﻿HOW 
  INSECTS 
  FLY 
  — 
  SNODQRASS 
  

  

  403 
  

  

  downstroke 
  (fig. 
  17 
  C), 
  and 
  must 
  then 
  reverse 
  itself 
  during 
  the 
  up- 
  

   stroke 
  (A). 
  The 
  ability 
  of 
  the 
  insect 
  vving 
  to 
  make 
  these 
  compound 
  

   movements 
  depends 
  in 
  part 
  upon 
  the 
  interrelations 
  of 
  the 
  articular 
  

   sclerites, 
  and 
  in 
  part 
  on 
  the 
  structure 
  of 
  the 
  wing 
  itself. 
  The 
  anterior 
  

   crowding 
  of 
  the 
  principal 
  wing 
  veins 
  stiffens 
  the 
  forward 
  part 
  of 
  the 
  

   wing 
  and 
  leaves 
  the 
  expanded 
  posterior 
  area 
  relatively 
  weak 
  and 
  flex- 
  

   ible. 
  When 
  the 
  wing 
  is 
  depressed, 
  therefore, 
  its 
  posterior 
  part 
  turns 
  

   upward 
  (fig. 
  17 
  C) 
  as 
  a 
  result 
  of 
  the 
  increased 
  air 
  pressure 
  below, 
  and 
  

   and 
  thus 
  gives 
  a 
  forward 
  thrust 
  to 
  the 
  wing, 
  with 
  the 
  result 
  that 
  the 
  

  

  Set. 
  

  

  Scl 
  

  

  Set' 
  

  

  Figure 
  18.— 
  Muscles 
  in 
  the 
  right 
  half 
  of 
  the 
  wing-beaj-ing 
  segments 
  of 
  a 
  grasshopper 
  (Dis- 
  

   sesteira) 
  as 
  seen 
  from 
  the 
  median 
  plane 
  of 
  the 
  body. 
  81 
  .112, 
  The 
  longitudinal 
  dorsal 
  

   muscles 
  which, 
  by 
  contraction, 
  arch 
  the 
  terga 
  plates 
  upward 
  and 
  thereby 
  depress 
  the 
  

   wings; 
  83, 
  84, 
  and 
  US, 
  the 
  tergo-sternal 
  depressors 
  of 
  the 
  tergal 
  plates, 
  which 
  indirectly 
  

   elevate 
  the 
  wings. 
  The 
  other 
  muscles 
  are 
  muscles 
  of 
  the 
  legs 
  and 
  of 
  the 
  sterna 
  

  

  anterior 
  margin 
  goes 
  downward 
  and 
  forward. 
  For 
  the 
  same 
  reasons 
  

   the 
  counterstroke 
  has 
  a 
  reverse 
  movement 
  (A). 
  

  

  It 
  is 
  possible 
  that 
  the 
  structure 
  of 
  the 
  wing 
  and 
  its 
  automatic 
  re- 
  

   sponse 
  to 
  air 
  pressure 
  enabled 
  the 
  primitive 
  winged 
  insects 
  to 
  fly 
  by 
  

   means 
  of 
  the 
  dorsal 
  and 
  tergo-sternal 
  segmental 
  musculature 
  alone. 
  

   Nearly 
  all 
  modern 
  insects, 
  however, 
  have 
  powerful 
  adjuncts 
  to 
  the 
  

   primitive 
  motor 
  mechanism 
  of 
  the 
  wings 
  in 
  the 
  pleural 
  muscles 
  that 
  

   affect 
  directly 
  the 
  movements 
  of 
  each 
  wing. 
  These 
  so-called 
  "direct 
  " 
  

   wing 
  muscles 
  (fig. 
  19, 
  E, 
  M' 
  , 
  M") 
  are 
  inserted 
  in 
  adult 
  insects 
  

   either 
  immediately 
  on 
  the 
  wing 
  bases 
  or 
  on 
  the 
  small 
  sclerites 
  

   beneath 
  the 
  wings 
  (figs. 
  13, 
  19, 
  Ba, 
  Sa) 
  or, 
  in 
  some 
  cases,' 
  on 
  a 
  lobe 
  

   of 
  the 
  lateral 
  wall 
  of 
  the 
  segment; 
  they 
  have 
  their 
  origins, 
  in 
  nearly 
  

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