﻿88 
  RUPERT 
  W. 
  JACK. 
  

  

  the 
  factors 
  that 
  determine 
  the 
  limits 
  of 
  these 
  ' 
  belts,' 
  but, 
  although 
  tsetse 
  are 
  

   undoubtedly 
  dependent 
  upon 
  the 
  blood 
  of 
  vertebrates 
  for 
  their 
  continued 
  existence, 
  

   all 
  recent 
  evidence 
  goes 
  to 
  show 
  that 
  the 
  most 
  important 
  element 
  is 
  the 
  physical 
  

   character 
  of 
  the 
  locality, 
  and 
  probably 
  its 
  suitability 
  as 
  a 
  breeding 
  ground." 
  These 
  

   remarks, 
  of 
  course, 
  refer 
  to 
  tsetse-flies 
  generally, 
  and 
  not 
  morsitans 
  in 
  particular, 
  

   but 
  they 
  leave 
  out 
  of 
  consideration 
  one 
  rather 
  obvious 
  fact, 
  namely, 
  that 
  the 
  limits 
  

   (of 
  morsitans 
  at 
  least) 
  are 
  not 
  necessarily 
  permanent. 
  Uninfested 
  country 
  adjacent 
  

   to 
  a 
  fly 
  area 
  may 
  be 
  just 
  as, 
  or 
  even 
  more, 
  favourable 
  to 
  the 
  pest 
  than 
  the 
  country 
  

   it 
  inhabits, 
  the 
  fact 
  being 
  demonstrated 
  beyond 
  question 
  by 
  its 
  becoming 
  heavily 
  

   infested 
  later. 
  This 
  has 
  been 
  occurring 
  constantly 
  in 
  Southern 
  Rhodesia 
  since 
  the 
  

   rinderpest, 
  the 
  limit 
  of 
  the 
  Sebungwe 
  belt 
  in 
  certain 
  parts, 
  for 
  instance, 
  year 
  by 
  year 
  

   traversing 
  country 
  obviously 
  favourable 
  on 
  both 
  sides 
  of 
  the 
  line. 
  The 
  character 
  

   of 
  the 
  locality, 
  probably 
  intimately 
  combined 
  with 
  the 
  incidence 
  of 
  a 
  suitable 
  food- 
  

   supply, 
  appears 
  to 
  account 
  for 
  the 
  permanent 
  limits 
  of 
  a 
  belt 
  ; 
  but 
  what 
  determines 
  

   the 
  transitory 
  limits 
  ? 
  Why 
  is 
  it 
  that 
  morsitans 
  does 
  not 
  spread 
  thinly 
  over 
  the 
  

   whole 
  of 
  its 
  potenial 
  area 
  instead 
  of 
  inhabiting 
  only 
  a 
  part 
  ? 
  A 
  fly 
  belt 
  has, 
  in 
  fact, 
  

   a 
  coherent 
  quality 
  of 
  its 
  own 
  which 
  needs 
  explaining. 
  Except 
  during 
  the 
  season 
  of 
  

   concentration 
  from 
  July 
  to 
  November 
  or 
  later, 
  the 
  insect 
  is 
  as 
  a 
  rule 
  by 
  no 
  means 
  

   confined 
  by 
  physical 
  conditions, 
  as 
  is 
  shown 
  by 
  the 
  fact 
  that 
  it 
  more 
  or 
  less 
  deserts 
  

   its 
  dry 
  season 
  haunts 
  and 
  spreads 
  for 
  some 
  miles 
  through 
  the 
  surrounding 
  forest. 
  

