586 
cent,  coini.ion  salt.  IJotii  tlie  pools  to  the  North  and  South  ot  the 
town  apiicrtr  lo  be  feci  b}'  spring-s. 
Cdlia  fharocusis  was  not  found  at  Heluan. 
In  previous  years,  cases  of  malaria  arc  said  to  have  occurred  in 
lleluan. 
During  the  collecting  work  carried  out  m  igoS  and  igog,a  nuuibcr 
of  natural  enemies  of  larvae  were  observed.  The  most  important  of 
these  arc  acpiatic  bugs  (Hemiptera),  commonly  known  as  ‘  back- 
swimmers  ’  (so  named  from  their  habit  of  swimming  on  their  backs), 
belonging  to  the  genus  Noionecla.  This  genus  has  a  wide 
geographical  distribution,  and  its  members  are  recorded  by  observers 
in  other  parts  of  the  world  as  natural  checks  on  the  incieasc  of 
Culicidae.  In  igo8  a  large  area  of  the  Ghezireh  grounds  of  the 
Khedc\ial  Agricultural  Society,  flooded  by  infiltration  watei,  was 
largely  freed  from  larvae  of  Culcx  spp.  and  CcllicL  pJuiT  ocnsis  through 
the  agency  of  these  ac^uatic  bugs,  and  the  same  has  been  observed  in 
other  localities. 
These  bugs  appear  to  increase  rapidly  and  to  become  cjuickly 
distributed  in  a  district.  The  adults  leave  the  water  with  ease  and 
take  flight,  but  if  in  the  coiu'se  of  their  flight  they  hit  an  obstacle  and 
fall,  they  appear  to  have  very  great  difficulty  in  again  taking  wing  or 
even  to  be  unable  to  do  so.  They  leave  the  w^ater  with  the  dorsal 
surface  upwards  making  a  slight  wTirring  noise. 
Another  but  rather  rare  member  of  the  same  family  of  water-bugs 
{No/oneclidae)  has  also  been  found  to  prey  upon  mosquito  larvae,  and 
although  these  insects  measure  but  2  mm.  in  length  they  will  attack 
and  destroy  almost  full-grown  mosquito  laiw^ae. 
Other  natural  enemies  noted  w-ere  \vater-boatmen  (Corixidae), 
aquatic  beetles  and  their  larvae,  and  the  larvae  and  nymphs  of 
dragon-flies  {Libel lulidae). 
The  ‘  back-swhmmers  ’  of  the  genus  Noionecla  must  undoubtedly  be 
jilaced  at  the  head  of  the  list.  It  is  unfortunate  that  these  useful 
in.sects  do  not  as  a  rule  inhabit  quite  shallow'  w'aters,  and  also  that 
their  mosquito  destroying  operations  are  hindered  to  some  extent,  b}' 
growths  of  grass  w'ater-plants  and  algae,  w'hich  protect  their  pre\'. 
With  regard  to  the  enemies  of  adult  mosquitoes,  bats  appear 
to  catch  them  at  sunset,  dragon-flies  probably  destroy  some  also,  but 
m  an)'  case  it  is  probable  that  the  greater  proportion  of  mosquitoes 
destroyed  in  this  way  are  males. 
