﻿210 
  R. 
  J. 
  TILLYARD. 
  

  

  sizes 
  of 
  different 
  individuals 
  during 
  the 
  same 
  ecdysis, 
  I 
  have 
  had 
  to 
  determine 
  the 
  

   number 
  of 
  instars 
  by 
  morphological 
  methods. 
  I 
  found 
  that 
  measurements 
  of 
  any 
  

   sort 
  were 
  not 
  to 
  be 
  relied 
  upon, 
  as 
  this 
  larva 
  is 
  so 
  soft-skinned 
  that 
  even 
  the 
  prothorax 
  

   and 
  the 
  back 
  part 
  of 
  the 
  head 
  swells 
  during 
  the 
  progress 
  of 
  each 
  ecdysis. 
  The 
  only 
  

   safe 
  method 
  seems 
  to 
  be 
  a 
  careful 
  study 
  of 
  the 
  chaetotaxy 
  and 
  of 
  the 
  form 
  of 
  the 
  

   spiracles. 
  By 
  making 
  careful 
  preparations 
  of 
  a 
  number 
  of 
  larvae 
  of 
  all 
  sizes 
  in 
  caustic 
  

   potash, 
  I 
  have 
  been 
  able 
  to 
  determine 
  that 
  there 
  are 
  certainly 
  five 
  instars. 
  This 
  

   is 
  remarkable, 
  since 
  all 
  other 
  Planipennia 
  have 
  only 
  three 
  or 
  four. 
  The 
  changes 
  

   in 
  the 
  general 
  shape 
  of 
  the 
  larva, 
  the 
  form 
  of 
  the 
  head, 
  prothorax 
  and 
  legs, 
  the 
  form 
  

   of 
  the 
  mouth-parts 
  and 
  antennae, 
  are 
  all 
  so 
  slight 
  as 
  to 
  be 
  practically 
  useless 
  in 
  this 
  

   connection. 
  As 
  regards 
  the 
  spiracles, 
  these 
  are 
  circular 
  in 
  the 
  newly-hatched 
  larva. 
  

   During 
  each 
  instar, 
  they 
  become 
  more 
  and 
  more 
  oval 
  ; 
  until, 
  in 
  the 
  last 
  larval 
  instar, 
  

   the 
  whole 
  series 
  is 
  of 
  a 
  very 
  elongate 
  oval 
  shape, 
  as 
  shown 
  in 
  Plate 
  iv, 
  fig. 
  7. 
  Cor- 
  

   related 
  with 
  the 
  changes 
  in 
  the 
  form 
  of 
  the 
  spiracles 
  is 
  a 
  much 
  more 
  definite 
  change 
  

   in 
  the 
  chaetotaxy. 
  The 
  newly 
  hatched 
  larva 
  possesses, 
  with 
  the 
  exception 
  of 
  the 
  

   specialised 
  end-segments 
  of 
  the 
  abdomen, 
  only 
  long 
  slender 
  primary 
  setae 
  (macrotrichia) 
  

   and 
  an 
  abundant 
  armature 
  of 
  minute 
  microtrichia 
  also. 
  At 
  the 
  second 
  instar, 
  a 
  

   number 
  of 
  secondary 
  setae 
  of 
  macrotrichial 
  type, 
  but 
  smaller 
  than 
  the 
  primary 
  setae, 
  

   occur 
  ; 
  these 
  increase 
  rapidly 
  in 
  number 
  with 
  each 
  instar, 
  until 
  they 
  completely 
  

   hide 
  the 
  original 
  primary 
  setae, 
  which 
  can 
  only 
  be 
  found 
  for 
  certain 
  in 
  one 
  or 
  two 
  

   special 
  areas 
  where 
  the 
  differentiation 
  of 
  the 
  two 
  types 
  of 
  setae 
  remains 
  marked. 
  

  

  For 
  the 
  determination 
  of 
  the 
  correct 
  instar, 
  I 
  found 
  the 
  metathorax 
  the 
  most 
  

   useful 
  segment. 
  It 
  has 
  the 
  double 
  advantage 
  of 
  having 
  a 
  chaetotaxy 
  closely 
  com- 
  

   parable 
  with 
  that 
  of 
  an 
  abdominal 
  segment, 
  and 
  secondary 
  setae 
  formed 
  on 
  a 
  short, 
  

   rather 
  stout 
  model, 
  quite 
  unlike 
  that 
  of 
  the 
  primary 
  ones. 
  Consequently 
  the 
  primary 
  

   setae 
  can 
  be 
  picked 
  out 
  through 
  all 
  instars, 
  and 
  the 
  invasion 
  of 
  the 
  secondary 
  setae 
  

