﻿Mechanism 
  of 
  the 
  Male 
  Genital 
  Tube 
  in 
  Coleoptera. 
  407 
  

  

  It 
  is 
  among 
  the 
  trilobe 
  forms 
  of 
  aedeagiis 
  that 
  the 
  sac 
  is 
  

   least 
  developed 
  ; 
  in 
  many 
  cases 
  it 
  cannot 
  be 
  distinguished 
  

   from 
  the 
  ejaculatory 
  duct, 
  and 
  in 
  others 
  it 
  is 
  only 
  differenti- 
  

   ated 
  by 
  a 
  slight 
  increase 
  in 
  size. 
  In 
  these 
  cases 
  it 
  lies 
  

   entirely 
  within 
  the 
  median 
  lobe. 
  With 
  the 
  enlargement 
  

   of 
  the 
  sac 
  modifications 
  have 
  taken 
  place 
  in 
  the 
  median 
  

   lobe 
  and 
  tegmen. 
  These 
  modifications 
  have 
  been 
  mainly 
  

   along 
  three 
  lines. 
  The 
  development 
  of 
  the 
  median 
  lobe 
  

   into 
  a 
  bulb 
  with 
  the 
  reduction 
  of 
  the 
  tegmen 
  (the 
  Staphylinid 
  

   type) 
  : 
  the 
  development 
  of 
  the 
  median 
  lobe 
  into 
  a 
  long 
  tube 
  

   (the 
  Phytophagoidea 
  type) 
  : 
  and 
  the 
  development 
  of 
  the 
  

   tegmen 
  into 
  a 
  more 
  or 
  less 
  tubular 
  organ 
  (the 
  Melo 
  'outbid 
  

  

  type). 
  

  

  Something 
  like 
  the 
  beginning 
  of 
  the 
  Staphylinid 
  type 
  can 
  

   be 
  seen 
  among 
  the 
  trilobe 
  forms 
  in 
  Syndesus 
  corniitKs* 
  but 
  

   it 
  is 
  among 
  the 
  Staphylinidae 
  that 
  this 
  type 
  reaches 
  its 
  

   most 
  complete 
  development. 
  In 
  that 
  family 
  such 
  forms 
  

   as 
  Gyrojjhaena 
  'pidcheUa 
  and 
  Zirophorus 
  bicornis 
  have 
  the 
  

   median 
  lobe 
  cylindrical 
  and 
  the 
  internal 
  sac 
  small, 
  but 
  

   in 
  the 
  latter 
  the 
  basal 
  portion 
  is 
  slightly 
  swollen 
  and 
  

   the 
  dorsal 
  surface 
  but 
  slightly 
  chitinised, 
  and 
  it 
  can 
  be 
  

   pressed 
  down 
  against 
  the 
  ventral 
  portion 
  ; 
  the 
  median 
  

   foramen 
  is 
  also 
  very 
  small. 
  The 
  line 
  of 
  development 
  is 
  

   for 
  the 
  apical 
  portion 
  of 
  the 
  median 
  lobe 
  to 
  shorten 
  and 
  

   the 
  basal 
  portion 
  to 
  become 
  enlarged 
  and 
  rounded, 
  the 
  

   ventral 
  and 
  dorsal 
  areas 
  to 
  become 
  chitinised, 
  especi- 
  

   ally 
  the 
  former, 
  with 
  a 
  band 
  of 
  membrane 
  separating 
  

   them. 
  The 
  dorsal 
  surface 
  can 
  be 
  brought 
  towards 
  the 
  

   ventral 
  surface 
  by 
  muscular 
  contraction. 
  Thus 
  a 
  complete 
  

   automatons 
  bulb 
  is 
  formed, 
  which 
  reaches 
  a 
  high 
  state 
  of 
  

   perfection 
  in 
  such 
  a 
  form 
  as 
  Xantholinns 
  glabratus. 
  In 
  

   many 
  Staphylinidae 
  if 
  the 
  aedeagus 
  be 
  dissected 
  out 
  of 
  a 
  

   freshly 
  killed 
  specimen 
  and 
  slight 
  pressure 
  be 
  placed 
  upon 
  

   the 
  bulb 
  the 
  sac 
  will 
  instantly 
  be 
  evaginated. 
  The 
  invagin- 
  

   ation 
  of 
  the 
  sac 
  is 
  done 
  by 
  the 
  contraction 
  of 
  muscles 
  be- 
  

   tween 
  certain 
  areas 
  on 
  the 
  sac 
  and 
  others 
  on 
  the 
  median 
  

   lobe. 
  But 
  there 
  are 
  innumerable 
  stages 
  between 
  these 
  forms 
  

   among 
  the 
  Staphyhnidae. 
  This 
  line 
  of 
  evolution 
  has 
  been 
  

   followed 
  in 
  other 
  groups 
  such 
  as 
  Pselaphidae 
  and 
  some 
  

   Malacoderms 
  (i. 
  e. 
  Telejjhorus 
  limbatus 
  and 
  Balanophcrus 
  

   mastersi). 
  

  

  * 
  For 
  figures 
  and 
  descriptions 
  of 
  the 
  forms 
  mentioned 
  the 
  reader 
  

   is 
  referred 
  to 
  Trans. 
  Ent. 
  Soc. 
  Lond., 
  1912, 
  pp. 
  477-642 
  and 
  Plates 
  

   XLII-LXXVIII. 
  

  

  