﻿HOW 
  INSECTS 
  FLY 
  SNODGRASS 
  

  

  389 
  

  

  II. 
  STRUCTURE 
  OF 
  THE 
  WINGS 
  

  

  It 
  is 
  a 
  long 
  wny 
  from 
  immovable 
  paranotal 
  lobes 
  to 
  fullj" 
  devel- 
  

   oped 
  wings 
  capable 
  of 
  sustained 
  flight. 
  In 
  the 
  first 
  step 
  toward 
  the 
  

   status 
  of 
  wings, 
  the 
  paranotal 
  lobes 
  of 
  insects 
  had 
  simultaneously 
  to 
  

   become 
  flexible 
  at 
  their 
  bases 
  and 
  to 
  acquire 
  a 
  motor 
  mechanism 
  

   that 
  would 
  impart 
  to 
  them 
  the 
  movements 
  necessary 
  in 
  organs 
  of 
  

   flight. 
  Flight, 
  it 
  must 
  be 
  observed, 
  is 
  not 
  to 
  be 
  accomplished 
  by 
  a 
  

   mere 
  up-and-down 
  flapping 
  of 
  a 
  pair 
  of 
  flat 
  appendages; 
  as 
  will 
  be 
  

   explained 
  later, 
  in 
  order 
  to 
  generate 
  forward 
  motion, 
  wings 
  must 
  be 
  

   capable 
  not 
  only 
  of 
  movements 
  in 
  a 
  vertical 
  direction, 
  but 
  also 
  of 
  

   some 
  degree 
  of 
  anterior 
  and 
  posterior 
  movement 
  accompanied 
  by 
  a 
  

   partial 
  rotation 
  on 
  their 
  long 
  axes. 
  The 
  mechanism 
  of 
  an 
  insect's 
  

   wings, 
  therefore, 
  must 
  be 
  such 
  as 
  to 
  produce 
  at 
  least 
  the 
  movements 
  

   necessary 
  for 
  progression 
  in 
  the 
  air. 
  Insects 
  that 
  fly 
  most 
  efficiently, 
  

   however, 
  have 
  evolved 
  an 
  apparatus 
  capable 
  also 
  of 
  controlled 
  

   flight, 
  of 
  sidewise 
  

   flight, 
  of 
  rearward 
  

   flight, 
  and 
  of 
  hover- 
  

   ing. 
  Finallj', 
  to 
  this 
  

   equipment 
  most 
  in- 
  

   sects 
  have 
  added 
  a 
  

   special 
  mechanism 
  

   for 
  folding 
  the 
  mngs 
  

   horizontally 
  over 
  the 
  

   body 
  when 
  not 
  in 
  

   use. 
  

  

  Winged 
  insects 
  

   may 
  be 
  divided 
  in- 
  

   to 
  those 
  that 
  keep 
  the 
  wings 
  extended 
  straight 
  out 
  at 
  the 
  sides 
  when 
  at 
  

   rest, 
  or 
  which 
  merely 
  close 
  them 
  vertically 
  over 
  the 
  back, 
  and 
  those 
  that 
  

   fold 
  the 
  wings 
  horizontally 
  over 
  the 
  body. 
  Among 
  modern 
  insects, 
  the 
  

   first 
  group 
  is 
  represented 
  by 
  the 
  dragon 
  flies 
  (figs. 
  7, 
  8) 
  and 
  the 
  I\ 
  I 
  ay 
  flies 
  

   (fig. 
  9) 
  ; 
  the 
  second 
  includes 
  most 
  of 
  the 
  other 
  insects. 
  In 
  the 
  Paleozoic 
  

   era, 
  the 
  insects 
  that 
  did 
  not 
  fold 
  the 
  wings 
  included 
  the 
  dragon 
  flies 
  and 
  

   May 
  flies 
  of 
  that 
  time, 
  and 
  also 
  the 
  members 
  of 
  the 
  Paleodictyoptera, 
  

   so 
  far 
  as 
  can 
  be 
  judged 
  from 
  their 
  fossil 
  remains. 
  These 
  insects 
  were 
  

   inhabitants 
  of 
  the 
  open, 
  watery 
  spaces, 
  and 
  probably 
  used 
  their 
  legs 
  

   principally 
  for 
  perching; 
  they 
  had 
  no 
  occasion 
  for 
  crawling 
  about 
  in 
  

   places 
  where 
  spread 
  wings 
  would 
  be 
  an 
  encumbrance 
  to 
  their 
  move- 
  

   ments. 
  Their 
  general 
  habits 
  were 
  evidently 
  similar 
  to 
  those 
  of 
  the 
  

   modern 
  May 
  flies 
  and 
  dragon 
  flies, 
  and 
  in 
  their 
  younger 
  stages 
  they 
  

   probably 
  lived 
  in 
  the 
  water 
  as 
  do 
  at 
  present 
  the 
  m.embers 
  of 
  these 
  two 
  

   groups 
  of 
  insects. 
  The 
  ancient 
  insects 
  that 
  folded 
  their 
  wings 
  hori- 
  

   zontally 
  over 
  the 
  back 
  when 
  not 
  in 
  use 
  included 
  the 
  roaches 
  and 
  

  

  FiGTJBE 
  6. 
  — 
  Theoretical, 
  diagrammatio 
  cross 
  section 
  through 
  a 
  thoracic 
  

   segment 
  of 
  a 
  hypothetical 
  insect 
  with 
  paranotal, 
  or 
  glider, 
  lobes 
  in 
  

   place 
  of 
  wings. 
  DMcl, 
  Dorsal 
  longitudinal, 
  intersegmental 
  muscles; 
  

   L, 
  base 
  of 
  leg; 
  Pi, 
  pleuron; 
  pnl, 
  paranotal 
  lobe; 
  S, 
  sternum 
  

  

  