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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  own 
  production; 
  in 
  terms 
  of 
  mechanics, 
  as 
  we 
  have 
  seen, 
  the 
  direc- 
  

   tion 
  from 
  which 
  a 
  current 
  of 
  air 
  is 
  drawn 
  by 
  a 
  stationary 
  object 
  is 
  

   the 
  direction 
  of 
  lowered 
  pressure, 
  while 
  the 
  opposite 
  is 
  that 
  of 
  

   increased 
  pressure. 
  According 
  to 
  the 
  observations 
  of 
  Demoll, 
  there- 
  

   fore, 
  when 
  an 
  insect 
  launches 
  itself 
  into 
  the 
  air 
  and 
  starts 
  the 
  

   vibration 
  of 
  its 
  wings, 
  there 
  is 
  at 
  once 
  created 
  before 
  it 
  and 
  above 
  

   it 
  a 
  region 
  of 
  decreased 
  pressure, 
  and 
  the 
  convergence 
  of 
  all 
  the 
  

   currents 
  behind 
  produces 
  here 
  a 
  region 
  of 
  greatly 
  increased 
  pressure. 
  

   The 
  lowered 
  pressure 
  above 
  counteracts 
  the 
  weight 
  of 
  the 
  insect; 
  

   the 
  increased 
  pressure 
  behind 
  drives 
  the 
  insect 
  forward 
  into 
  the 
  

   low-pressure 
  region 
  in 
  front. 
  Thus, 
  Demoll 
  points 
  out, 
  while 
  the 
  

   soaring 
  bird, 
  with 
  large 
  outstretched 
  wings 
  practically 
  stationary, 
  

   rests 
  upon 
  the 
  air, 
  the 
  flying 
  insect, 
  with 
  small 
  but 
  rapidly 
  moving 
  

   wings, 
  is 
  suspended 
  in 
  the 
  air. 
  The 
  bird, 
  a 
  glider 
  plane, 
  or 
  a 
  kite 
  is 
  

   borne 
  up 
  by 
  increase 
  of 
  pressure 
  below 
  ; 
  the 
  insect, 
  on 
  the 
  other 
  hand, 
  is 
  

   sucked 
  up 
  into 
  the 
  air 
  and 
  held 
  suspended 
  by 
  the 
  partial 
  vacuum 
  

   that 
  its 
  wings 
  create 
  above 
  it. 
  The 
  flying 
  mechanism 
  of 
  the 
  insect, 
  

   therefore, 
  is 
  comparable 
  with 
  that 
  of 
  a 
  helicopter 
  airplane, 
  except 
  

   that 
  in 
  the 
  insect 
  the 
  wings 
  neatly 
  combine 
  the 
  function 
  of 
  two 
  sets 
  

   of 
  propellers 
  working 
  at 
  right 
  angles 
  to 
  each 
  other. 
  If 
  both 
  pro- 
  

   pellers 
  of 
  a 
  bi-motored 
  airplane 
  were 
  directed 
  forward 
  and 
  upward, 
  

   the 
  machine 
  would 
  resemble 
  an 
  insect 
  — 
  if 
  it 
  could 
  fly. 
  The 
  body 
  

   weight 
  of 
  an 
  insect 
  is 
  so 
  distributed 
  that 
  the 
  center 
  of 
  gravity 
  lies 
  

   behind 
  the 
  wing 
  bases, 
  usually 
  at 
  the 
  base 
  of 
  the 
  abdomen. 
  (See 
  

   Demoll, 
  1918.) 
  

  

  The 
  driving 
  force 
  of 
  the 
  insect's 
  wing 
  movements 
  probably 
  depends 
  

   upon 
  the 
  angle 
  at 
  which 
  the 
  wing 
  surfaces 
  cut 
  the 
  air. 
  Slow-flying 
  

   insects 
  with 
  broad 
  wings, 
  such 
  as 
  the 
  butterflies 
  and 
  grasshoppers, 
  

   keep 
  the 
  wing 
  surfaces 
  almost 
  horizontal 
  and 
  fly 
  more 
  in 
  the 
  manner 
  

   of 
  small 
  birds 
  with 
  comparatively 
  few 
  strokes 
  of 
  the 
  Avings 
  in 
  any 
  

   unit 
  of 
  time. 
  Some 
  of 
  the 
  large 
  swallowtail 
  butterflies 
  even 
  soar 
  

   for 
  short 
  distances 
  with 
  the 
  wings 
  held 
  stationary. 
  The 
  more 
  swiftly 
  

   flying 
  insects, 
  however, 
  having 
  narrow 
  wings, 
  turn 
  the 
  wing 
  surfaces 
  

   more 
  nearly 
  vertical 
  with 
  each 
  stroke, 
  whether 
  up 
  or 
  down, 
  but 
  as 
  

   Ritter 
  (1911) 
  says, 
  "the 
  insect 
  flies 
  fastest 
  when 
  the 
  downstroke 
  

   approaches 
  a 
  vertical 
  direction," 
  because 
  the 
  greater 
  the 
  speed 
  the 
  

   more 
  the 
  curve 
  of 
  the 
  upstroke 
  is 
  drawn 
  forward 
  in 
  the 
  direction 
  of 
  

   flight. 
  

  

  The 
  speed 
  of 
  insect 
  flight 
  may 
  be 
  very 
  high, 
  considering 
  the 
  small 
  

   size 
  of 
  insects. 
  Demoll 
  (1918) 
  gives 
  a 
  table 
  of 
  the 
  rate 
  at 
  which 
  

   different 
  species 
  fly, 
  obtained 
  by 
  setting 
  individuals 
  at 
  liberty 
  in 
  a 
  

   room 
  lighted 
  by 
  one 
  window 
  and 
  recording 
  the 
  time 
  in 
  which 
  they 
  

   flew 
  direct 
  from 
  the 
  dark 
  side 
  of 
  the 
  room 
  to 
  the 
  light. 
  Among 
  the 
  

   swiftest 
  flying 
  insects, 
  according 
  to 
  this 
  test, 
  are 
  the 
  hawk 
  moths 
  

   (Sphingidae), 
  a 
  horsefly 
  (Tabanus 
  hovinus), 
  and 
  the 
  dragon 
  flies. 
  

  

  