﻿HOW 
  INSECTS 
  FLY 
  — 
  SNODGRASS 
  

  

  387 
  

  

  could 
  have 
  served 
  some 
  important 
  function. 
  The 
  idea 
  that 
  comes 
  

   most 
  easily 
  to 
  the 
  imagination 
  is 
  that 
  the 
  fanlike 
  extensions 
  of 
  the 
  

   body 
  wall 
  constituted 
  a 
  glider 
  apparatus, 
  enabling 
  its 
  possessor 
  to 
  

   sail 
  downward 
  from 
  elevated 
  positions, 
  or 
  perhaps 
  from 
  one 
  elevation 
  

   to 
  another, 
  as 
  do 
  the 
  modern 
  flying 
  squirrels 
  with 
  their 
  parachute- 
  

   hke 
  folds 
  of 
  skin 
  stretched 
  between 
  the 
  legs 
  on 
  the 
  sides 
  of 
  the 
  body. 
  

  

  Something 
  of 
  the 
  possibilities 
  of 
  a 
  sailing 
  or 
  gUding 
  insect 
  may 
  be 
  

   demonstrated 
  with 
  a 
  model 
  cut 
  from 
  a 
  piece 
  of 
  thin 
  cardboard 
  ac- 
  

   cording 
  to 
  the 
  pattern 
  of 
  Figure 
  4, 
  and 
  given 
  a 
  proper 
  ballast 
  by 
  

   attaching 
  a 
  weight 
  beneath. 
  When 
  passively 
  released 
  from 
  any 
  altitude 
  

   the 
  cardboard 
  model 
  drops 
  straight 
  down; 
  when 
  

   given 
  a 
  forward 
  impulse, 
  however, 
  it 
  can 
  be 
  

   made 
  to 
  sail 
  downward 
  with 
  a 
  graceful 
  for- 
  

   ward 
  glide 
  extending 
  a 
  distance 
  varjdng 
  with 
  

   the 
  height 
  from 
  which 
  it 
  is 
  projected. 
  The 
  

   ballast 
  is 
  most 
  important; 
  it 
  may 
  be 
  a 
  crum- 
  

   pled 
  piece 
  of 
  sheet 
  lead 
  held 
  on 
  with 
  a 
  pin 
  or 
  

   thread, 
  but 
  it 
  must 
  have 
  just 
  a 
  certain 
  weight, 
  

   determined 
  by 
  clipping 
  it 
  down 
  to 
  the 
  most 
  

   effective 
  size, 
  and 
  it 
  must 
  be 
  attached 
  beneath 
  

   a 
  point 
  near 
  the 
  center 
  of 
  the 
  thorax. 
  

  

  From 
  the 
  performance 
  of 
  this 
  model 
  we 
  may 
  

   conclude 
  that 
  two 
  conditions 
  are 
  essential 
  to 
  

   make 
  gliding 
  possible, 
  aside 
  from 
  the 
  posses- 
  

   sion 
  of 
  a 
  glider 
  apparatus. 
  The 
  first 
  condition 
  

   demands 
  that 
  the 
  center 
  of 
  gravity 
  be 
  in 
  the 
  

   region 
  of 
  the 
  body 
  supporting 
  the 
  glider 
  lobes; 
  

   the 
  second, 
  that 
  the 
  creature 
  must 
  have 
  the 
  

   ability 
  to 
  project 
  itself 
  into 
  the 
  air. 
  We 
  would 
  

   draw 
  the 
  inference, 
  therefore, 
  that 
  insects 
  

   equipped 
  with 
  glider 
  lobes, 
  if 
  the 
  lobes 
  were 
  

   confined 
  to 
  the 
  thorax, 
  had 
  slender 
  abdomens 
  

   and 
  carried 
  the 
  weight 
  of 
  their 
  viscera 
  more 
  

   evenly 
  distributed 
  in 
  the 
  body 
  than 
  do 
  many 
  

   of 
  the 
  more 
  specialized 
  insects 
  of 
  the 
  present 
  

   time, 
  and 
  furthermore, 
  that 
  the 
  gliding 
  in- 
  

   sects 
  had 
  well-developed 
  legs 
  which 
  enabled 
  them 
  to 
  run 
  swiftly 
  off 
  

   the 
  end 
  of 
  a 
  support, 
  or 
  to 
  launch 
  themselves 
  into 
  the 
  air 
  by 
  a 
  

   vigorous 
  leap. 
  Many 
  of 
  the 
  modern 
  grasshoppers 
  do 
  not 
  depart 
  

   far 
  from 
  this 
  primitive 
  mode 
  of 
  fiight, 
  except 
  in 
  that 
  they 
  combine 
  

   an 
  excellent 
  pair 
  of 
  saltatorial 
  legs 
  with 
  a 
  pair 
  of 
  expansive 
  sails 
  that 
  

   can 
  perform 
  also, 
  in 
  a 
  weak 
  manner, 
  as 
  organs 
  of 
  true 
  flight. 
  

  

  The 
  ques-tion 
  always 
  arises, 
  or 
  is 
  deliberately 
  brought 
  up 
  to 
  embar- 
  

   rass 
  every 
  speculation 
  in 
  evolution, 
  as 
  to 
  how 
  organs 
  could 
  have 
  been 
  

   82322— 
  3U 
  26 
  

  

  FiGUEE 
  4.— 
  Outline 
  of 
  a 
  mode 
  

   that 
  can 
  be 
  made, 
  when 
  prop- 
  

   erly 
  ballasted, 
  to 
  show 
  some- 
  

   thing 
  of 
  the 
  possibilities 
  of 
  a 
  

   glider 
  insect 
  

  

  