130 
There  was  not  a  single  infected  specimen 
among  the  thousands  of  rats,  Mus  decuma- 
nus  or  albiventris,  examined  while  1  was 
director  of  the  bactériologie  Institute  of  S. 
Paulo,  though  there  were  two  infected  speci¬ 
mens  of  Holochilus  vulpinus,  among  the  very 
few  rats  belonging  to  other  species. 
Whatever  the  mode  of  infection,  it  seems 
evident  that  the  larvae  can  only  attack  a 
few,  selected  species. 
The  parasitism  is  well  supported  though 
the  larvae  may  attain  the  size  of  the  hosts 
head;  this  may  be  due  to  the  fact  that 
the  parasite  is  not  free  in  the  subcutaneous 
tissue,  but  emprisoned  in  a  sac,  probably 
formed  by  a  sebacious  gland  or  another  dila¬ 
ted  follicle  of  the  derma.  As  can  be  seen  in 
Dermatobia,  the  larvae  have  prickles  or  rough 
scales  and  provoke  a  sero-purulent  secretion 
on  which  they  live.  When  the  larva  is  remo¬ 
ved,  the  secretion  stops  at  once,  long  before 
the  wound  is  shut.  Cellulitis  is  sometimes 
observed  on  the  scalp  of  children,  but  it  is 
due  to  abnormal  conditions  and  does  not 
occur  in  loose  skinned  animals,  which  do  not 
try  to  get  rid  of  the  parasite  by  violent 
means.  1  do  not  think  it  probable  that  the 
Cuterebra  larvae  living  in  the  scrota  of  North- 
American  squirrels  provoke  castration.  If  the 
testicules  have  not  simply  been  displaced  to 
the  abdomen,  it  is  more  likely  that  they 
were  removed  by  another  male  of  the  same 
species,  as  is  often  the  case  in  domestic  rab¬ 
bits.  The  development  of  the  cutaneous  larvae 
is  slow  and,  if  their  parasitism  were  not 
well  supported,  the  continuity  of  the  species 
would  be  endangered. 
The  material  for  a  study  on  the  larvae  is 
scant  and  generally  badly  preserved,  except 
that  of  the  already  rather  well  known  Der¬ 
matobia,  so  I  will  not  enter  into  a  discus¬ 
sion  of  their  characteristics. 
It  is  impossible,  as  yet,  to  be  even  sure 
of  the  genus  to  which  larvae  belong.  Cutere- 
bra,  Bogeria  and  Rogenhofera  are  all  cuta¬ 
neous  parasites  of  Rodents.  For  Pseudoga¬ 
metes  it  has  never  been  proved  and  I  know 
of  no  host  of  cutaneous  larvae  in  Petropolis, 
where  my  specimens  v/ere  caught.  If  there 
[  were  one,  it  could  hardly  escape  detection, 
on  account  of  the  relative  frequency  of  the  spe¬ 
cies  and  the  large  size  the  larvae  must  attain, 
especially  as  the  region  of  Petropolis  is  well 
known. 
Sex  differences  in  American  Oestrinae. 
Biological  notes. 
It  is  very  difficult  to  determine  the  sex 
of  dry  specimens  of  our  Oestrinae.  The  size 
and  distance  betiveen  the  eyes  give  no  reliable 
clue,  specially  whees  only  one  sex  is  present. 
Some  have  very  small  antennae,  but  this 
does  not  seem  to  be  a  sex  character;  it  is 
rather  due  to  imperfect  distension.  (Like  the 
wings,  the  antennae  only  develop  after  the 
fly  has  left  the  puparium  and  are  small  and 
shrivelled  at  the  time  of  the  eedysis).  The 
females  of  C.  apicalis  have  a  dimple  in  the 
terminal  article  of  the  antennae;  this  is  not 
found  in  the  male  but  may  also  be  wanting 
in  other  species.  In  Pseudogametes  semiater, 
the  female  has  a  bigger  abdomen  and  is 
althogether  larger  than  the  male,  but  the  dif¬ 
ference  is  not  always  very  accentuated  in 
this  and  in  other  species. 
The  ventral  sclerites  of  our  Oestrinae 
are  quite  reduced;  the  dorsal  ones  pass  on 
the  under  side,  so  that  the  lateral  membranes 
become  ventral.  In  dried  specimens  they  are 
retracted,  and  with  them  also  the  posterior 
extremity  of  the  abdomen,  so  that  the  geni¬ 
tal  appendices  are  hidden.  In  recently  trans¬ 
formed  specimens  the  abdomen  is  distended 
by  liquid  so  that  after  pressure,  or  even  dire- 
tly,  the  segments,  which  later  on  will  be 
retracted,  may  show  plainly. 
It  is  then  easy  to  distinguish  the  fe¬ 
male  Dermatobia,  by  its  ovipositor,  from  the 
male  which  has  a  distinct  and  rather  com¬ 
plicated  sexual  structure  of  dark  chitin.  The 
'female  also  protrudes  her  ovipositor,  when 
she  hovers  around  animals. 
In  order  to  determine  the  sex  of  dried 
specimens,  the  last  ventral  segment  may  be 
removed,  or  the  contents  of  the  abdomen 
examined  ;  in  the  female  they  consist  almost 
entirely  of  eggs.  By  the  use  of  these  methods, 
