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MR.  J.  O.  WESTWOOD  ON  NYCTERIBIA. 
the  almost  rudimental  state  of  the  prothorax  in  the  true  Dipterous  Insects,  it  seems  clear 
(especially  when  the  remaining  portions  of  the  thorax  are  examined)  that  the  space  in- 
closed by  this  fine  line  constitutes  all  that  remains  of  the  prothorax,  giving  insertion, 
however,  to  its  ordinary  attachments,  viz.  the  head  and  the  pair  of  anterior  legs. 
The  central  portion  of  the  dorsum  of  the  thorax  is  inclosed  by  a  narrow  crustaceous 
line,  and  is  occupied  by  an  oval  plate,  rather  dilated  towards  the  abdomen,  and  com- 
posed of  a  brownish  coriaceous  membrane,  slightly  depressed  in  the  middle  in  a  dried 
specimen,  and  offering  a  slight  transverse  elevation  in  the  centre.  Latreille  describes 
this  portion  as  forming  a  dorsal  channel,  and  having  its  posterior  extremity  terminated 
in  the  common  French  species  "  par  une  partie  elevee,  formant  le  capuchon,"  in  which 
the  head,  when  thrown  back,  is  received.  I  have  seen  nothing  of  this  capuchon,  and 
but  little  of  the  channel,  in  any  of  the  specimens  which  I  have  examined,  and  quite 
agree  with  M.  Dufour  in  regarding  them  merely  as  being  occasioned  by  the  desiccation 
of  the  insects  after  death :  the  ' '  groove  down  the  middle  [of  the  thorax]  to  receive 
the  head,"  described  by  Mr.  Curtis,  is  doubtless  attributable  to  the  same  cause. 
On  each  side  of  this  central  portion,  about  midway  on  each  side,  is  to  be  observed 
another  slender  crustaceous  bar,  directed  obhquely  towards  the  head,  and  extending  to 
the  sides  of  the  dorsum  of  the  thorax,  whence  it  is  prolonged  nearly  in  a  line  to  the  place 
of  insertion  of  the  fore  legs,  thus  inclosing  on  each  side  an  elongate  and  somewhat  tri- 
angular plate  of  a  whitish  colour,  the  anterior  margins  of  which  do  not  extend  to  the 
margin  of  the  thorax,  permitting  the  pectus  to  be  seen  from  above.  To  the  narrow 
posterior  extremity  of  this  lateral  portion  is  attached  the  base  of  the  intermediate  pair 
of  legs  ;  and  behind  these,  on  each  side,  a  pair  of  short  and  narrow  portions,  similarly 
separated,  are  to  be  observed,  to  the  exterior  of  which  the  basal  portion  of  the  posterior 
legs  is  attached.  Hence  it  appears  to  me  that  the  central  and  anterior  lateral  triangular 
plates  represent  the  dorsum  of  the  mesothorax,  and  the  small  posterior  lateral  ones  that 
of  the  metathorax. 
The  disposition  of  these  portions  of  the  thorax  is,  however,  very  different  on  the  ven- 
tral surface.  This  is  quite  flat,  and  of  a  uniform  crustaceous  texture,  of  a  somewhat  oval 
form,  without  the  least  indication  of  the  insertion  of  the  legs,  and  having  a  central  lon- 
gitudinal line  running  from  the  anterior  to  the  posterior  extremity.  M.  Dufour  de- 
scribes it  as  "  un  plastron  d'une  seule  piece but  I  have  uniformly  found  an  impressed 
line  of  division  extending  from  the  posterior  base  of  the  intermediate  legs,  and  running 
parallel  with  the  anterior  margin,  thus  exhibiting  the  pectus  of  the  mesothorax  in  the 
form  of  a  lunate  plate,  and  that  of  the  metathorax  as  much  more  extensive. 
But  the  most  remarkable  organ  connected  with  the  thorax  is  a  pair  of  pectinated 
processes  placed  between  the  base  of  the  anterior  and  intermediate  legs,  and  received 
in  a  cavity  (formed  by  the  lateral  productions  of  the  dorsum  and  pectus  of  the  anterior 
parts  of  the  thorax),  one  on  each  side  of  the  thorax.    On  detaching  one  of  the  inter- 
