﻿450 
  W. 
  F. 
  FISKE. 
  

  

  A 
  delicate 
  " 
  natural 
  balance 
  " 
  is 
  thus 
  created, 
  which 
  remains 
  in 
  a 
  state 
  of 
  great 
  

   stability 
  as 
  long 
  as 
  nothing 
  interferes 
  with 
  the 
  free 
  dispersion 
  of 
  flies. 
  Super-density 
  

   (or 
  " 
  outbreak 
  ' 
  or 
  " 
  epidemic 
  ") 
  of 
  fly 
  is 
  impossible. 
  It 
  cannot 
  increase 
  so 
  as 
  to 
  

   cause 
  direct 
  injury 
  to 
  its 
  host 
  species 
  through 
  excessive 
  blood-letting 
  ; 
  it 
  cannot 
  

   even 
  increase 
  so 
  as 
  to 
  cause 
  its 
  host 
  animals 
  excessive 
  annoyance. 
  

  

  But 
  if 
  dispersion 
  is 
  for 
  any 
  reason 
  obstructed 
  or 
  interfered 
  with, 
  the 
  mechanism 
  

   of 
  the 
  balance 
  between 
  the 
  fly 
  and 
  it 
  host 
  species 
  is 
  thrown 
  out 
  of 
  gear, 
  and 
  its 
  

   density 
  continues 
  to 
  increase 
  until 
  in 
  their 
  own 
  protection 
  its 
  hosts 
  are 
  forced 
  to 
  

   abandon 
  their 
  haunts. 
  The 
  fly 
  thus 
  becomes 
  an 
  " 
  enemy 
  " 
  of 
  its 
  hosts 
  in 
  actuality. 
  

   It 
  has 
  " 
  broken 
  out 
  ' 
  : 
  or 
  become 
  'epidemic," 
  in 
  the 
  terminology 
  of 
  economic 
  

   entomology. 
  It 
  has 
  increased 
  to 
  a 
  state 
  of 
  super-density 
  when 
  the 
  flies 
  have 
  become 
  

   the 
  real 
  deprivative 
  enemies 
  of 
  each 
  other, 
  just 
  as 
  flies 
  of 
  the 
  genus 
  Tabanus 
  may 
  

   become 
  the 
  deprivative 
  enemies 
  of 
  tsetse-flies. 
  One 
  tsetse 
  can 
  be 
  the 
  deprivative 
  

   or 
  competitive 
  enemy 
  of 
  another 
  as 
  easily 
  as 
  a 
  Tabanus 
  can 
  be 
  the 
  enemy 
  of 
  a 
  tsetse. 
  

  

  On 
  a 
  very 
  few 
  occasions 
  only 
  were 
  such 
  conditions 
  found 
  to 
  prevail 
  in 
  the 
  region 
  

   covered 
  by 
  these 
  investigations. 
  The 
  first 
  instance 
  was 
  on 
  the 
  island 
  of 
  Kimmi, 
  

   which 
  alone 
  of 
  all 
  the 
  islands 
  visited 
  — 
  some 
  70 
  in 
  all 
  — 
  appeared 
  to 
  constitute 
  almost 
  

   entirely 
  a 
  protected 
  area. 
  There 
  was 
  no 
  opportunity 
  for 
  dispersion 
  from 
  it 
  unless 
  

   the 
  flies 
  struck 
  out 
  over 
  open 
  water, 
  which 
  they 
  were 
  plainly 
  loath 
  to 
  do, 
  and 
  there 
  

   was 
  hardly 
  a 
  spot 
  on 
  the 
  island 
  where 
  the 
  flies 
  could 
  fail 
  to 
  find 
  both 
  shelter 
  and 
  

   breeding 
  grounds 
  within 
  easy 
  reach. 
  There 
  was 
  no 
  apparent 
  reason 
  why 
  density 
  

   should 
  not 
  increase 
  under 
  such 
  circumstances 
  to 
  the 
  point 
  of 
  super- 
  density, 
  which 
  

   would 
  either 
  injure 
  the 
  host 
  animals 
  or 
  force 
  them 
  to 
  abandon 
  the 
  district. 
  

  

  Conditions 
  encountered 
  on 
  this 
  island 
  in 
  January 
  1914 
  indicated 
  that 
  something 
  

   of 
  this 
  sort 
  was 
  actually 
  taking 
  place. 
  The 
  infestation 
  was 
  heavy 
  (37*2 
  according 
  

   to 
  methods 
  finally 
  accepted 
  for 
  estimating 
  male 
  density) 
  and 
  the 
  female 
  percentage 
  

   was 
  very 
  high 
  (63 
  '5 
  in 
  total 
  catch 
  and 
  55*3 
  at 
  the 
  lowest 
  estimate). 
  The 
  flies 
  were 
  

   as 
  ravenous 
  as 
  the 
  sex 
  ratio 
  indicates, 
  and 
  host 
  animals 
  seen 
  during 
  3-|- 
  days 
  spent 
  

   on 
  the 
  island 
  by 
  Dr. 
  Carpenter 
  and 
  me 
  totalled 
  a 
  single 
  Varanus 
  seen 
  by 
  him 
  and 
  

   a 
  single 
  hippo 
  heard 
  crashing 
  about 
  in 
  a 
  reed 
  thicket 
  by 
  me. 
  

  

  No 
  crocodiles 
  were 
  seen, 
  but 
  there 
  was 
  abundant 
  evidence 
  of 
  their 
  very 
  recent 
  

   presence 
  in 
  the 
  form 
  of 
  nesting 
  places 
  with 
  hatched 
  egg-shells, 
  and 
  runways 
  through 
  

   the 
  reed 
  fringe 
  along 
  the 
  fore-shore. 
  There 
  was 
  also 
  plenty 
  of 
  evidence 
  of 
  the 
  recent 
  

   presence 
  of 
  Varanus, 
  the 
  mounds 
  thrown 
  up 
  by 
  which 
  were 
  more 
  numerous 
  on 
  

   this 
  than 
  on 
  any 
  other 
  island 
  visited, 
  except 
  Manene. 
  It 
  seemed 
  not 
  at 
  all 
  improbable 
  

   that 
  the 
  fly 
  had 
  actually 
  increased 
  to 
  such 
  super-density 
  as 
  to 
  force 
  even 
  the 
  crocodiles 
  

   to 
  abandon 
  the 
  island, 
  temporarily, 
  until 
  the 
  density 
  of 
  the 
  pest 
  should 
  be 
  reduced. 
  

  

  This 
  idea 
  was 
  sustained 
  by 
  the 
  failure 
  of 
  the 
  experiment 
  cited 
  in 
  Sect. 
  VI 
  (c), 
  

   Table 
  XXIII 
  (p. 
  402), 
  in 
  which 
  tethered 
  crocodile 
  and 
  Varanus 
  (on 
  this 
  same 
  island 
  

   at 
  this 
  same 
  time) 
  became 
  so 
  quickly 
  intractable 
  under 
  attack 
  that 
  experiments 
  had 
  

   to 
  be 
  discontinued. 
  

  

  The 
  idea 
  also 
  suggested 
  the 
  following 
  experiment 
  : 
  — 
  • 
  

  

  If, 
  as 
  appeared 
  probable, 
  super-density 
  of 
  fly 
  had 
  caused 
  the 
  host 
  animals 
  to 
  

   abandon 
  the 
  island 
  temporarily, 
  the 
  sudden 
  deprivation 
  of 
  food 
  would 
  result 
  in 
  a 
  

  

  