﻿INVESTIGATIONS 
  INTO 
  THE 
  BIONOMICS 
  OF 
  GLOSSINA 
  PALPALIS. 
  455 
  

  

  zone 
  where 
  conditions 
  of 
  life 
  are 
  positively 
  unfavourable. 
  In 
  this 
  surrounding 
  

   zone 
  loss 
  through 
  mortality 
  exceeds 
  gain 
  through 
  natural 
  increase, 
  but 
  in 
  addition 
  

   there 
  is 
  gain 
  through 
  immigration 
  from 
  the 
  centres 
  of 
  infestation. 
  A 
  perfect 
  

   " 
  natural 
  balance 
  " 
  is 
  thus 
  established 
  and 
  perpetually 
  maintained, 
  so 
  long 
  as 
  flies 
  

   are 
  free 
  to 
  disperse 
  from 
  the 
  infested 
  centres 
  into 
  the 
  surrounding 
  zone. 
  If 
  dispersion 
  

   is 
  interfered 
  with, 
  the 
  mechanism 
  of 
  this 
  balance 
  is 
  thrown 
  out 
  of 
  gear 
  ; 
  but 
  this 
  

   very 
  rarely 
  happens 
  in 
  nature 
  so 
  far 
  as 
  I 
  am 
  aware 
  it 
  failed 
  to 
  function 
  in 
  this 
  

   region 
  only 
  on 
  a 
  few 
  small 
  islands, 
  such 
  as 
  Kimmi, 
  where 
  the 
  conditions 
  of 
  life 
  

   appear 
  to 
  be 
  so 
  favourable 
  that 
  super-density 
  results 
  at 
  frequent 
  intervals, 
  causing 
  

   the 
  host 
  animals 
  to 
  abandon 
  the 
  locality 
  temporarily 
  until 
  the 
  density 
  of 
  fly 
  is 
  reduced. 
  

   Here 
  we 
  have 
  a 
  natural 
  balance 
  constructed 
  on 
  an 
  entirely 
  different 
  mechanical 
  

   principle, 
  which 
  operates 
  as 
  effectually, 
  but 
  not 
  nearly 
  so 
  smoothly, 
  as 
  in 
  the 
  

   majority 
  of 
  cases, 
  in 
  which 
  it 
  is 
  the 
  flies 
  and 
  not 
  the 
  host 
  animals 
  that 
  are 
  moved 
  

   to 
  disperse 
  from 
  over-densely 
  infested 
  localities. 
  

  

  The 
  role 
  played 
  by 
  natural 
  enemies 
  and 
  destructive 
  factors 
  generally 
  is 
  to 
  destroy 
  

   the 
  insects, 
  and 
  to 
  prevent 
  the 
  existence 
  of 
  the 
  species 
  everywhere 
  beyond 
  the 
  limits 
  

   of 
  dispersion 
  from 
  protected 
  localities. 
  The 
  protection 
  of 
  shelter 
  and 
  breeding 
  

   grounds 
  suffices 
  against 
  all 
  destructive 
  factors, 
  animate 
  and 
  inanimate, 
  except 
  possibly 
  

   and 
  very 
  rarely, 
  against 
  extraordinary 
  numbers 
  of 
  spiders 
  or 
  dragonflies. 
  The 
  

   protective 
  factors 
  in 
  the 
  bionomics 
  of 
  the 
  species 
  are 
  far 
  more 
  specific 
  in 
  their 
  

   nature, 
  and 
  easily 
  studied, 
  than 
  the 
  destructive. 
  It 
  is, 
  therefore 
  inadvisible 
  to 
  

   study 
  the 
  destructive 
  factors 
  specifically. 
  It 
  is 
  possible 
  to 
  utilise 
  them 
  for 
  controlling 
  

   the 
  density 
  of 
  the 
  pest 
  easily 
  enough 
  by 
  merely 
  depriving 
  it 
  of 
  its 
  specific 
  protection 
  

   against 
  them, 
  but 
  altogether 
  impossible 
  to 
  expect 
  them 
  to 
  penetrate 
  protected 
  

   precincts, 
  or 
  to 
  utilise 
  them 
  otherwise 
  than 
  in 
  the 
  manner 
  indicated 
  by 
  the 
  facts. 
  

  

  It 
  is 
  possible 
  to 
  argue 
  successfully 
  that 
  the 
  species 
  is 
  normally 
  controlled 
  in 
  range 
  

   and 
  density 
  by 
  the 
  amount 
  of 
  available 
  food, 
  provided 
  that 
  the 
  word 
  " 
  available 
  " 
  

   is 
  broadly 
  enough 
  interpreted. 
  Food, 
  to 
  be 
  available, 
  must 
  be 
  within 
  reach 
  of 
  the 
  

   insects 
  from 
  protection 
  (of 
  both 
  sorts). 
  It 
  is 
  also 
  necessary 
  that 
  the 
  host 
  animals 
  

   should 
  be 
  complacent 
  under 
  attack, 
  for 
  there 
  is 
  no 
  host 
  of 
  the 
  insect 
  which 
  cannot 
  

   protect 
  itself 
  against 
  attack 
  if 
  it 
  will. 
  Monkeys 
  are 
  " 
  unavailable 
  " 
  as 
  hosts, 
  not 
  

   because 
  their 
  blood 
  is 
  less 
  suitable 
  as 
  food, 
  nor 
  because 
  the 
  flies 
  are 
  unable 
  to 
  draw 
  

   it, 
  but 
  entirely 
  because 
  the 
  animals 
  are 
  so 
  active 
  in 
  protecting 
  themselves. 
  Sheep 
  

   and 
  goats 
  are 
  more 
  complacent 
  and 
  less 
  active 
  in 
  protecting 
  themselves 
  than 
  

   monkeys, 
  but 
  much 
  less 
  complacent 
  and 
  more 
  actively 
  self-protective 
  than 
  oxen 
  

   or 
  antelopes. 
  Men 
  vary 
  greatly 
  in 
  temperament, 
  but 
  the 
  average 
  man 
  stands 
  

   between 
  goats 
  and 
  sheep 
  on 
  the 
  one 
  hand 
  and 
  oxen 
  and 
  antelopes 
  on 
  the 
  other. 
  

   The 
  large 
  reptiles 
  are 
  the 
  most 
  complacent 
  of 
  all, 
  and 
  most 
  available 
  as 
  hosts, 
  

   despite 
  the 
  fact 
  that 
  the 
  flies 
  experience 
  measurably 
  greater 
  difficulty 
  in 
  drawing 
  

   blood 
  from 
  them. 
  Availability 
  .of 
  food 
  thus 
  varies 
  greatly 
  with 
  the 
  species, 
  and 
  

   with 
  the 
  temperament 
  of 
  the 
  individual 
  host, 
  and 
  also 
  with 
  circumstances 
  and 
  

   conditions 
  of 
  time 
  and 
  place. 
  If 
  large 
  Tabanid 
  flies 
  are 
  annoying 
  oxen, 
  they 
  become 
  

   less 
  available 
  as 
  sources 
  of 
  food 
  ; 
  if 
  Glossina 
  itself 
  increases 
  in 
  local 
  density, 
  its 
  host 
  

   animals 
  become 
  less 
  available. 
  This 
  is 
  strikingly 
  exemplified 
  by 
  human 
  behaviour. 
  

   If 
  there 
  are 
  very 
  few 
  flies 
  a 
  man 
  pays 
  little 
  or 
  no 
  attention 
  to 
  them, 
  and 
  they 
  have 
  

  

  