and 
  

   that 
  as 
  a 
  rule 
  they 
  prefer 
  large 
  leaves." 
  

  

  Williston 
  (N. 
  American 
  Diptera, 
  3rd 
  edition, 
  1908, 
  p. 
  242) 
  remarks 
  

   '' 
  The 
  flies 
  of 
  this 
  small 
  family 
  have 
  been 
  seen 
  dancing 
  in 
  the 
  air 
  in 
  

   small 
  swarms, 
  or 
  running 
  about 
  on 
  the 
  leaves 
  of 
  underbush." 
  In 
  1909 
  

   I 
  wrote 
  that 
  Platypezidce 
  " 
  may 
  sometimes 
  be 
  seen 
  running 
  on 
  broad 
  

   leaves 
  under 
  trees, 
  travelling 
  round 
  and 
  round 
  in 
  little 
  circles, 
  and 
  

   expending 
  a 
  vast 
  amount 
  of 
  energy 
  without 
  any 
  obvious 
  purpose 
  or 
  

   perceptible 
  result." 
  

  

  This 
  habit 
  of 
  running 
  on 
  broad 
  leaves 
  is 
  indeed 
  extremely 
  charac- 
  

   teristic 
  of 
  the 
  family, 
  or 
  at 
  least 
  of 
  such 
  species 
  as 
  I 
  have 
  encountered 
  

   in 
  this 
  country. 
  The 
  peculiarity 
  of 
  the 
  movement 
  is 
  the 
  extreme 
  

   rapidity 
  with 
  which 
  it 
  is 
  usually 
  executed, 
  and 
  which 
  seems 
  unnecessary 
  

   in 
  a 
  mere 
  reconnaissance 
  or 
  search 
  for 
  food. 
  Moreover 
  it 
  is 
  generally 
  

   confined 
  to 
  a 
  comparatively 
  small 
  area, 
  and 
  seems 
  unconnected 
  with 
  

   courtship. 
  

  

  The 
  description 
  given 
  above, 
  of 
  their 
  running 
  " 
  in 
  little 
  circles," 
  

   is 
  in 
  the 
  case 
  of 
  the 
  species 
  here 
  mentioned 
  not 
  quite 
  correct, 
  as 
  its 
  

   course 
  is 
  more 
  often 
  a 
  long 
  ellipse 
  or 
  a 
  nearly 
  straight 
  to-and-fro 
  path, 
  

   while 
  sometimes 
  it 
  consists 
  of 
  a 
  series 
  of 
  darts 
  in 
  varying 
  directions 
  

   across 
  a 
  central 
  space 
  of 
  more 
  or 
  less 
  fixed 
  extent. 
  The 
  area 
  covered 
  

   by 
  these 
  rapidly 
  alternating 
  movements 
  may 
  be 
  about 
  three 
  or 
  four 
  

   inches 
  square, 
  or 
  may 
  be 
  only 
  about 
  one 
  square 
  inch, 
  according 
  to 
  the 
  

   degree 
  to 
  which 
  the 
  insect 
  is 
  disturbed. 
  It 
  seems 
  to 
  take 
  to 
  flight 
  only 
  

   as 
  a 
  last 
  resource. 
  

  

  In 
  the 
  field 
  this 
  curious 
  habit 
  is 
  so 
  constantly 
  displayed 
  as 
  to 
  afford 
  

   on 
  most 
  occasions 
  an 
  easy 
  means 
  of 
  " 
  spotting 
  " 
  a 
  Platypezid 
  at 
  sight, 
  

  

  ( 
  279 
  ) 
  

  

  t2 
  

  

  