﻿418 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  legs 
  or 
  abdomen. 
  Steering, 
  therefore, 
  Stellwaag 
  concludes, 
  is 
  evi- 
  

   dently 
  accomplished 
  by 
  a 
  differential 
  action 
  in 
  the 
  wings 
  themselves. 
  

  

  With 
  the 
  insects 
  held 
  on 
  pins, 
  it 
  is 
  possible 
  to 
  observe 
  the 
  action 
  of 
  

   the 
  wings 
  directly; 
  and 
  to 
  make 
  the 
  wing 
  movements 
  more 
  evident 
  

   Stellwaag 
  employed 
  the 
  method 
  used 
  by 
  Marey 
  of 
  attaching 
  bits 
  of 
  

   gold 
  leaf 
  to 
  the 
  wing 
  tips 
  and 
  throwing 
  a 
  strong 
  light 
  on 
  them 
  while 
  

   in 
  motion. 
  In 
  many 
  cases, 
  he 
  says, 
  not 
  only 
  the 
  plane 
  of 
  the 
  wings 
  is 
  

   seen 
  to 
  be 
  altered 
  as 
  the 
  insects 
  revolves 
  itself 
  on 
  the 
  pin, 
  but 
  there 
  

   is 
  also 
  visible 
  a 
  change 
  in 
  the 
  amplitude 
  of 
  the 
  wing 
  strokes 
  on 
  one 
  

   side 
  or 
  the 
  other. 
  From 
  these 
  experiments 
  it 
  is 
  evident 
  that 
  the 
  

   flying 
  insect 
  must 
  control 
  the 
  direction 
  of 
  its 
  flight 
  after 
  the 
  manner 
  of 
  

   a 
  rower 
  in 
  a 
  boat, 
  who, 
  in 
  the 
  absence 
  of 
  a 
  helmsman, 
  keeps 
  to 
  his 
  

   course 
  or 
  alters 
  it 
  by 
  changes 
  in 
  the 
  manipulation 
  of 
  the 
  oars. 
  

  

  The 
  muscles 
  of 
  the 
  insect 
  concerned 
  in 
  the 
  differential 
  action 
  of 
  the 
  

   wings 
  must 
  be 
  the 
  pleural 
  muscles 
  of 
  the 
  alar 
  segments, 
  which 
  are 
  

   those 
  of 
  the 
  basalar 
  and 
  subalar 
  sclerites 
  (fig. 
  19, 
  E, 
  M' 
  , 
  and 
  M"), 
  

   since 
  these 
  muscles 
  alone 
  have 
  specific 
  connections 
  with 
  the 
  wings. 
  

   The 
  longitudinal 
  and 
  vertical 
  muscles 
  of 
  the 
  wing-bearing 
  segments, 
  

   though 
  potent 
  effectors 
  of 
  wing 
  movements, 
  can 
  not 
  unequallj'^ 
  

   distribute 
  their 
  influence 
  between 
  the 
  two 
  sides 
  of 
  a 
  segment. 
  

  

  It 
  is 
  not 
  surprising 
  that 
  insects 
  should 
  be 
  experts 
  on 
  the 
  wing, 
  con- 
  

   sidering 
  that 
  they 
  have 
  been 
  flying 
  for 
  several 
  hundred 
  million 
  years, 
  

   but 
  still 
  we 
  are 
  inclined 
  to 
  marvel 
  when 
  we 
  see 
  them 
  perform 
  feats 
  that 
  

   are 
  as 
  yet 
  quite 
  impossible 
  for 
  our 
  newly 
  developed, 
  heavier-than-air 
  

   fiying 
  machines. 
  In 
  addition 
  to 
  their 
  ability 
  to 
  steer 
  themselves 
  

   adroitly 
  in 
  forward 
  flight, 
  many 
  insects 
  can 
  go 
  into 
  reverse 
  gear 
  and 
  

   fly 
  directly 
  backward 
  without 
  altering 
  the 
  position 
  of 
  their 
  bodies, 
  and, 
  

   moreover, 
  they 
  have 
  also 
  some 
  mechanism 
  of 
  adjustment 
  by 
  which 
  

   they 
  can 
  fly 
  sidewise, 
  either 
  to 
  the 
  right 
  or 
  left, 
  at 
  right 
  angles 
  to 
  the 
  

   body 
  axis. 
  The 
  dragon 
  flies 
  are 
  particularly 
  adept 
  in 
  these 
  modes 
  of 
  

   flight, 
  but 
  man}^ 
  of 
  the 
  smaller 
  insects, 
  such 
  as 
  the 
  flies 
  and 
  bees, 
  are 
  

   quite 
  equal 
  to 
  the 
  dragon 
  flies 
  in 
  being 
  able 
  to 
  dart 
  suddenly 
  to 
  the 
  side 
  

   or 
  rearward 
  while 
  the 
  head 
  still 
  points 
  in 
  the 
  direction 
  of 
  the 
  arrested 
  

   forward 
  flight. 
  Reversed 
  and 
  lateral 
  flying 
  is 
  probably 
  controlled 
  also 
  

   by 
  the 
  pleural 
  muscles 
  of 
  the 
  flight 
  mechanism, 
  which 
  alone 
  can 
  give 
  

   an 
  altered 
  or 
  differential 
  action 
  to 
  the 
  wings; 
  but 
  it 
  is 
  remarkable 
  that 
  

   organs 
  so 
  evidently 
  fasliioned 
  for 
  forward 
  flight, 
  as 
  are 
  the 
  wings 
  of 
  

   insects, 
  can 
  function 
  efficiently 
  for 
  producing 
  motion 
  in 
  other 
  direc- 
  

   tions. 
  

  

  Still 
  another 
  feat 
  that 
  many 
  insects 
  perform 
  on 
  the 
  wing 
  with 
  seem- 
  

   ing 
  ease 
  is 
  hovering. 
  Keeping 
  the 
  wings 
  in 
  rapid 
  movement, 
  the 
  insect 
  

   remains 
  without 
  other 
  motion 
  suspended 
  at 
  one 
  point 
  in 
  the 
  air, 
  even 
  

   maintaining 
  its 
  position 
  i 
  the 
  face 
  of 
  a 
  slight 
  breeze. 
  Presumably, 
  in 
  

   hovering, 
  the 
  wings 
  are 
  vibrated 
  approximately 
  in 
  a 
  horizontal 
  plane, 
  

   thus 
  creating 
  a 
  region 
  of 
  decreased 
  air 
  pressure 
  above 
  the 
  body 
  'out 
  not 
  

  

  