﻿388 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  evolved 
  in 
  their 
  earlier 
  stages 
  when 
  they 
  were 
  too 
  small 
  to 
  serve 
  any 
  

   specific 
  function; 
  and 
  if 
  we 
  can 
  not 
  answer 
  this 
  question 
  it 
  is 
  taken 
  to 
  

   invalidate 
  the 
  assumption 
  imphed 
  by 
  the 
  theory. 
  Certainly, 
  para- 
  

   notal 
  lobes 
  on 
  the 
  thorax 
  could 
  not 
  have 
  had 
  any 
  locomotory 
  function 
  

   until 
  they 
  were 
  large 
  enough 
  to 
  serve 
  as 
  gliders. 
  And 
  yet, 
  many 
  

   modern 
  insects 
  have 
  lateral 
  extensions 
  of 
  the 
  back 
  plate 
  of 
  the 
  

   prothorax, 
  such 
  as 
  occur 
  frequently 
  in 
  the 
  beetles, 
  which 
  appear 
  to 
  

   be 
  of 
  no 
  particular 
  use 
  to 
  their 
  possessors, 
  and 
  in 
  some 
  species 
  of 
  

   mantids 
  the 
  back 
  plate 
  of 
  the 
  prothorax 
  is 
  expanded 
  into 
  a 
  large, 
  flat 
  

   shield. 
  (Fig. 
  5.) 
  Utility 
  probably 
  guides 
  the 
  course 
  of 
  an 
  organ's 
  

   evolution, 
  but 
  it 
  does 
  not 
  necessarily 
  determine 
  its 
  inception. 
  Speci- 
  

   fic 
  structures 
  may 
  develop 
  for 
  no 
  practical 
  reason 
  at 
  all. 
  Some 
  writers 
  

   have 
  supposed 
  that 
  the 
  paranotal 
  lobes 
  of 
  insects 
  served 
  first 
  as 
  gills 
  

   ~-x 
  ..-- 
  for 
  breathing 
  in 
  the 
  water 
  but 
  this 
  

  

  theory 
  must 
  assume 
  that 
  insects 
  had 
  

   an 
  aquatic 
  stage 
  in 
  their 
  phylogenj^, 
  of 
  

   which 
  there 
  is 
  no 
  evidence. 
  

  

  There 
  are 
  existing 
  to-day 
  certain 
  

   small 
  insects, 
  known 
  as 
  the 
  Apterygota 
  

   because 
  they 
  not 
  only 
  lack 
  wings 
  but 
  

   they 
  contain 
  no 
  evidence 
  in 
  their 
  body 
  

   structure 
  of 
  being 
  descended 
  from 
  

   winged 
  ancestors. 
  They 
  are 
  appar- 
  

   ently 
  direct 
  descendants 
  of 
  the 
  un- 
  

   known, 
  primitive, 
  wingless 
  progenitors 
  

   of 
  all 
  the 
  insects. 
  Indirectly 
  the 
  Ap- 
  

   terygota 
  furnish 
  evidence 
  of 
  the 
  evolu- 
  

   tion 
  of 
  wings 
  in 
  the 
  winged 
  insects, 
  or 
  

   Pterygota, 
  from 
  paranotal 
  lobes. 
  The 
  

   Pterygota 
  are 
  characterized 
  by 
  a 
  special 
  

   type 
  of 
  structure 
  in 
  the 
  plates 
  forming 
  

   the 
  lateral, 
  or 
  pleural, 
  walls 
  of 
  the 
  thoracic 
  segments, 
  the 
  pleural 
  struc- 
  

   ture 
  being 
  essentially 
  the 
  same 
  in 
  the 
  wingless 
  prothorax 
  as 
  in 
  the 
  two 
  

   wing-bearing 
  segments. 
  In 
  the 
  Apterygota 
  the 
  corresponding 
  plates 
  

   are 
  quite 
  different 
  from 
  those 
  of 
  the 
  winged 
  insects, 
  and 
  differ 
  much 
  in 
  

   the 
  several 
  apterygote 
  families. 
  We 
  must 
  conclude, 
  therefore, 
  that 
  

   the 
  peculiar 
  structure 
  of 
  the 
  pleural 
  walls 
  of 
  the 
  thoracic 
  segments 
  in 
  

   the 
  Pterygota, 
  and 
  their 
  basic 
  uniformity 
  in 
  the 
  three 
  segments 
  of 
  the 
  

   thorax 
  means 
  that 
  the 
  walls 
  of 
  these 
  segments 
  once 
  served 
  a 
  common 
  

   purpose. 
  This 
  purpose 
  evidently 
  was 
  the 
  support 
  of 
  a 
  pair 
  of 
  para- 
  

   notal 
  lobes 
  on 
  each 
  thoracic 
  segment. 
  (Fig. 
  6.) 
  The 
  lack 
  of 
  pleural 
  

   plates 
  in 
  the 
  lateral 
  walls 
  of 
  the 
  abdominal 
  segments 
  similar 
  to 
  those 
  

   of 
  the 
  thorax 
  suggests 
  that 
  paranotal 
  lobes 
  of 
  the 
  abdomen, 
  if 
  present, 
  

   never 
  reached 
  a 
  size 
  of 
  functional 
  importance. 
  

  

  FiGVRE 
  ii.—A 
  mantis 
  from 
  Ecuador 
  with 
  

   l^rge 
  lateral 
  extensions 
  of 
  the 
  back 
  plate 
  

   of 
  the 
  prothorax 
  

  

  