﻿402 
  

  

  ANNUAL 
  EEPOET 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  might 
  be 
  lowered 
  by 
  raising 
  the 
  piston. 
  AH 
  modern 
  insects, 
  with 
  a 
  

   few 
  exceptions, 
  produce 
  the 
  downstroke 
  of 
  the 
  wings 
  at 
  least 
  in 
  part 
  

   by 
  the 
  contraction 
  of 
  the 
  dorsal 
  thoracic 
  muscles. 
  (Fig. 
  17 
  C, 
  A.) 
  

   In 
  response 
  to 
  the 
  new 
  function 
  thus 
  thrust 
  upon 
  them, 
  these 
  muscles 
  

   in 
  the 
  two 
  wing-bearing 
  segments 
  have 
  become 
  greatly 
  enlarged. 
  

   (Fig. 
  18,81 
  , 
  112.) 
  The 
  elevation 
  of 
  the 
  wings, 
  on 
  the 
  other 
  hand 
  (fig. 
  

   17 
  A), 
  involves 
  a 
  depression 
  of 
  the 
  back 
  plate 
  {T)oi 
  the 
  wing-support- 
  

   ing 
  segment, 
  and 
  this 
  is 
  accomplished 
  by 
  a 
  contraction 
  of 
  the 
  ver- 
  

   tical, 
  or 
  tergo-sternal, 
  muscles 
  of 
  the 
  segment 
  (C), 
  just 
  as 
  the 
  

  

  pump 
  handle 
  might 
  be 
  raised 
  

   by 
  pulling 
  down 
  on 
  the 
  pis- 
  

   ton. 
  It 
  is 
  a 
  question 
  wheth- 
  

   er 
  the 
  tergo-sternal 
  muscles 
  

   of 
  the 
  wing-bearing 
  seg- 
  

   ments 
  were 
  a 
  part 
  of 
  the 
  

   primitive 
  musculature 
  of 
  the 
  

   segments, 
  or 
  whether 
  they 
  

   are 
  speciaUj^ 
  acquired 
  wing 
  

   muscles; 
  but, 
  whatever 
  their 
  

   origin, 
  they 
  are 
  always 
  very 
  

   large 
  in 
  the 
  two 
  segments 
  

   (mesothorax 
  and 
  metatho- 
  

   rax) 
  that 
  carry 
  the 
  wings 
  

   (fig. 
  18, 
  83, 
  84, 
  113), 
  while 
  

   they 
  are 
  either 
  absent 
  or 
  

   doubtfully 
  represented 
  in 
  

   other 
  segments 
  of 
  the 
  body. 
  

   The 
  segmental 
  muscles 
  

   that 
  move 
  the 
  wings 
  owe 
  

   their 
  efficiency 
  to 
  the 
  fact 
  

   that 
  each 
  wing 
  is 
  pivoted 
  a 
  

   short 
  distance 
  beyond 
  its 
  

   base 
  on 
  a 
  strong 
  fulcrum 
  of 
  

   the 
  lateral 
  wall 
  of 
  the 
  seg- 
  

   ment. 
  (Fig. 
  17 
  A, 
  IFF.) 
  The 
  

   degree 
  of 
  movement 
  in 
  the 
  back 
  plate, 
  or 
  tergum 
  (T), 
  of 
  the 
  segment 
  

   necessary 
  to 
  produce 
  the 
  up-and-down 
  strokes 
  of 
  the 
  wings 
  is 
  very 
  

   small. 
  In 
  freshly 
  killed 
  specimens 
  of 
  flies 
  and 
  bees 
  the 
  wings 
  respond 
  

   instantly 
  to 
  the 
  least 
  downward 
  pressure 
  on 
  the 
  back 
  of 
  the 
  segment, 
  

   or 
  to 
  a 
  gentle 
  lengthwise 
  compression 
  of 
  the 
  thorax. 
  

  

  True 
  flight, 
  as 
  we 
  have 
  already 
  observed, 
  is 
  not 
  to 
  be 
  accomplished 
  

   by 
  mere 
  upstrokes 
  and 
  downstrokes 
  of 
  the 
  wings. 
  Each 
  wing 
  must 
  

   have 
  a 
  propeller 
  movement 
  in 
  order 
  to 
  produce 
  motion 
  through 
  the 
  

   air 
  ; 
  the 
  plane 
  of 
  the 
  wdng 
  must 
  dip 
  forward 
  and 
  downward 
  with 
  each 
  

  

  Figure 
  17. 
  — 
  Diagrams 
  suggesting 
  the 
  indirect 
  action 
  of 
  

   the 
  segmental 
  muscles 
  on 
  the 
  wings, 
  and 
  the 
  torsion 
  of 
  

   the 
  wings 
  in 
  flight, 
  anterior 
  view. 
  A, 
  The 
  wings 
  ele- 
  

   vated 
  on 
  pleural 
  wing 
  processes 
  (,WP) 
  by 
  depression 
  of 
  

   tergum 
  ( 
  T) 
  caused 
  by 
  contraction 
  of 
  tergo-sternal 
  mus- 
  

   cles 
  (C); 
  hind 
  margins 
  of 
  wings 
  deflected. 
  B, 
  C, 
  The 
  

   wings 
  depressed 
  by 
  an 
  elevation 
  of 
  tergum 
  caused 
  by 
  

   contraction 
  of 
  dorsal 
  longitudinal 
  muscles 
  {A); 
  hind 
  

   margins 
  of 
  wings 
  elevated 
  

  

  