﻿HOW 
  INSECTS 
  PLY 
  — 
  SNODGRASS 
  

  

  399 
  

  

  thickening 
  (a) 
  of 
  the 
  membrane 
  beneath 
  the 
  wing 
  base 
  with 
  the 
  humeral 
  

   angle 
  of 
  the 
  wing; 
  the 
  subalare 
  is 
  similarly 
  connected 
  (b) 
  with 
  the 
  second 
  

   axillary 
  sclerite. 
  

  

  The 
  wing 
  regions. 
  — 
  The 
  wings 
  of 
  all 
  insects 
  are 
  more 
  or 
  less 
  as3^m- 
  

   metrical 
  in 
  form; 
  not 
  only 
  is 
  the 
  front 
  margin 
  of 
  each 
  wing 
  always 
  of 
  

   a 
  different 
  contour 
  from 
  the 
  hind 
  margin, 
  but 
  the 
  pattern 
  of 
  the 
  ante- 
  

   rior 
  veins 
  never 
  matches 
  that 
  of 
  the 
  posterior 
  veins. 
  There 
  is 
  a 
  tend- 
  

   ency 
  for 
  the 
  anterior 
  veins 
  to 
  become 
  crowded 
  toward 
  the 
  forward 
  

   margin 
  in 
  such 
  a 
  manner 
  as 
  to 
  give 
  rigidity 
  to 
  the 
  front 
  half 
  of 
  the 
  wing 
  

   while 
  the 
  posterior 
  veins 
  are 
  more 
  widely 
  spaced, 
  allowing 
  of 
  a 
  flexibil- 
  

   ity 
  in 
  the 
  rear 
  half 
  of 
  the 
  wing. 
  This 
  arrangement 
  of 
  the 
  veins, 
  

   together 
  with 
  the 
  antero-posterior 
  difference 
  in 
  the 
  form 
  of 
  the 
  wing, 
  

   is 
  clearly 
  an 
  adaptation 
  for 
  giving 
  greater 
  efficiency 
  to 
  the 
  wings 
  as 
  

   organs 
  of 
  forward 
  flight, 
  notwithstanding 
  the 
  fact 
  that 
  many 
  insects 
  

   can 
  fly 
  backward 
  and 
  sidewise. 
  As 
  a 
  result 
  of 
  the 
  functional 
  differ- 
  

   ences 
  in 
  the 
  parts 
  of 
  the 
  wing, 
  the 
  wing 
  area 
  becomes 
  dift'erentiated 
  

  

  d 
  hf 
  

  

  Figure 
  14. 
  — 
  Principal 
  wing 
  regions 
  in 
  insects 
  that 
  flex 
  the 
  wings, 
  a-b. 
  Base 
  

   of 
  axillary 
  region; 
  Ax, 
  axillary 
  region; 
  bf, 
  basal 
  fold 
  of 
  wing, 
  plica 
  basalis; 
  c, 
  

   apex 
  of 
  axillary 
  region; 
  rf, 
  point 
  of 
  articulation 
  of 
  radia 
  vein 
  with 
  second 
  axil- 
  

   lary 
  (see 
  fig. 
  12); 
  jf, 
  jugal 
  fold, 
  plica 
  jugalis; 
  Ju, 
  jugal 
  region; 
  Rm, 
  remigium, 
  

   remigial 
  region; 
  V, 
  vannus, 
  vannal 
  region; 
  vf, 
  vannal 
  fold, 
  plica 
  vannalis 
  

  

  structurally 
  into 
  several 
  regions. 
  The 
  wing 
  regions 
  are 
  particularly 
  

   well 
  defined 
  in 
  insects 
  that 
  flex 
  the 
  wings, 
  because 
  the 
  wing 
  fold- 
  

   ing 
  could 
  not 
  be 
  allowed 
  to 
  interfere 
  with 
  the 
  function 
  of 
  flying, 
  and, 
  

   therefore, 
  the 
  adaptations 
  for 
  flexing 
  have 
  to 
  follow 
  the 
  structural 
  plan 
  

   primarily 
  laid 
  down 
  for 
  purposes 
  of 
  flight. 
  

  

  We 
  have 
  already 
  seen 
  that 
  there 
  is 
  at 
  the 
  base 
  of 
  each 
  wing 
  an 
  

   axillary 
  region, 
  represented 
  by 
  the 
  second 
  plate 
  of 
  the 
  dragon 
  fly's 
  

   wing 
  (fig. 
  11 
  A, 
  AxP), 
  which 
  becomes 
  broken 
  up 
  into 
  a 
  number 
  of 
  

   definite 
  axillary 
  sclerites 
  in 
  the 
  wing-flexing 
  insects 
  (fig. 
  12). 
  The 
  

   region 
  of 
  the 
  axillary 
  sclerites 
  has 
  in 
  general 
  the 
  form 
  of 
  a 
  scalene 
  

   triangle 
  (fig. 
  14, 
  Ax) 
  with 
  its 
  base 
  (a 
  — 
  6) 
  against 
  the 
  body 
  and 
  its 
  

   longer 
  side 
  anterior 
  to 
  the 
  apex 
  (c). 
  The 
  base 
  of 
  the 
  triangle 
  is 
  the 
  

   hinge 
  of 
  the 
  wing 
  with 
  the 
  body 
  ; 
  the 
  apex 
  represents 
  the 
  distal 
  end 
  of 
  

   the 
  third 
  axillary 
  sclerite 
  (fig. 
  12, 
  SAx), 
  which 
  carries 
  the 
  bases 
  of 
  the 
  

   vannal 
  veins; 
  the 
  point 
  d 
  on 
  the 
  anterior 
  side 
  of 
  the 
  triangle 
  (fig. 
  14) 
  

  

  