﻿INVESTIGATIONS 
  INTO 
  THE 
  BIONOMICS 
  OF 
  GLOSSINA 
  PALPALIS. 
  457 
  

  

  It 
  is 
  useless, 
  however, 
  to 
  discuss 
  ways 
  and 
  means 
  for 
  reducing 
  obnoxious 
  density 
  

   of 
  this 
  pest 
  until 
  some 
  decision 
  has 
  been 
  reached 
  concerning 
  what 
  constitutes 
  obnoxious 
  

   density. 
  It 
  has 
  been 
  shown 
  by 
  three 
  investigations 
  that 
  the 
  density 
  of 
  infestations 
  

   by 
  this 
  insect, 
  as 
  measured 
  by 
  the 
  males 
  to 
  be 
  caught 
  " 
  per 
  boy 
  per 
  hour," 
  ranges 
  

   all 
  the 
  way 
  from 
  less 
  than 
  0*1 
  to 
  more 
  than 
  150'0. 
  A 
  locality 
  is 
  no 
  less 
  truly 
  infested 
  

   if 
  density 
  is 
  O'l 
  than 
  if 
  it 
  is 
  150*0 
  — 
  the 
  difference 
  is 
  wholly 
  one 
  of 
  degree. 
  We 
  know 
  

   that 
  a 
  density 
  of 
  0*1 
  to 
  TO 
  is 
  innocuous, 
  because 
  density 
  in 
  populated 
  districts 
  along 
  

   shore 
  appears 
  never 
  to 
  have 
  been 
  reduced 
  below 
  these 
  figures, 
  despite 
  which 
  human 
  

   trypanosomiasis 
  formerly 
  prevalent 
  — 
  when 
  density 
  was 
  much 
  higher 
  — 
  seems 
  

   completely 
  to 
  have 
  disappeared. 
  There 
  is 
  good 
  reason 
  to 
  believe 
  that 
  a 
  density 
  of 
  

   6*0 
  is 
  ordinarily 
  safe 
  and 
  sanitary, 
  but 
  that 
  one 
  of 
  15 
  - 
  or 
  20*0 
  would 
  ordinarily 
  be 
  

   dangerous. 
  Admittedly 
  it 
  would 
  be 
  desirable 
  for 
  purely 
  sanitary 
  reasons 
  to 
  reduce 
  

   density 
  to 
  everywhere, 
  but 
  the 
  expense 
  would 
  be 
  so 
  enormous 
  as 
  to 
  render 
  it 
  

   completely 
  impracticable. 
  Practical 
  measures 
  must 
  combine 
  efficiency 
  with 
  economy, 
  

   and 
  the 
  most 
  economical 
  measures 
  are 
  such 
  as 
  do 
  not 
  carry 
  reduction 
  in 
  density 
  

   of 
  the 
  pest 
  beyond 
  what 
  is 
  necessary 
  for 
  sanitary 
  reasons. 
  

  

  XI. 
  Effect 
  of 
  Fluctuating 
  Lake 
  Level 
  on 
  G-lossina 
  palpalis. 
  

   In 
  Sect. 
  VII 
  (e) 
  mention 
  was 
  made 
  of 
  the 
  effect 
  of 
  fluctuating 
  lake-level 
  on 
  range 
  

   and 
  density 
  of 
  Glossina 
  palpalis. 
  Following 
  are 
  data 
  bearing 
  on 
  this 
  point, 
  which 
  

   could 
  not 
  well 
  be 
  included 
  in 
  that 
  section, 
  and 
  from 
  which 
  certain 
  conclusions 
  may 
  

   be 
  drawn, 
  as 
  presented. 
  

  

  XI 
  (a). 
  Fluctuations 
  in 
  Level 
  of 
  Victoria 
  Nyanza. 
  

  

  Victoria 
  Nyanza 
  is 
  usually 
  highest 
  in 
  May 
  and 
  lowest 
  in 
  November, 
  with 
  a 
  tendency 
  

   to 
  rise 
  in 
  December 
  and 
  to 
  fall 
  in 
  February. 
  The 
  average 
  yearly 
  fluctuation 
  is 
  about 
  

   1J 
  feet. 
  But 
  the 
  movements 
  are 
  very 
  irregular, 
  and 
  in 
  1906, 
  instead 
  of 
  falling 
  in 
  

   February 
  it 
  rose 
  steadily 
  from 
  November 
  1905 
  until 
  the 
  following 
  May, 
  and 
  reached 
  

   an 
  unprecedented 
  height. 
  Again 
  in 
  1910 
  it 
  failed 
  to 
  rise 
  in 
  December, 
  but 
  continued 
  

   to 
  fall 
  steadily 
  until 
  it 
  reached 
  an 
  unprecedentedly 
  low 
  level 
  in 
  February 
  1911. 
  The 
  

   maximum 
  in 
  May 
  1906 
  was 
  about 
  4J 
  feet 
  above 
  the 
  minimum 
  in 
  February 
  1911, 
  

   and 
  was 
  nearly 
  3 
  feet 
  higher 
  than 
  the 
  maximum 
  from 
  1911 
  to 
  1913 
  inclusive. 
  

  

  The 
  fluctuations 
  during 
  the 
  ten 
  year 
  period 
  ending 
  in 
  1913 
  are 
  shown 
  on 
  a 
  

   graph 
  recently 
  published 
  by 
  Dr. 
  H. 
  Lyndhurst 
  Duke 
  (Bull. 
  Ent. 
  Res. 
  IX, 
  p. 
  270). 
  

  

  XI 
  (6). 
  Effect 
  of 
  Fluctuations 
  on 
  Shelter 
  and 
  Breeding 
  Grounds 
  of 
  Tsetse. 
  

  

  The 
  high 
  level 
  of 
  1906 
  uprooted 
  much 
  vegetation 
  along 
  the 
  shores 
  and 
  washed 
  

   clean 
  the 
  sand 
  and 
  gravel 
  beaches 
  that 
  in 
  1913-14 
  lay 
  some 
  3 
  feet 
  higher 
  than 
  and 
  

   anywhere 
  up 
  to 
  200 
  yards 
  inland 
  from 
  the 
  foreshore. 
  . 
  Following 
  recession 
  of 
  the 
  

   water, 
  new 
  vegetation 
  sprung 
  up 
  to 
  shade 
  the 
  newly 
  washed 
  sands 
  and 
  gravels, 
  and 
  

   ideal 
  breeding 
  grounds 
  were 
  created 
  where 
  none, 
  perhaps, 
  existed 
  before. 
  But 
  

   already 
  in 
  1913 
  and 
  1914 
  these 
  new 
  breeding 
  grounds 
  were 
  beginning 
  to 
  deteriorate- 
  

   Leaf- 
  mould 
  was 
  accumulating 
  and 
  covering 
  the 
  sand, 
  vegetation 
  was 
  massing 
  above 
  it 
  

   and 
  keeping 
  it 
  cold 
  and 
  damp 
  ; 
  and 
  already 
  some 
  of 
  these 
  (breeding 
  places 
  had 
  

   become 
  second-rate 
  or 
  quite 
  unattractive. 
  They 
  were 
  fast 
  becoming 
  like 
  the 
  deposits 
  

   of 
  sand 
  and 
  gravel 
  that 
  lay 
  beyond 
  the 
  flood 
  mark 
  of 
  1906, 
  which 
  are 
  nearly 
  always 
  

   hidden 
  from 
  sight 
  beneath 
  blankets 
  of 
  mould 
  and 
  quite 
  valueless 
  as 
  protection 
  to 
  tsetse. 
  

  

  