﻿372 
  W. 
  F. 
  FISKE. 
  

  

  It 
  is 
  impracticable 
  to 
  include 
  in 
  the 
  graph 
  data 
  concerning 
  environmental 
  features 
  

   responsible 
  for 
  the 
  striking 
  variations 
  in 
  male 
  density 
  and 
  sex 
  ratio 
  shown 
  by 
  the 
  

   curves. 
  In 
  general 
  they 
  are 
  as 
  follows 
  : 
  

  

  Points 
  1 
  to 
  4 
  inclusive 
  are 
  typical 
  of 
  shore 
  infested 
  to 
  slightly 
  above 
  the 
  average 
  

   extent 
  for 
  this 
  general 
  region. 
  This 
  reach 
  of 
  1 
  J 
  miles 
  lies 
  on 
  the 
  western 
  shore 
  of 
  the 
  

   bay, 
  and 
  is 
  densely 
  wooded, 
  with 
  bits 
  of 
  open 
  shore 
  alternating 
  with 
  reed-grown 
  and 
  

   jungly 
  reaches. 
  There 
  were 
  some 
  sand 
  and 
  gravel 
  deposits 
  affording 
  good 
  breeding 
  

   grounds. 
  

  

  Points 
  5 
  to 
  8 
  inclusive 
  : 
  — 
  The 
  forest 
  continues, 
  but 
  there 
  is 
  no 
  more 
  open 
  shore, 
  

   nor 
  any 
  breeding 
  places. 
  Instead 
  there 
  is 
  a 
  fringe 
  of 
  floating 
  sudd 
  (papyrus 
  and 
  

   saw-grass) 
  lying 
  off-shore 
  and 
  no 
  open 
  space 
  between 
  it 
  and 
  the 
  massive 
  shelter 
  of 
  

   the 
  forest 
  behind. 
  Catches 
  were 
  made 
  from 
  a 
  canoe 
  outside 
  the 
  sudd, 
  which, 
  with 
  

   the 
  exception 
  of 
  <.;rass, 
  is 
  the 
  most 
  repulsive 
  type 
  of 
  vegetation. 
  

  

  Only 
  the 
  hungry 
  food-hunting 
  flies 
  (principally 
  females) 
  pass 
  beyond 
  the 
  limits 
  

   of 
  the 
  open 
  shore 
  at 
  point 
  4. 
  The 
  relatively 
  idle 
  males 
  turn 
  back. 
  

  

  The 
  minimum 
  density 
  of 
  males 
  and 
  maximum 
  percentage 
  of 
  females 
  is 
  recorded 
  

   from 
  point 
  6. 
  At 
  point 
  7 
  males 
  are 
  more 
  numerous 
  and 
  female 
  percentage 
  has 
  

   fallen, 
  indicating 
  that 
  a 
  new 
  fly 
  colony 
  is 
  being 
  approached. 
  The 
  catch 
  at 
  point 
  8 
  

   confirms 
  this 
  and 
  makes 
  it 
  certain 
  that 
  males 
  are 
  coming 
  into 
  the 
  repulsively 
  sheltered 
  

   reach 
  from 
  the 
  other 
  direction. 
  

  

  At 
  point 
  9 
  there 
  is 
  a 
  break 
  in 
  the 
  sudd, 
  and 
  an 
  open 
  grassy 
  bit 
  of 
  fore-shore, 
  

   scattered 
  with 
  bushes 
  and 
  backed 
  by 
  massive 
  forest. 
  Shelter 
  is 
  attractive, 
  but 
  as 
  

   yet 
  no 
  breeding 
  grounds 
  occur. 
  The 
  excellent 
  shelter 
  makes 
  for 
  a 
  sharp 
  increase 
  

   in 
  male 
  density 
  and 
  a 
  corresponding 
  decrease 
  in 
  female 
  percentage. 
  

  

  At 
  point 
  10, 
  the 
  forest, 
  which 
  has 
  continued 
  unbroken 
  until 
  now, 
  ceases 
  and 
  open 
  

   grassland 
  comes 
  down 
  to 
  near 
  the 
  water's 
  edge. 
  The 
  foreshore 
  is 
  open, 
  with 
  scattered 
  

   bushes, 
  and 
  deposits 
  of 
  beach 
  sand 
  afford 
  excellent 
  breeding 
  grounds. 
  For 
  a 
  short 
  

   space 
  between 
  points 
  9 
  and 
  10 
  this 
  series 
  of 
  open 
  sandy 
  belts 
  is 
  backed 
  by 
  the 
  massive 
  

   shelter 
  of 
  the 
  forest. 
  This 
  combination 
  affords 
  complete 
  protection 
  to 
  the 
  flies, 
  

   both 
  as 
  pupae 
  and 
  adults, 
  and 
  forms 
  the 
  centre 
  of 
  the 
  colony, 
  or 
  centre 
  of 
  infestation 
  

   for 
  the 
  shore 
  on 
  either 
  side. 
  

  

  Beyond 
  point 
  10 
  the 
  open 
  fore-shore, 
  with 
  sand 
  deposits 
  and 
  the 
  slight 
  shelter 
  

   of 
  scattered 
  bush, 
  continues 
  (as 
  at 
  point 
  10), 
  but 
  is 
  no 
  longer 
  backed 
  by 
  massive 
  

   shelter. 
  The 
  effect 
  on 
  male 
  density 
  and 
  female 
  percentage 
  is 
  precisely 
  as 
  caused 
  

   by 
  the 
  fringe 
  of 
  sudd. 
  The 
  idle 
  males 
  turn 
  back 
  when 
  massive 
  shelter 
  ends, 
  and 
  only 
  

   the 
  food-hunting 
  flies, 
  in 
  part, 
  continue. 
  

  

  At 
  point 
  14 
  is 
  a 
  relatively 
  thick 
  mass 
  of 
  bush, 
  which 
  tempts 
  a 
  few 
  males 
  to 
  linger, 
  

   but 
  not 
  for 
  long, 
  and 
  density 
  does 
  not 
  rise 
  again 
  until, 
  some 
  distance 
  further 
  on, 
  

   forest 
  shelter 
  is 
  again 
  encountered. 
  

  

  There 
  is 
  not 
  the 
  slightest 
  doubt 
  that 
  in 
  these 
  observation 
  points 
  high 
  percentage 
  

   of 
  females 
  coupled 
  with 
  low 
  density 
  of 
  males 
  is 
  due 
  to 
  insufficient 
  or 
  unattractive 
  

   sheltering 
  vegetation, 
  leading 
  to 
  a 
  partial 
  segregation 
  of 
  the 
  food- 
  hunting 
  flies 
  of 
  

   both 
  sexes 
  from 
  the 
  relatively 
  idle 
  and 
  satiated 
  male? 
  

  

  