﻿DISTRIBUTION 
  OF 
  THE 
  AIR-TUBES. 
  193 
  

  

  placed. 
  The 
  nuclei 
  can 
  be 
  seen 
  in 
  Anabrus 
  in 
  tracheae 
  that 
  have 
  been 
  

   mounted 
  in 
  balsam, 
  without 
  being 
  stained, 
  for 
  the 
  flat 
  cells 
  are 
  sur- 
  

   charged 
  with 
  unusually 
  large, 
  highly 
  refringent, 
  reddish-brown 
  pigment 
  

   granules 
  of 
  nearly 
  uniform 
  size 
  (Fig. 
  Gl), 
  hence 
  the 
  nuclei 
  appear 
  as 
  clear 
  

   ovals 
  in 
  the 
  midst 
  of 
  the 
  dark 
  bodies 
  of 
  the 
  cells. 
  

  

  The 
  spiral 
  threads 
  are 
  unusually 
  delicate 
  in 
  Anabrus 
  (Fig. 
  57), 
  and 
  

   lie 
  quite 
  close 
  together. 
  

  

  In 
  the 
  finer 
  branches 
  of 
  the 
  tracheal 
  system 
  the 
  structure 
  is 
  slightly 
  

   modified 
  (Fig. 
  8). 
  In 
  the 
  first 
  place, 
  the 
  nuclei 
  are 
  farther 
  apart, 
  

   showing 
  that 
  the 
  cells 
  are 
  much 
  larger 
  than 
  in 
  the 
  main 
  tubes, 
  and 
  the 
  

   nuclei 
  appear 
  much 
  elongated, 
  though 
  their 
  volume 
  does 
  not 
  seem 
  much, 
  

   if 
  at 
  all, 
  changed. 
  The 
  fine 
  branches 
  divide 
  mainly 
  by 
  forking. 
  In 
  

   the 
  crotch 
  of 
  the 
  fork 
  there 
  often 
  sits 
  a 
  triangular 
  nucleus 
  of 
  entirely 
  

   different 
  shape 
  from 
  those 
  on 
  the 
  other 
  parts 
  of 
  the 
  respiratory 
  appa- 
  

   ratus 
  (Fig 
  In'). 
  

  

  The 
  peculiar 
  elongated 
  shape 
  of 
  the 
  nuclei 
  on 
  the 
  finer 
  tracheae 
  ren- 
  

   ders 
  it 
  possible 
  to 
  follow 
  the 
  course 
  of 
  the 
  delicate 
  air 
  tubes 
  (in 
  stained 
  

   preparations) 
  through 
  the 
  other 
  tissues 
  with 
  considerable 
  ease. 
  Never- 
  

   theless 
  it 
  is 
  advantageous 
  in 
  studying 
  the 
  distribution 
  of 
  the 
  tracheae 
  in 
  

   the 
  various 
  organs 
  to 
  examine 
  them 
  immediately 
  after 
  the 
  insect 
  is 
  killed, 
  

   because 
  they 
  are 
  then 
  injected 
  with 
  air, 
  so 
  that 
  under 
  the 
  microscope 
  the 
  

   large 
  tubes 
  appear 
  silvery 
  and 
  the 
  fine 
  branchlets 
  as 
  dark 
  lines 
  in 
  the 
  

   fresh 
  tissue. 
  It 
  will 
  then 
  be 
  found 
  that 
  their 
  distribution 
  is 
  almost 
  as 
  

   characteristic 
  of 
  the 
  single 
  organs 
  as 
  is 
  the 
  course 
  of 
  the 
  blood-vessels 
  

   in 
  vertebrates. 
  Williams 
  247 
  has 
  reported 
  some 
  observations 
  on 
  this 
  sub- 
  

   ject, 
  but 
  his 
  statements 
  are 
  generally 
  received 
  with 
  some 
  questioning. 
  

  

  Sir 
  John 
  Lubbock 
  248 
  has 
  published 
  a 
  valuable 
  and 
  extensive 
  memoir 
  

   on 
  the 
  distribution 
  of 
  the 
  tracheae, 
  containing 
  the 
  results 
  of 
  observations 
  

   on 
  a 
  very 
  large 
  number 
  of 
  insects 
  of 
  all 
  orders. 
  As 
  far 
  as 
  I 
  have 
  been 
  

   able 
  to 
  express 
  my 
  own 
  results 
  in 
  general 
  terms, 
  I 
  believe 
  they 
  are 
  

   confirmed 
  by 
  the 
  facts 
  recorded 
  by 
  Lubbock. 
  I 
  find 
  that 
  the 
  distribu- 
  

   tion 
  of 
  the 
  tracheae 
  depends, 
  first, 
  upon 
  the 
  shape 
  of 
  the 
  organs, 
  and, 
  

   secondly, 
  upon 
  the 
  size 
  of 
  those 
  whose 
  size 
  is 
  variable 
  j 
  whereby 
  it 
  must 
  

   be 
  remembered 
  that 
  the 
  trachea 
  7 
  , 
  as 
  far 
  as 
  at 
  present 
  known, 
  are 
  exclusively 
  

   confined 
  to 
  the 
  connective 
  tissue, 
  including, 
  of 
  course, 
  the 
  fat 
  body. 
  No 
  

   epithelium 
  is 
  ever 
  penetrated 
  by 
  the 
  air 
  tubes 
  in 
  any 
  instances 
  known 
  

   to 
  me, 
  through 
  either 
  my 
  own 
  observations 
  or 
  the 
  writings 
  of 
  others. 
  

   I 
  give 
  descriptions 
  of 
  the 
  distribution 
  of 
  the 
  tracheae 
  in 
  certain 
  organs 
  

   of 
  Caloptenus 
  and 
  (Edipoda. 
  Around 
  the 
  large 
  organs 
  (intestine, 
  sexual 
  

   organs), 
  with 
  interior 
  cavities, 
  the 
  tracheae 
  ramify 
  in 
  all 
  directions, 
  as 
  

   on 
  the 
  ovary, 
  for 
  instance, 
  Fig. 
  13, 
  forking 
  so 
  that 
  the 
  branches 
  diverge 
  

   at 
  a 
  wide 
  angle. 
  In 
  the 
  organs 
  which 
  have 
  muscular 
  walls, 
  like 
  the 
  

   oviduct 
  (Fig. 
  14), 
  for 
  example, 
  the 
  tracheae 
  run 
  straight 
  when 
  the 
  walls 
  

   are 
  distended, 
  but 
  have 
  a 
  sinuous 
  course, 
  as 
  in 
  the 
  figure 
  (14), 
  when 
  

  

  247 
  Williams: 
  Ann. 
  Mag. 
  Xat. 
  Hist. 
  (1854), 
  vol. 
  xiii, 
  p. 
  194. 
  

  

  u 
  * 
  Lubbock: 
  Distribution 
  of 
  the 
  trachea; 
  of 
  insects. 
  Phil. 
  Trans. 
  18G0, 
  vol. 
  xsiii, 
  p. 
  194. 
  

   13 
  L 
  

  

  