﻿HOW 
  INSECTS 
  FLY 
  SNODGEASS 
  407 
  

  

  the 
  anterior 
  part 
  of 
  the 
  wing 
  as 
  it 
  turns 
  it 
  shghtly 
  forward. 
  The 
  

   mechanism 
  of 
  deflection, 
  including 
  the 
  basal 
  ar 
  sclerite 
  and 
  its 
  muscle, 
  

   has 
  been 
  called 
  the 
  pronator 
  apparatus 
  of 
  the 
  wing. 
  The 
  movement 
  

   of 
  pronation 
  accompanies 
  the 
  depression 
  of 
  the 
  wing. 
  (Fig. 
  17 
  C.) 
  

   The 
  reverse 
  movement, 
  or 
  the 
  deflection 
  of 
  the 
  posterior 
  part 
  of 
  the 
  

   wing 
  accompanying 
  the 
  upstroke 
  (fig. 
  17 
  C), 
  is 
  probably 
  caused 
  

   mostly 
  by 
  air 
  pressure 
  on 
  the 
  expanded, 
  flexible 
  posterior 
  area 
  of 
  the 
  

   wing 
  surface, 
  but 
  it 
  is 
  likely 
  that 
  the 
  tension 
  of 
  the 
  posterior 
  pleural 
  

   muscle 
  (figs. 
  13, 
  19, 
  M"), 
  pulling 
  on 
  the 
  second 
  axillary 
  sclerite 
  poste- 
  

   rior 
  to 
  the 
  fulcrum, 
  contributes 
  to 
  the 
  posterior 
  deflection 
  of 
  the 
  wing 
  

   during 
  the 
  upstroke. 
  

  

  The 
  motion 
  of 
  each 
  wing 
  in 
  flight 
  is, 
  then, 
  the 
  resultant 
  of 
  its 
  several 
  

   elemental 
  movements. 
  During 
  the 
  downstroke 
  the 
  wing 
  goes 
  from 
  

   above 
  downward 
  and 
  forward; 
  its 
  anterior 
  margin 
  is 
  deflected 
  and 
  

   its 
  posterior 
  area 
  turns 
  upward. 
  (Fig. 
  17 
  C.) 
  During 
  the 
  upstroke, 
  

   the 
  wing 
  goes 
  upward 
  and 
  backward, 
  and 
  its 
  posterior 
  surface 
  is 
  

   deflected 
  (A). 
  

  

  By 
  comparing 
  the 
  movements 
  of 
  an 
  insect's 
  wings 
  in 
  motion 
  with 
  

   the 
  action 
  of 
  the 
  blades 
  of 
  an 
  airplane 
  propeller, 
  it 
  will 
  be 
  seen 
  that 
  

   there 
  is 
  a 
  similarity 
  between 
  the 
  two. 
  The 
  planes 
  of 
  the 
  propeller 
  

   blades 
  are 
  so 
  turned 
  that 
  each 
  blade 
  in 
  rotating 
  to 
  the 
  right 
  cuts 
  

   downward 
  through 
  the 
  air 
  with 
  a 
  forward 
  slant, 
  while 
  on 
  the 
  left 
  it 
  

   goes 
  upward 
  with 
  a 
  rearward 
  slant. 
  The 
  edge 
  of 
  the 
  blade 
  that 
  

   opposes 
  the 
  air, 
  moreover, 
  is 
  beveled 
  in 
  such 
  a 
  manner 
  that 
  it 
  lies 
  in 
  

   the 
  plane 
  of 
  rotation. 
  The 
  insect 
  wing 
  differs 
  from 
  the 
  propeller 
  

   blade 
  in 
  that, 
  by 
  its 
  flexibility, 
  it 
  can 
  successively 
  adapt 
  the 
  different 
  

   parts 
  of 
  its 
  surface 
  to 
  the 
  same 
  changes 
  in 
  relative 
  position 
  during 
  an 
  

   up-and-down 
  movement 
  that 
  the 
  rigid 
  propeller 
  blade 
  assumes 
  in 
  

   revolution. 
  The 
  mechanical 
  effect 
  produced 
  by 
  the 
  two 
  instruments 
  

   is 
  the 
  same, 
  as 
  we 
  shall 
  see 
  in 
  the 
  next 
  section 
  of 
  this 
  paper. 
  

  

  As 
  a 
  result 
  of 
  the 
  compound 
  movements 
  of 
  the 
  vibrating 
  insect 
  

   wing, 
  the 
  tip 
  of 
  the 
  wing, 
  if 
  the 
  insect 
  is 
  held 
  stationary, 
  describes 
  a 
  

   curve 
  having 
  the 
  form 
  of 
  a 
  figure 
  8. 
  This 
  fact 
  has 
  long 
  been 
  known; 
  

   it 
  was 
  first 
  demonstrated 
  visibly 
  by 
  Marey 
  (1869, 
  1874) 
  who 
  attached 
  

   bits 
  of 
  gold 
  leaf 
  to 
  the 
  wing 
  tips 
  of 
  a 
  wasp 
  and 
  observed 
  the 
  luminous 
  

   figures 
  described 
  when 
  a 
  beam 
  of 
  strong 
  light 
  was 
  thrown 
  on 
  the 
  

   vibrating 
  wings. 
  The 
  movements 
  of 
  the 
  wings 
  have 
  since 
  been 
  studied 
  

   more 
  accurately, 
  however, 
  by 
  mechanical 
  devices 
  in 
  which 
  the 
  vibrat- 
  

   ing 
  tips 
  are 
  allowed 
  to 
  touch 
  lightly 
  the 
  surface 
  of 
  a 
  moving 
  sheet 
  of 
  

   smoked 
  paper, 
  thus 
  inscribing 
  a 
  record 
  of 
  their 
  movements 
  which 
  can 
  

   be 
  more 
  carefully 
  examined. 
  The 
  most 
  successful 
  studies 
  of 
  this 
  kind 
  

   are 
  those 
  of 
  Ritter 
  (1911) 
  who 
  constructed 
  an 
  apparatus 
  that 
  would 
  

   rapidly 
  slide 
  a 
  small 
  board 
  covered 
  with 
  blackened 
  paper 
  past 
  a 
  

   blowfly 
  secured 
  in 
  such 
  a 
  position 
  that 
  the 
  tip 
  of 
  a 
  vibrating 
  wing 
  

  

  