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  REPOKT 
  UNITED 
  STATES 
  ENTOMOLOGICAL 
  COMMISSION. 
  

  

  sensation, 
  and 
  hence 
  they 
  continue 
  to 
  move 
  upward 
  until 
  they 
  reach 
  a 
  

   current 
  of 
  air 
  of 
  sufficient 
  strength 
  to 
  bear 
  them 
  onward 
  ; 
  they 
  continue 
  

   to 
  float 
  on 
  this 
  until 
  exhausted, 
  a 
  change 
  in 
  the 
  condition 
  of 
  the 
  atmo- 
  

   sphere 
  causes 
  them 
  to 
  descend, 
  or 
  a 
  suitable 
  place 
  for 
  depositing 
  eggs 
  or 
  

   procuring 
  food 
  is 
  observed. 
  

  

  That 
  a 
  dry 
  condition 
  of 
  the 
  atmosphere 
  does 
  originate 
  this 
  migrating 
  

   instinct 
  I 
  think 
  is 
  evident 
  not 
  only 
  from 
  the 
  fact 
  that 
  migratory 
  locusts 
  

   are 
  only 
  found 
  in 
  dry 
  and 
  arid 
  regions, 
  but 
  from 
  the 
  additional 
  fact 
  that 
  

   in 
  excessively 
  dry 
  years 
  we 
  see 
  in 
  this 
  country 
  Acridium 
  americanum, 
  

   Caloptenus 
  differ 
  entialis, 
  C. 
  atlantis, 
  and 
  C. 
  femur-rubrum, 
  exhibiting 
  a 
  

   strong 
  tendency 
  to 
  mass 
  together 
  and 
  migrate. 
  Even 
  their 
  forms 
  appear 
  

   to 
  be 
  modified 
  so 
  as 
  to 
  adapt 
  them 
  to 
  this 
  purpose. 
  C. 
  femur-rubrum 
  y 
  

   under 
  such 
  circumstances, 
  I 
  am 
  satisfied 
  from 
  numerous 
  observations 
  

   made 
  in 
  the 
  last 
  twenty 
  years, 
  undergoes 
  certain 
  modifications 
  which 
  

   bring 
  it 
  nearer 
  C. 
  spretus. 
  This, 
  as 
  a 
  matter 
  of 
  course, 
  is 
  only 
  distinctly 
  

   apparent 
  when 
  two 
  or 
  three 
  unusually 
  dry 
  years 
  follow 
  in 
  succession. 
  

   One 
  fact 
  noticed, 
  and 
  for 
  which 
  no 
  satisfactory 
  explanation 
  can 
  so 
  far 
  

   be 
  given, 
  is 
  the 
  tendency 
  (among 
  the 
  Acridians) 
  of 
  the 
  last 
  segment 
  of 
  the 
  

   male 
  to 
  become 
  elongated 
  and 
  pointed, 
  and 
  generally 
  to 
  become 
  notched 
  

   at 
  the 
  tip, 
  or 
  where 
  there 
  is 
  a 
  notch, 
  as 
  in 
  the 
  species 
  of 
  Cyrtacanthacris 
  

   {Acridium 
  americanum 
  and 
  allies) 
  to 
  change 
  from 
  the 
  square 
  or 
  U 
  form 
  to 
  

   the 
  sharp 
  or 
  Y 
  form. 
  Whether 
  the 
  same 
  thing 
  is 
  true 
  with 
  reference 
  to 
  

   the 
  (Edipodeans 
  I 
  am 
  unable 
  to 
  say 
  ; 
  so 
  far 
  as 
  my 
  observations 
  extend, 
  

   I 
  have 
  observed 
  nothing 
  of 
  the 
  kind 
  in 
  any 
  species 
  of 
  this 
  group. 
  209 
  

  

  Another 
  fact 
  worthy 
  of 
  notice 
  is 
  that 
  as 
  a 
  very 
  general 
  rule 
  — 
  so 
  far 
  as 
  

   I 
  am 
  aware 
  without 
  exception 
  — 
  the 
  elytra 
  are 
  spotted, 
  the 
  spots 
  usually 
  

   more 
  or 
  less 
  quadrate. 
  

  

  A 
  careful 
  comparison 
  of 
  the 
  internal 
  anatomy 
  of 
  the 
  migratory 
  and 
  

   closely 
  allied 
  non-migratory 
  species 
  would 
  probably 
  reveal 
  some 
  con- 
  

   stant 
  peculiarity 
  which 
  would 
  assist 
  in 
  explaining 
  how 
  the 
  climatic 
  con- 
  

   ditions 
  mentioned 
  bring 
  about 
  this 
  disposition 
  or 
  propensity 
  to 
  migrate. 
  

   I 
  am 
  inclined 
  to 
  think 
  it 
  is 
  largely 
  due 
  to 
  the 
  effect 
  these 
  atmospheric- 
  

   conditions 
  have 
  on 
  the 
  air-tubes 
  and 
  air-sacs. 
  The 
  vivi-dissections 
  made 
  

   by 
  Mr. 
  Packard, 
  and 
  recorded 
  in 
  our 
  First 
  Report, 
  tend 
  to 
  confirm 
  this 
  

   view 
  by 
  showing 
  the 
  important 
  bearing 
  the 
  numerous 
  and 
  large 
  air-sacs 
  

   must 
  have 
  on 
  the 
  flight 
  of 
  the 
  insect 
  ; 
  and 
  the 
  large 
  amount 
  of 
  tracheal 
  

   surface 
  brought 
  in 
  contact 
  with 
  the 
  air 
  and 
  thereby 
  rendered 
  sensible 
  to 
  

   its 
  changes. 
  

  

  Our 
  conclusion, 
  therefore, 
  is 
  that 
  the 
  migratory 
  habit 
  or 
  instinct 
  of 
  lo- 
  

   custs, 
  whether 
  in 
  the 
  Eastern 
  or 
  Western 
  Continent, 
  is 
  directly 
  attributa- 
  

   ble 
  to 
  the 
  arid 
  condition 
  of 
  the 
  area 
  in 
  which 
  they 
  originate 
  ; 
  that 
  the 
  unus- 
  

   ually 
  dry 
  and 
  rarified 
  state 
  of 
  the 
  atmosphere 
  is 
  the 
  chief 
  factor 
  in 
  originat- 
  

   ing 
  this 
  instinct 
  ; 
  that 
  while 
  it 
  affects, 
  to 
  a 
  greater 
  or 
  less 
  extent, 
  all 
  parts 
  

   of 
  the 
  insect 
  its 
  chief 
  influence 
  is 
  produced 
  by 
  its 
  effect 
  on 
  the 
  trachea? 
  and 
  

   air-sacs. 
  It 
  also 
  must 
  have 
  a 
  tendency 
  to 
  harden 
  the 
  integuments 
  and 
  to 
  

  

  ' 
  M 
  ' 
  I 
  have 
  since 
  noticed 
  the 
  saxno 
  tendency 
  iu 
  (K. 
  atrox. 
  

  

  