﻿358 
  W. 
  F. 
  FISKE. 
  

  

  It 
  is 
  believed, 
  however, 
  that 
  the 
  ratio 
  in 
  the 
  catch 
  on 
  Lugazi 
  after 
  the 
  hosts 
  had 
  

   been 
  banished 
  from 
  the 
  island 
  (57*2 
  per 
  cent, 
  of 
  females) 
  is 
  not 
  far 
  from 
  representing 
  

   the 
  real 
  ratio 
  between 
  the 
  sexes 
  on 
  that 
  particular 
  islet. 
  The 
  more 
  active 
  males 
  

   pretty 
  certainly 
  run 
  greater 
  risks, 
  and 
  do 
  not 
  on 
  the 
  average 
  live 
  so 
  long 
  as 
  tjie 
  less 
  

   active 
  females, 
  and 
  the 
  real 
  sex 
  ratio 
  would 
  be 
  determined 
  by 
  respective 
  longevity 
  

   of 
  the 
  sexes 
  — 
  probably 
  a 
  variable 
  quantity. 
  

  

  An 
  explanation 
  is 
  thus 
  provided 
  for 
  any 
  ratio 
  between 
  the 
  sexes 
  of 
  caught 
  flies 
  

   from, 
  say, 
  60 
  per 
  cent, 
  to 
  70 
  per 
  cent, 
  of 
  females 
  down, 
  but 
  doubtfully 
  for 
  female 
  

   percentages 
  of 
  70, 
  80 
  or 
  even 
  90, 
  such 
  as 
  are 
  occasionally 
  encountered. 
  The 
  explana- 
  

   tion 
  for 
  these 
  excessively 
  high 
  percentages 
  was 
  a 
  mystery 
  until 
  long 
  after 
  the 
  

   conclusion 
  of 
  the 
  Lugazi 
  experiment, 
  when 
  investigations 
  into 
  the 
  movements 
  of 
  

   flies 
  along 
  the 
  shore 
  of 
  a 
  lake 
  or 
  stream 
  provided 
  the 
  explanation 
  (see 
  Sect, 
  III) 
  

   and 
  at 
  the 
  same 
  time 
  additional 
  confirmation 
  of 
  the 
  conclusions, 
  tentatively 
  reached 
  

   through 
  the 
  Lugazi 
  experiment, 
  that 
  the 
  percentage 
  of 
  females 
  in 
  caught 
  flies 
  may 
  

   serve 
  as 
  an 
  index 
  to 
  the 
  abundance 
  of 
  food 
  (or 
  density 
  of 
  host-animals) 
  in 
  the 
  

   region 
  where 
  the 
  catch 
  is 
  made. 
  This 
  conclusion 
  is 
  undoubtedly 
  correct, 
  provided 
  

   that 
  the 
  catch 
  is 
  truly 
  representative 
  of 
  the 
  district 
  or 
  region 
  ; 
  it 
  is 
  not 
  correct 
  if 
  

   the 
  catch 
  is 
  made 
  under 
  specific 
  conditions 
  (with 
  respect 
  to 
  shelter, 
  etc.) 
  described 
  

   in 
  Sect. 
  III. 
  

  

  II. 
  The 
  Variable 
  Behaviour 
  of 
  Glossina 
  palpalis 
  towards 
  Man 
  and 
  

   Domestic 
  Animals 
  as 
  correlated 
  with 
  Variations 
  in 
  Sex 
  Ratio. 
  

  

  The 
  conclusions 
  tentatively 
  reached 
  through 
  the 
  foregoing 
  experiments 
  are 
  of 
  

   interest 
  in 
  a 
  vital 
  connection. 
  They 
  suggest 
  that 
  if 
  the 
  female 
  percentage 
  is 
  low, 
  

   food 
  must 
  be 
  plentiful, 
  and 
  man 
  would 
  be 
  less 
  liable 
  to 
  attack. 
  Moreover, 
  when 
  

   a 
  great 
  majority 
  of 
  the 
  flies 
  are 
  feeding 
  regularly 
  on 
  reptilian 
  hosts 
  — 
  which 
  do 
  not 
  

   carry 
  the 
  virus 
  of 
  the 
  human 
  disease 
  — 
  many 
  flies, 
  or 
  a 
  dense 
  infestation 
  by 
  fly, 
  

   would 
  be 
  less 
  injurious 
  to 
  man 
  and 
  less 
  liable 
  to 
  transmit 
  disease 
  from 
  man 
  to 
  

   man 
  than 
  if 
  there 
  were 
  only 
  a 
  few 
  flies, 
  or 
  a 
  light 
  infestation, 
  feeding 
  principally 
  

   or 
  exclusively 
  on 
  man, 
  or 
  on 
  animals 
  which 
  may 
  carry 
  the 
  virus 
  of 
  human 
  disease. 
  

  

  Therefore, 
  if 
  the 
  conclusions 
  are 
  correct, 
  the 
  injuriousness 
  of 
  Glossina 
  palpalis 
  to 
  a 
  

   population 
  living 
  in 
  constant 
  contact 
  with 
  it 
  would 
  be 
  subject 
  to 
  variations 
  indepen- 
  

   dently 
  of 
  any 
  variations 
  in 
  density 
  of 
  the 
  fly, 
  or 
  of 
  any 
  other 
  factor 
  than 
  the 
  

   abundance 
  of 
  host 
  animals 
  incapable 
  of 
  harbouring 
  the 
  virus 
  of 
  human 
  disease, 
  and 
  

   if 
  it 
  were 
  attempted 
  to 
  reduce 
  the 
  density 
  of 
  fly, 
  with 
  the 
  object 
  of 
  minimising 
  its 
  

   injuriousness, 
  through 
  measures 
  directed 
  against 
  its 
  host 
  animals, 
  the 
  results 
  might 
  

   be 
  the 
  opposite 
  of 
  those 
  desired. 
  Through 
  a 
  campaign 
  of 
  extermination 
  directed 
  

   against 
  its 
  reptilian 
  host, 
  its 
  density 
  might 
  be 
  reduced, 
  but 
  its 
  injuriousness 
  increased. 
  

  

  Several 
  experiments 
  were 
  conducted 
  and 
  many 
  observations 
  were 
  made 
  which 
  

   have 
  a 
  bearing 
  on 
  the 
  point, 
  and 
  which 
  were 
  also 
  designed 
  to 
  confirm 
  the 
  conclusions 
  

   to 
  the 
  Lula 
  and 
  Lugazi 
  experiments 
  cited 
  above 
  ; 
  for 
  if 
  these 
  conclusions 
  are 
  correct, 
  

   there 
  must 
  be 
  a 
  conspicuous 
  correlation 
  between 
  variations 
  in 
  female 
  percentage 
  

   and 
  in 
  the 
  persistency 
  with 
  which 
  tsetse-flies 
  press 
  their 
  attacks 
  upon 
  man. 
  

  

  In 
  part, 
  these 
  experiments 
  and 
  observations 
  are 
  cited 
  in 
  this 
  immediate 
  connection 
  

   (as 
  confirmatory 
  of 
  the 
  preceding 
  conclusion) 
  ; 
  but 
  they 
  are 
  further 
  cited 
  in 
  subse- 
  

   quent 
  pages 
  in 
  other 
  connections. 
  

  

  