﻿414 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  29 
  

  

  plane 
  and 
  thereby 
  spreads 
  the 
  wing. 
  Some 
  insects 
  may 
  be 
  seen 
  to 
  

   extend 
  the 
  wings 
  deliberately 
  before 
  taking 
  flight, 
  but 
  with 
  most 
  

   species 
  flight 
  is 
  practically 
  simultaneous 
  with 
  the 
  wing 
  expansion. 
  

  

  V. 
  FLIGHT 
  

  

  A 
  rotating 
  electric 
  fan 
  throws 
  out 
  a 
  current 
  of 
  air 
  because 
  its 
  inertia 
  

   is 
  greater 
  than 
  the 
  resistance 
  of 
  the 
  air. 
  An 
  airplane 
  moves 
  through 
  

   the 
  air 
  because 
  the 
  force 
  of 
  its 
  inertia 
  is 
  less 
  than 
  the 
  force 
  of 
  the 
  air 
  

   pressure 
  created 
  by 
  its 
  revolving 
  propeller. 
  In 
  other 
  words, 
  any 
  

   object 
  that 
  can 
  create 
  a 
  current 
  in 
  the 
  surrounding 
  medium, 
  if 
  held 
  

   stationary, 
  will 
  move 
  itself 
  through 
  the 
  medium 
  contrary 
  to 
  the 
  direc- 
  

   tion 
  of 
  the 
  potential 
  current 
  if 
  the 
  force 
  of 
  the 
  latter 
  is 
  greater 
  than 
  the 
  

   inertia 
  of 
  the 
  object. 
  Motion 
  is 
  the 
  result 
  of 
  the 
  difference 
  in 
  density, 
  

   or 
  pressure, 
  in 
  the 
  medium 
  created 
  by 
  the 
  propeller 
  mechanism 
  on 
  

   opposite 
  sides 
  of 
  it; 
  the 
  object 
  moves 
  toward 
  the 
  region 
  of 
  lowered 
  

   pressure. 
  

  

  Flight 
  by 
  any 
  heavier-than-air 
  machine 
  or 
  animal 
  that 
  does 
  not 
  

   depend 
  on 
  currents 
  or 
  rising 
  columns 
  of 
  air 
  requires 
  a 
  mechanism 
  

   capable 
  not 
  only 
  of 
  producing 
  a 
  forward 
  motion, 
  but 
  also 
  of 
  creating 
  a 
  

   lifting 
  force 
  sufficient 
  to 
  overcome 
  the 
  pull 
  of 
  gravity. 
  Soaring 
  birds 
  

   and 
  gliding 
  planes 
  keep 
  themselves 
  aloft 
  after 
  the 
  manner 
  of 
  a 
  kite; 
  

   the 
  extent 
  of 
  their 
  area 
  that 
  they 
  oppose 
  to 
  the 
  air 
  sustains 
  them 
  on 
  the 
  

   air 
  pressure 
  created 
  beneath. 
  The 
  ordinary 
  airplane 
  is 
  driven 
  only 
  

   forward 
  by 
  the 
  direct 
  action 
  of 
  its 
  propeller; 
  it 
  is 
  lifted 
  by 
  the 
  areas 
  of 
  

   decreased 
  pressure 
  created 
  above 
  its 
  slanting 
  wings 
  by 
  the 
  force 
  of 
  its 
  

   forward 
  motion. 
  With 
  most 
  insects 
  the 
  area 
  of 
  the 
  wings 
  is 
  far 
  too 
  

   small, 
  in 
  proportion 
  to 
  the 
  size 
  and 
  weight 
  of 
  the 
  insect's 
  body, 
  to 
  have 
  

   much 
  value 
  as 
  a 
  planing 
  surface, 
  and, 
  moreover, 
  the 
  wings 
  are 
  the 
  

   active 
  elements 
  in 
  the 
  motor 
  mechanism. 
  A 
  small-winged 
  insect, 
  

   therefore, 
  can 
  neither 
  soar 
  in 
  the 
  manner 
  of 
  the 
  larger 
  birds, 
  nor 
  can 
  it 
  

   sustain 
  itself 
  in 
  the 
  way 
  the 
  moving 
  airplane 
  does. 
  The 
  wings 
  of 
  

   insects 
  must 
  furnish 
  not 
  only 
  the 
  driving 
  power, 
  but 
  a 
  lifting 
  force 
  as 
  

   well, 
  which 
  is 
  to 
  say, 
  the 
  movement 
  of 
  the 
  wings 
  must 
  create 
  a 
  region 
  

   of 
  lowered 
  pressure 
  both 
  before 
  and 
  above 
  the 
  body 
  of 
  the 
  insect. 
  

  

  An 
  interesting 
  and 
  instructive 
  study 
  of 
  the 
  effect 
  of 
  the 
  wing 
  move- 
  

   ments 
  of 
  insects 
  on 
  the 
  surrounding 
  air 
  has 
  been 
  made 
  by 
  Demoll 
  

   (1918). 
  By 
  means 
  of 
  a 
  simple 
  apparatus 
  consisting 
  of 
  a 
  frame 
  with 
  

   several 
  horizontal 
  cross 
  bars 
  on 
  which 
  were 
  suspended 
  rows 
  of 
  fine 
  

   owl 
  feathers, 
  Demoll 
  was 
  able 
  to 
  demonstrate 
  the 
  direction 
  of 
  the 
  

   air 
  currents 
  created 
  by 
  the 
  wiags 
  in 
  vibration 
  when 
  the 
  insect 
  itself 
  

   is 
  held 
  stationary. 
  The 
  lightness 
  of 
  the 
  feathers 
  made 
  the 
  latter 
  

   delicately 
  responsive 
  to 
  any 
  disturbance 
  of 
  the 
  air 
  in 
  their 
  immediate 
  

   viciaity, 
  and 
  thus 
  the 
  air 
  currents 
  set 
  up 
  by 
  the 
  whirring 
  wings 
  of 
  an 
  

   insect, 
  secured 
  by 
  the 
  body 
  in 
  such 
  manner 
  that 
  the 
  wing 
  movements 
  

   would 
  not 
  be 
  hindered, 
  were 
  registered 
  in 
  the 
  displacement 
  of 
  the 
  

  

  