﻿HOW 
  INSECTS 
  FLY 
  SNODGRASS 
  405 
  

  

  The 
  fifth 
  set 
  of 
  wing 
  muscles 
  are 
  the 
  wing 
  flexors. 
  The 
  flexor 
  of 
  

   each 
  wing 
  is 
  usually 
  a 
  small 
  muscle, 
  or 
  a 
  group 
  of 
  small 
  muscles 
  (fig. 
  

   19, 
  D), 
  the 
  fibers 
  of 
  which 
  have 
  their 
  origin 
  on 
  the 
  pleural 
  wall 
  of 
  the 
  

   segment 
  and 
  are 
  inserted 
  directly 
  on 
  the 
  third 
  axillary 
  sclerite 
  of 
  the 
  

   wing 
  base 
  (fig. 
  12, 
  3 
  Ax). 
  The 
  flexor 
  muscles 
  are 
  present 
  in 
  nearly 
  all 
  

   insects, 
  including 
  the 
  dragon 
  flies 
  which 
  do 
  not 
  fold 
  the 
  wings; 
  in 
  

   insects 
  having 
  a 
  special 
  flexor 
  mechanism 
  in 
  the 
  wing 
  base 
  they 
  appear 
  

   to 
  accomplish 
  the 
  entire 
  movement 
  of 
  flexion. 
  

  

  Various 
  other 
  small 
  muscles 
  are 
  often 
  associated 
  with 
  the 
  wings; 
  

   they 
  arise 
  on 
  the 
  upper 
  parts 
  of 
  the 
  pleural 
  walls 
  of 
  the 
  wing-bearing 
  

   segments 
  and 
  are 
  inserted 
  either 
  on 
  the 
  tergum 
  near 
  the 
  wings 
  or 
  

   directly 
  on 
  the 
  wing 
  bases. 
  Since 
  these 
  muscles 
  are 
  not 
  of 
  constant 
  

   occurrence 
  and 
  differ 
  in 
  different 
  groups 
  of 
  insects, 
  they 
  need 
  not 
  be 
  

   considered 
  in 
  a 
  general 
  discussion. 
  

  

  IV. 
  THE 
  WING 
  MOVEMENTS 
  

  

  The 
  motions 
  of 
  insects' 
  wings 
  fall 
  into 
  two 
  distinct 
  categories; 
  

   those 
  of 
  one 
  include 
  the 
  movements 
  of 
  flight, 
  those 
  of 
  the 
  second 
  

   embrace 
  the 
  movements 
  of 
  flexion 
  and 
  extension. 
  The 
  flight 
  move- 
  

   ments 
  are 
  common 
  to 
  all 
  winged 
  insects; 
  the 
  movements 
  of 
  flexion 
  

   and 
  extension 
  pertain 
  only 
  to 
  insects 
  that 
  fold 
  the 
  wings 
  horizontally 
  

   over 
  the 
  back 
  when 
  not 
  in 
  use. 
  

  

  The 
  movements 
  qf 
  flight. 
  — 
  The 
  movements 
  of 
  the 
  wings 
  that 
  make 
  

   flight 
  possible 
  consist 
  of 
  an 
  vpsiroke, 
  a 
  doivnstroke, 
  a, 
  forward 
  movement, 
  

   a 
  reanoard 
  movement, 
  and 
  a 
  'partial 
  rotation 
  of 
  each 
  wing 
  on 
  its 
  long 
  axis. 
  

  

  It 
  was 
  formerly 
  supposed 
  that 
  the 
  torsion 
  of 
  the 
  wings, 
  including 
  

   the 
  horizontal 
  and 
  rotary 
  movements, 
  is 
  entirely 
  the 
  result 
  of 
  air 
  

   pressure 
  on 
  the 
  wings 
  as 
  they 
  arc 
  vibrated 
  in 
  a 
  vertical 
  direction. 
  

   This 
  idea 
  was 
  elaborated 
  particularly 
  by 
  Marey 
  (1874). 
  There 
  is 
  

   no 
  doubt 
  that 
  the 
  wings 
  do 
  respond 
  by 
  a 
  differential 
  action 
  in 
  their 
  

   planes 
  to 
  air 
  pressure 
  alone. 
  It 
  has 
  been 
  shown 
  by 
  Bull 
  (1904a) 
  

   that 
  the 
  wing 
  of 
  a 
  dragon 
  fly 
  attached 
  to 
  a 
  vibratory 
  apparatus 
  in 
  a 
  

   vacuum 
  jar 
  takes 
  on 
  the 
  rotary 
  movement 
  automatically 
  when 
  air 
  

   is 
  admitted. 
  Later, 
  however. 
  Bull 
  (1910) 
  observed 
  that 
  the 
  stump 
  

   from 
  which 
  a 
  wing 
  has 
  been 
  severed 
  in 
  a 
  living 
  insect 
  is 
  deflected 
  

   forward 
  with 
  the 
  downstroke 
  and 
  takes 
  a 
  reverse 
  position 
  during 
  

   the 
  upstroke. 
  It 
  is 
  now 
  conceded 
  by 
  students 
  of 
  the 
  insect 
  wing 
  

   mechanism 
  that 
  the 
  movements 
  of 
  the 
  wing, 
  though 
  in 
  part 
  auto- 
  

   matically 
  possible 
  by 
  air 
  pressure, 
  are 
  all 
  produced, 
  or 
  at 
  least 
  

   augmented, 
  by 
  the 
  action 
  of 
  the 
  wing 
  muscles. 
  Directive 
  move- 
  

   ments 
  of 
  the 
  wings, 
  it 
  must 
  be 
  observed, 
  are 
  only 
  possible 
  through 
  

   muscular 
  and 
  nervous 
  control. 
  

  

  The 
  upstroke 
  of 
  the 
  wing, 
  as 
  we 
  have 
  seen, 
  is 
  produced 
  by 
  the 
  

   simple 
  device 
  of 
  depressing 
  the 
  back 
  plate, 
  or 
  tergum, 
  of 
  the 
  segment 
  

   bearing 
  the 
  wings( 
  fig. 
  17 
  A), 
  the 
  action 
  being 
  the 
  result 
  of 
  a 
  contrac- 
  

  

  