﻿DEVELOPMENT 
  OF 
  THE 
  BARK 
  BEETLES. 
  281 
  

  

  A 
  feature 
  we 
  liave 
  observed 
  in 
  these 
  eggs, 
  which 
  are 
  very 
  transpa- 
  

   rent, 
  is 
  the 
  protophismic 
  threads 
  (th) 
  which 
  connect 
  the 
  cells 
  of 
  the 
  

   amnion 
  with 
  one 
  or 
  two 
  of 
  those 
  of 
  the 
  serous 
  membrane. 
  These 
  are 
  

   also 
  seen 
  in 
  Fig. 
  2a. 
  They 
  appear 
  usually 
  to 
  be 
  forked 
  after 
  originating 
  

   from 
  the 
  amniotic 
  cells. 
  I, 
  have 
  never 
  seen 
  these 
  threads 
  before, 
  nor 
  

   have 
  they 
  been 
  noticed 
  by 
  embryologists, 
  so 
  far 
  as 
  I 
  am 
  aware. 
  

  

  Fig. 
  3 
  is 
  a 
  dorsal 
  view 
  of 
  the 
  embryo, 
  representing 
  the 
  procephalic 
  

   lobes, 
  or 
  two 
  sides 
  of 
  the 
  epicranium 
  or 
  head 
  j 
  the 
  clypeus 
  in 
  Fig. 
  2 
  {cl) 
  

   is 
  seen 
  to 
  be 
  a 
  lobe 
  projecting 
  over 
  the 
  antennae 
  (ant). 
  

  

  (Fig. 
  4, 
  drawn 
  by 
  Dr. 
  Gissler, 
  represents 
  a 
  farther 
  advanced 
  stage, 
  in 
  

   Hylurgops, 
  the 
  endof 
  the 
  abdomen 
  (ah) 
  having 
  retreated 
  from 
  the 
  back 
  

   of 
  the 
  head.) 
  

  

  Fig. 
  5 
  represents 
  an 
  embryo 
  Xyleborus, 
  observed 
  June 
  3d. 
  The 
  head 
  

   is 
  now 
  distinctly 
  differentiated 
  from 
  the 
  rest 
  of 
  the 
  body 
  ; 
  the 
  indica- 
  

   tions 
  of 
  the 
  head-segments 
  have 
  disappeared, 
  the 
  four 
  pairs 
  of 
  append- 
  

   ages 
  (1-4) 
  being 
  clearly 
  indicated, 
  as 
  well 
  as 
  the 
  clypeus 
  {cly). 
  In 
  this 
  

   embryo 
  the 
  indications 
  of 
  eleven 
  pairs 
  of 
  spiracles 
  were 
  observed 
  (sp^- 
  

   sp^^), 
  and 
  it 
  is 
  interesting 
  to 
  note 
  that 
  in 
  each 
  thoracic 
  segment 
  there 
  is 
  

   at 
  first 
  a 
  pair 
  of 
  spiracles, 
  though 
  the 
  prothoracic 
  pair 
  is 
  the 
  largest 
  and 
  

   best 
  developed. 
  The 
  spiracles 
  are 
  superficially 
  composed 
  of 
  a 
  circle 
  of 
  

   ectodermal 
  cells, 
  which 
  surround 
  a 
  depression 
  of 
  tubes, 
  resulting 
  from 
  

   a 
  local 
  inpushing 
  or 
  invagination 
  of 
  the 
  ectoblast. 
  Whether 
  the 
  double 
  

   series 
  of 
  granules 
  (/r), 
  which 
  appear 
  to 
  originate 
  from 
  the 
  stigmata, 
  are 
  

   the 
  germs 
  of 
  the 
  trachese 
  or 
  not, 
  we 
  were 
  unable 
  to 
  definitely 
  state, 
  but 
  

   I 
  am 
  inclined 
  to 
  think 
  that 
  they 
  are. 
  

  

  The 
  presence 
  at 
  first 
  of 
  three 
  pairs 
  of 
  thoracic 
  embryonic 
  stigmata 
  is 
  a 
  

   matter 
  of 
  great 
  interest, 
  because, 
  as 
  seen 
  in 
  Figs. 
  8 
  and 
  9, 
  the 
  late 
  embryo 
  

   or 
  freshly 
  hatched 
  larva 
  has 
  but 
  a 
  single 
  pair 
  of 
  stigmata, 
  i, 
  e., 
  those 
  sit- 
  

   uated 
  in 
  the 
  prothorax. 
  Now 
  it 
  is 
  difficult 
  to 
  explain 
  the 
  presence 
  of 
  

   these 
  temporary, 
  embryonic, 
  meso 
  thoracic 
  and 
  metathoracic 
  stigmata, 
  

   unless 
  the 
  ancestors 
  of 
  these 
  and 
  other 
  insects 
  had 
  them 
  in 
  each 
  segment 
  

   of 
  the 
  thorax. 
  In 
  this 
  connection 
  we 
  may 
  refer 
  the 
  reader 
  to 
  our 
  article 
  

   on 
  the 
  primitive 
  number 
  of 
  spiracles 
  in 
  winged 
  insects, 
  published 
  in 
  the 
  

   American 
  Naturalist^ 
  vol. 
  8, 
  Sept. 
  1874, 
  in 
  which 
  we 
  took 
  the 
  ground 
  

   that 
  the 
  normal, 
  primitive 
  number 
  of 
  pairs 
  of 
  spiracles 
  is 
  eleven, 
  and 
  

   that 
  probably 
  the 
  larvae 
  of 
  the 
  different 
  groux)S 
  of 
  winged 
  insects 
  had 
  

   originally 
  a 
  pair 
  on 
  each 
  thoracic 
  segment. 
  

  

  The 
  remaining 
  figures 
  were 
  drawn 
  by 
  Dr. 
  G. 
  F. 
  Gissler, 
  and 
  represent 
  

   the 
  later 
  stages 
  of 
  the 
  embryo 
  Hylurgops. 
  In 
  Fig. 
  7 
  the 
  mouth-parts 
  

   are 
  elaborated, 
  the 
  labial 
  and 
  maxillary 
  palpi 
  being 
  differentiated. 
  

  

  Fig. 
  8 
  represents 
  a 
  freshly 
  hatched 
  Hylurgops, 
  which 
  is 
  1.5 
  ™™ 
  in 
  length. 
  

   The 
  head 
  is 
  very 
  large, 
  while 
  the 
  spiracles 
  are 
  distinct, 
  and 
  the 
  stomach 
  

   (s^), 
  intestines 
  (i), 
  and 
  rectum 
  (r) 
  are 
  distinctly 
  visible. 
  It 
  will 
  be 
  seen 
  that 
  

   the 
  rudimentary 
  feet 
  of 
  the 
  early 
  embryo 
  have 
  disappeared. 
  When 
  the 
  

   larvae 
  hatch-, 
  as 
  soon 
  as 
  they 
  are 
  free 
  from 
  the 
  shell, 
  according 
  to 
  Dr. 
  

   Gissler's 
  observations, 
  they 
  attach 
  themselves 
  to 
  the 
  surface 
  of 
  the 
  bark 
  

  

  