   As 
  far 
  as 
  the 
  writer 
  has 
  been 
  able 
  to 
  ascertain 
  its 
  dispersing 
  range 
  does 
  not 
  exceed 
  

   two 
  or 
  three 
  miles, 
  although 
  there 
  would 
  seem 
  to 
  be 
  no 
  obvious 
  reason 
  why 
  it 
  should 
  

   not 
  be 
  considerably 
  greater. 
  Again, 
  the 
  males 
  are 
  undoubtedly 
  carried 
  in 
  all 
  directions 
  

   up 
  to 
  ten 
  miles, 
  or 
  possibly 
  more, 
  by 
  the 
  movements 
  of 
  animals 
  and 
  human 
  beings, 
  

   but 
  as 
  Lamborn 
  has 
  shown 
  in 
  Nyasaland, 
  they 
  have 
  both 
  the 
  instinct 
  and 
  the 
  power 
  

   to 
  return 
  to 
  the 
  point 
  whence 
  they 
  were 
  carried. 
  The 
  females 
  seem 
  only 
  to 
  seek 
  

   animals 
  and 
  human 
  beings 
  for 
  the 
  purpose 
  of 
  feeding, 
  and 
  they 
  are 
  probably 
  rarely 
  

   carried 
  any 
  considerable 
  distance. 
  A 
  strong 
  " 
  homing 
  instinct 
  " 
  has 
  been 
  proved 
  

   in 
  respect 
  of 
  the 
  males, 
  and 
  is 
  to 
  be 
  inferred, 
  at 
  least 
  over 
  a 
  comparatively 
  short 
  

   distance, 
  in 
  respect 
  of 
  the 
  females. 
  

  

  The 
  instinct 
  of 
  a 
  fly 
  to 
  return 
  to 
  its 
  haunts 
  if 
  it 
  strays 
  or 
  is 
  carried 
  into 
  unfavourable 
  

   country, 
  and 
  the 
  instinct 
  of 
  the 
  males 
  to 
  return 
  to 
  the 
  common 
  haunt, 
  where 
  the 
  

   females 
  are 
  to 
  be 
  found, 
  are 
  quite 
  comprehensible 
  ; 
  but 
  they 
  do 
  not 
  explain 
  why 
  

   favourable 
  country 
  adjacent 
  to 
  fly 
  areas 
  is 
  frequently 
  free 
  of 
  the 
  pest. 
  The 
  fact 
  of 
  

   the 
  matter 
  seems 
  to 
  be 
  that 
  the 
  fly 
  is 
  absolutely 
  unadventurous, 
  if 
  the 
  term 
  be 
  

   permissible, 
  not 
  ranging 
  very 
  far 
  from 
  its 
  provedly 
  favourable 
  dry 
  season 
  haunts, 
  

   nor 
  allowing 
  itself 
  to 
  be 
  carried 
  beyond 
  the 
  range 
  of 
  its 
  power 
  to 
  return. 
  It 
  appears 
  

   to 
  the 
  writer 
  that 
  this 
  is 
  to 
  be 
  explained, 
  not 
  by 
  the 
  lack 
  of 
  any 
  tendency 
  to 
  

   wander, 
  but 
  by 
  the 
  possession 
  of 
  a 
  definite 
  instinct 
  to 
  avoid 
  wandering 
  ; 
  as 
  it 
  is 
  to 
  be 
  

   supposed 
  that 
  in 
  the 
  absence 
  of 
  such 
  a 
  controlling 
  instinct 
  the 
  flies 
  under 
  the 
  stimulus 
  

   of 
  hunger 
  would 
  tend 
  to 
  range 
  far 
  and 
  wide 
  in 
  search 
  of 
  food, 
  as 
  they 
  well 
  might 
  do 
  

   without 
  encountering 
  unfavourable 
  conditions 
  during 
  nearly 
  eight 
  months 
  of 
  the 
  year. 
  

  

  Assuming 
  the 
  existence 
  of 
  a 
  strong 
  instinct 
  to 
  avoid 
  undue 
  wandering, 
  an 
  explana- 
  

   tion 
  of 
  its 
  development 
  on 
  the 
  basis 
  of 
  benefit 
  to 
  the 
  species 
  must 
  surely 
  be 
  possible. 
  

   One 
  of 
  the 
  results 
  is 
  a 
  more 
  or 
  less 
  gregarious 
  habit, 
  but 
  it 
  is 
  difficult 
  to 
  perceive 
  what 
  

   benefit 
  the 
  fly 
  derives 
  from 
  this. 
  Other 
  things 
  being 
  favourable 
  a 
  single 
  fly 
  is 
  

  

  