﻿TSETE 
  FLY 
  IN 
  SOUTHERN 
  RHODESIA. 
  89 
  

  

  undoubtedly 
  quite 
  capable 
  of 
  looking 
  after 
  itself. 
  The 
  writer 
  formerly 
  inclined 
  to 
  the 
  

   opinion 
  that 
  owing 
  to 
  the 
  slow 
  rate 
  of 
  breeding 
  a 
  gregarious 
  habit, 
  in 
  enabling 
  the 
  

   sexes 
  to 
  meet 
  and 
  mate 
  with 
  the 
  maximum 
  certainty 
  and 
  regularity, 
  might 
  be 
  of 
  

   considerable 
  value 
  ; 
  but 
  this 
  theory 
  would 
  seem 
  to 
  be 
  untenable 
  in 
  the 
  light 
  of 
  the 
  

   fact 
  that 
  there 
  is 
  no 
  evidence 
  to 
  show 
  that 
  separate 
  matings 
  are 
  necessary 
  for 
  each 
  

   act 
  of 
  reproduction, 
  the 
  probability 
  being 
  that 
  one 
  act 
  of 
  copulation 
  suffices 
  for 
  the 
  

   female's 
  life- 
  time. 
  The 
  following 
  suggestion 
  is 
  put 
  forward 
  for 
  consideration, 
  

   namely 
  the 
  value 
  of 
  parental 
  experience. 
  If 
  there 
  is 
  one 
  thing 
  proved 
  in 
  connection 
  

   with 
  tsetse-fly 
  it 
  is 
  its 
  lack 
  of 
  adaptability. 
  Under 
  favourable 
  conditions 
  of 
  forest 
  

   and 
  food-supply 
  it 
  frequently 
  occurs 
  in 
  great 
  numbers, 
  but 
  it 
  is 
  altogether 
  absent 
  

   from 
  large 
  tracts 
  of 
  country 
  not 
  affording 
  its 
  very 
  particular 
  requirements 
  as 
  to 
  con- 
  

   ditions, 
  and 
  tends 
  to 
  vanish 
  entirely 
  from 
  its 
  habitat 
  if 
  conditions 
  be 
  modified 
  to 
  its 
  

   disadvantage. 
  Given 
  a 
  wandering 
  habit 
  without 
  a 
  " 
  homing 
  instinct 
  " 
  a 
  considerable 
  

   proportion 
  of 
  the 
  flies 
  would 
  tend 
  to 
  stray 
  into 
  unfavourable 
  country 
  and 
  perish, 
  

   and 
  it 
  appears 
  conceivable 
  that 
  this 
  fact 
  may 
  have 
  led 
  to 
  the 
  development 
  of 
  a 
  strong 
  

   instinct 
  against 
  wandering, 
  which 
  functions 
  whether 
  the 
  surrounding 
  country 
  be 
  

   favourable 
  or 
  not. 
  In 
  the 
  vast 
  majority 
  of 
  cases 
  the 
  fact 
  of 
  a 
  fly 
  being 
  born 
  at 
  any 
  

   particular 
  spot 
  proves 
  the 
  suitability 
  of 
  the 
  locality 
  to 
  the 
  species, 
  and, 
  as 
  in 
  the 
  

   case 
  of 
  a 
  certain 
  conservative 
  type 
  of 
  human 
  being, 
  what 
  is 
  good 
  enough 
  for 
  the 
  

   parents 
  may 
  be 
  judged 
  good 
  enough 
  for 
  the 
  off-spring. 
  

  

  The 
  Manner 
  in 
  which 
  Fly-Belts 
  extend. 
  

  

  This 
  is 
  a 
  matter 
  which 
  has 
  received 
  some 
  attention 
  in 
  this 
  territory 
  for 
  a 
  

   number 
  of 
  years 
  past. 
  The 
  following 
  rules 
  are 
  in 
  accordance 
  with 
  the 
  evidence 
  

   accumulated, 
  but 
  must 
  be 
  regarded 
  at 
  present 
  as 
  merely 
  tentative 
  : 
  — 
  

  

  (1) 
  The 
  advance 
  does 
  not 
  take 
  the 
  form 
  of 
  disconnected 
  offshoots 
  at 
  a 
  distance 
  

   from 
  the 
  main 
  belt, 
  but 
  the 
  whole 
  movement 
  is 
  analogous 
  to 
  that 
  of 
  a 
  rising 
  flood, 
  

   flowing 
  along 
  favourable 
  channels 
  and 
  gradually 
  extending 
  the 
  flooded 
  area 
  ; 
  

   (2) 
  when 
  the 
  line 
  of 
  advance 
  is 
  interrupted 
  in 
  regard 
  to 
  permanent 
  shade, 
  as 
  in 
  

   crossing 
  a 
  watershed, 
  the 
  movement 
  is 
  confined 
  to 
  the 
  wet 
  season 
  ; 
  (3) 
  large 
  

   numbers 
  of 
  flies 
  are 
  necessary 
  at 
  the 
  previous 
  dry 
  season 
  limit 
  before 
  a 
  moderately 
  

   wide 
  region 
  affording 
  only 
  summer 
  shade 
  can 
  be 
  crossed 
  ; 
  (4) 
  the 
  fly 
  can 
  only 
  

   spread 
  in 
  the 
  presence 
  of 
  considerable 
  quantities 
  of 
  the 
  larger 
  mammals. 
  

  

  The 
  observations 
  leading 
  to 
  these 
  conclusions 
  have 
  been 
  confined 
  to 
  Southern 
  

   Rhodesia, 
  but 
  with 
  the 
  exception 
  of 
  the 
  last 
  they 
  appear 
  to 
  be 
  due 
  to 
  the 
  same 
  cause, 
  

   which 
  is 
  independent 
  of 
  locality, 
  namely, 
  that 
  the 
  flies 
  tend 
  to 
  remain 
  in 
  or 
  return 
  to 
  

   one 
  particular 
  spot. 
  The 
  recently 
  observed 
  facts 
  in 
  this 
  territory 
  are 
  altogether 
  

   opposed 
  to 
  the 
  theory 
  that 
  fly 
  moves 
  into 
  and 
  establishes 
  itself 
  in 
  new 
  districts 
  with 
  

   game, 
  as 
  has 
  been 
  stated 
  by 
  hunters 
  and 
  explorers 
  in 
  the 
  past. 
  Newly 
  infested 
  spots 
  

   may 
  be 
  disconnected 
  by 
  a 
  few 
  miles 
  from 
  the 
  former 
  limits 
  of 
  the 
  belt 
  during 
  the 
  

   season 
  of 
  concentration, 
  but 
  the 
  belt 
  is 
  continuous 
  during 
  the 
  season 
  of 
  dispersion. 
  

  

  The 
  fact 
  of 
  fly 
  being 
  only 
  able 
  to 
  advance 
  from 
  one 
  dry 
  season 
  haunt 
  to 
  another 
  

   in 
  the 
  wet 
  season 
  is 
  easily 
  understood 
  on 
  the 
  basis 
  of 
  the 
  seasonal 
  dispersion 
  of 
  the 
  

   insect, 
  which 
  enables 
  larvae 
  to 
  be 
  deposited 
  in 
  the 
  intervening 
  forest 
  until 
  the 
  neigh- 
  

   bourhood 
  of 
  the 
  next 
  river, 
  or 
  other 
  suitable 
  dry 
  season 
  haunt 
  is 
  reached. 
  When 
  

   the 
  next 
  season 
  of 
  concentration 
  arrives, 
  the 
  pest 
  presumably 
  concentrates 
  in 
  both 
  

   (C572) 
  g 
  

  

  