   on 
  to 
  the 
  specialised 
  clear 
  areas, 
  or 
  pinacula, 
  which 
  originally 
  carried 
  certain 
  groups 
  

   of 
  primary 
  setae 
  only, 
  can 
  be 
  followed 
  up 
  with 
  precision. 
  On 
  the 
  abdominal 
  segments, 
  

   the 
  secondary 
  setae, 
  though 
  at 
  first 
  somewhat 
  smaller 
  than 
  the 
  primary, 
  are 
  of 
  the 
  

   same 
  slender 
  form, 
  and 
  by 
  the 
  third 
  instar 
  there 
  is 
  so 
  little 
  difference 
  in 
  size 
  that 
  

   the 
  task 
  of 
  picking 
  out 
  the 
  one 
  from 
  the 
  other 
  is 
  quite 
  hopeless. 
  

  

  A 
  point 
  of 
  great 
  interest 
  in 
  this 
  larva 
  is 
  the 
  fact 
  that 
  the 
  primary 
  setae 
  can 
  most 
  

   certainly 
  be 
  homologised 
  with 
  those 
  of 
  the 
  Lepidopterous 
  larva, 
  as 
  I 
  have 
  also 
  found 
  

   to 
  be 
  the 
  case 
  with 
  the 
  larva 
  of 
  the 
  Mecopteron 
  Chorista 
  (though 
  this 
  result 
  has 
  not 
  

   yet 
  been 
  published). 
  The 
  attention 
  of 
  Lepidopterists 
  is 
  drawn 
  to 
  this 
  fact, 
  which 
  

   emphasises, 
  by 
  a 
  further 
  striking 
  character, 
  the 
  close 
  affinity 
  that 
  I 
  have 
  already 
  

   insisted 
  upon 
  as 
  belonging 
  to 
  the 
  whole 
  of 
  the 
  Panorpoid 
  Orders. 
  It 
  also 
  throws 
  

   some 
  light 
  upon 
  the 
  vexed 
  question 
  of 
  the 
  seta 
  named 
  iheta 
  in 
  Fracker's 
  nomen- 
  

   clature, 
  which 
  only 
  occurs 
  in 
  the 
  second 
  instar 
  of 
  Hepialidae, 
  and 
  is 
  therefore 
  con- 
  

   sidered 
  as 
  sub-primary 
  by 
  that 
  author 
  (1915, 
  p. 
  34). 
  This 
  seta 
  is 
  absent 
  throughout 
  

   in 
  Ithone, 
  so 
  that 
  no 
  evidence 
  is 
  forthcoming 
  for 
  its 
  existence 
  in 
  the 
  Planipennia. 
  

   Its 
  late 
  arrival, 
  therefore, 
  in 
  the 
  Lepidoptera 
  is 
  probably 
  due 
  to 
  its 
  having 
  been 
  

   added 
  to 
  the 
  original 
  chaetotaxy 
  after 
  the 
  definite 
  evolution 
  of 
  the 
  Lepidopterous 
  

   larval 
  type. 
  

  

  A 
  short 
  account 
  will 
  now 
  be 
  given 
  of 
  each 
  larval 
  instar, 
  with 
  special 
  attention 
  

   to 
  the 
  changes 
  in 
  spiracles 
  and 
  chaetotaxy. 
  

  

  First 
  Larval 
  Instar 
  (Plate 
  iv, 
  fig. 
  2; 
  text-figs. 
  2-5). 
  

  

  The 
  newly 
  hatched 
  larva 
  is 
  actually 
  about 
  twice 
  as 
  long 
  as 
  the 
  egg 
  in 
  which 
  it 
  

   was 
  confined 
  ; 
  though, 
  owing 
  to 
  its 
  curvature, 
  it 
  does 
  not 
  appear 
  so 
  long 
  as 
  this. 
  

   The 
  head 
  and 
  prothorax 
  are 
  of 
  about 
  equal 
  size, 
  and 
  considerably 
  larger 
  than 
  the 
  

   other 
  segments. 
  The 
  general 
  form 
  is*melolonthoid 
  ; 
  the 
  larva 
  is 
  obviously 
  adapted 
  

   only 
  for 
  a 
  burrowing 
  existence, 
  being 
  very 
  awkward 
  and 
  ungainly 
  if 
  placed 
  on 
  a 
  flat 
  

   surface. 
  In 
  attempting 
  to 
  walk, 
  it 
  keeps 
  its 
  abdomen 
  arched, 
  but 
  presses 
  its 
  thorax 
  

   down 
  flat 
  ; 
  in 
  such 
  an 
  attitude, 
  it 
  can 
  only 
  make 
  slow 
  and 
  very 
  awkward 
  progression. 
  

  

  