﻿Histology 
  of 
  the 
  Scent-organs 
  in 
  the 
  Genus 
  Hydroptila. 
  423 
  

  

  (Plate 
  XXII, 
  figs. 
  1 
  and 
  2). 
  They 
  are 
  pyriform 
  with 
  long 
  

   dehoate 
  stalks, 
  and 
  tlie 
  outer 
  expanded 
  portion 
  is 
  clothed 
  

   with 
  filaments 
  apparently 
  similar 
  to 
  those 
  in 
  the 
  scent- 
  

   scales 
  of 
  certain 
  Pierine 
  butterflies. 
  Apart 
  from 
  their 
  

   shape 
  and 
  position 
  they 
  possess 
  a 
  feature 
  which 
  I 
  have 
  been 
  

   unable 
  to 
  find 
  in 
  any 
  other 
  species 
  examined. 
  Each 
  has 
  

   beneath 
  its 
  socket 
  a 
  large, 
  granular, 
  heavily 
  nucleated 
  cell, 
  

   distinguishable 
  by 
  its 
  size 
  and 
  structure 
  from 
  the 
  other 
  

   cells 
  of 
  the 
  layer 
  in 
  which 
  it 
  lies. 
  There 
  are 
  some 
  twelve 
  

   of 
  these 
  androconia 
  on 
  each 
  side 
  of 
  the 
  centre 
  line. 
  There 
  

   is 
  no 
  trace 
  of 
  hairs 
  or 
  brushes. 
  Structures 
  somewhat 
  

   resembling 
  the 
  " 
  brush 
  bags 
  " 
  in 
  other 
  species 
  are 
  present, 
  

   but 
  without 
  hairs. 
  These 
  are 
  the 
  four 
  eversible 
  tubercles 
  

   already 
  referred 
  to. 
  There 
  are 
  two 
  of 
  these 
  on 
  each 
  side, 
  

   and 
  when 
  fully 
  expanded 
  they 
  extend 
  to 
  a 
  considerable 
  

   length, 
  as 
  may 
  be 
  seen 
  on 
  reference 
  to 
  Mr. 
  Mosely's 
  photo- 
  

   graphs. 
  The 
  material 
  of 
  these 
  tubercles 
  is 
  an 
  extremely 
  

   delicate 
  chitin, 
  not 
  of 
  the 
  same 
  nature 
  throughout 
  its 
  

   entire 
  length, 
  since 
  the 
  proximal 
  portion 
  takes 
  a 
  different 
  

   stain 
  from 
  that 
  of 
  the 
  remainder. 
  

  

  A 
  section 
  through 
  the 
  partly 
  everted 
  tubercle 
  (fig. 
  3) 
  

   shows 
  a 
  few 
  scattered 
  cells 
  on 
  the 
  inner 
  side. 
  These 
  are 
  

   probably 
  hypodermal 
  cells 
  which 
  have 
  been 
  displaced 
  in 
  

   the 
  process 
  of 
  preparation 
  . 
  The 
  whole 
  tissue 
  of 
  the 
  tubercle 
  

   when 
  not 
  fully 
  extended 
  is 
  thrown 
  into 
  a 
  multiphcity 
  of 
  

   interlocking 
  folds. 
  When 
  retracted 
  it 
  is 
  inverted 
  and 
  not 
  

   merely 
  collaj)sed, 
  and 
  this 
  remark 
  apphes 
  to 
  those 
  eversible 
  

   tubercles 
  in 
  other 
  species, 
  which 
  are 
  fined 
  with 
  hairs 
  and 
  

   form 
  brush 
  bags. 
  A 
  similar 
  inversion 
  takes 
  place 
  in 
  the 
  

   brush 
  bags 
  of 
  the 
  Danaine 
  butterflies, 
  but 
  it 
  can 
  there 
  be 
  

   accounted 
  for 
  by 
  the 
  presence 
  of 
  a 
  muscle 
  attached 
  to 
  the 
  

   apex 
  of 
  the 
  bag, 
  and 
  acting 
  as 
  a 
  retractor 
  when 
  the 
  fluid 
  

   pressure 
  is 
  released. 
  It 
  is 
  a 
  remarkable 
  fact 
  that 
  no 
  such 
  

   muscle 
  can 
  be 
  found 
  in 
  any 
  of 
  the 
  species 
  of 
  Hydroptila. 
  

   This 
  is 
  not 
  due 
  to 
  any 
  fault 
  of 
  the 
  preparation. 
  Muscle 
  

   tissue 
  is 
  amongst 
  the 
  easiest 
  of 
  all 
  to 
  recognise 
  in 
  insect 
  

   sections, 
  and 
  in 
  my 
  preparations 
  other 
  tiny 
  muscles 
  inside 
  

   the 
  head 
  are 
  easily 
  observable. 
  

  

  So 
  far 
  I 
  am 
  unable 
  to 
  account 
  for 
  the 
  inversion 
  of 
  the 
  

   retracted 
  tubercle, 
  unless 
  it 
  is 
  to 
  be 
  explained 
  by 
  some 
  

   complicated 
  condition 
  of 
  a 
  variable 
  coefficient 
  of 
  elasticity 
  

   in 
  different 
  parts 
  of 
  the 
  membrane. 
  The 
  fact 
  of 
  its 
  differen- 
  

   tial 
  staining 
  already 
  alluded 
  to 
  does 
  not 
  help 
  us, 
  as 
  this 
  

   occurs 
  only 
  in 
  the 
  present 
  species. 
  

  

  