288 
On  several  occasions  we  thought  that  they  showed  a  more  decided  preference 
for  khaki  than  for  the  dark  skin  of  the  native,  but  this  may  have  been  due  to  the 
fact  that  we  walked  ahead  of  the  carriers  and  so  attracted  the  fly  first. 
More  or  less  seasonal  variation  is  observed  in  the  case  of  this  fly, 
and  in  some  localities  apparently  depends  upon  the  winds,  for  during 
the  South-west  monsoon  they  cannot  be  found  on  many  elevated 
spots  where  they  exist  at  other  times  of  the  year. 
When  we  reached  the  top  of  the  plateau  at  the  Lovu  river,  at  the  end  of  May 
igcS,  Cl.  niorsilans  was  present,  but  during  August  we  were  unable  to  get 
specimens,  although  there  were  plenty  in  the  valley  below.  In  October,  when  the 
winds  had  ceased,  they  could  again  be  found  at  the  edge  of  the  plateau. 
Some  of  the  officials  who  have  been  many  years  in  the  country, 
deny  that  there  is  any  great  difference  in  the  numbers  of  the  fly  in  the 
wet  and  dry  seasons.  We  were  never  long  enough  at  any  one  place 
to  make  any  observations  on  this  point.  One  official  said  that  he  had 
noticed  that  the  fly  was  absent  in  the  part  of  his  district  where  the 
soil  was  clayey,  and  that  they  could  be  found  in  surrounding  areas 
where  the  soil  was  sandy.  If  this  observation  is  correct,  it  might  be 
explained  by  the  larvae  being  unable  to  burrow  into  the  stiffer  soil  in 
order  to  pupate.  Another  said  that  they  could  always  be  found  in 
‘  mopani  ’  bush,  i.e.,  forest  composed  of  these  trees,  which  are  rather 
far  apart  and  grow  with  straight  trunks.  The  ground  beneath  them 
is  covered  with  a  scanty  carpet  of  grass,  devoid  of  undergrowth,  as 
the  soil  is  sandy  and  not  very  fertile. 
Gl.  morsitans  requires  much  less  shade  than  Gl.  palpalis,  and  is 
not  commonly  found  in  such  close  connection  with  water  as  this 
species  ;  indeed,  they  may  be  taken  miles  away  from  it. 
Near  Chongola’s  village,  on  the  Luapula  river,  the  country  is  one  series  o! 
immense,  bare,  and  absolutely  level  fiats  separated  by  very  small  patches  of  bush. 
Gl.  mnrsiians  abounds  in  this  bush,  and  also  in  the  small,  bush-clad  ant-bills  which 
dot  the  plains.  They  fly  from  these  ant-hills  to  attack  the  passing  caravans,  but 
are  never  seen  in  the  absolutely  bare  flats,  unless  they  have  been  carried  there  on 
the  natives.  We  travelled  through  this  country,  in  the  dry  season,  for  seven  hours 
on  one  occasion,  before  reaching  a  water  hole. 
Ensor*  has  noted  that  the  two  species  have  selective  spheres  and 
that  they  do  not  encroach  on  one  another’s  territory,  and  while  our 
observations  agree,  on  the  whole,  with  this,  we  have  caught  the  two 
flies  together  under  circumstances  which  would  preclude  the 
possibility  of  Gl.  morsitans  having  followed  us  to  the  water. 
*  I'.nsor,  1908.  Wellcome  Lab.  Reports,  No.  3. 
