27^ 
Annual  ]\epnrt  for  1904  of  the  Zooloijist. 
crops  which  it  is  so  important  to  stamp  out  on  its  first  appear- 
ance. Its  rate  of  inci’ease  at  the  beginning  is  comparatively 
slow,  but  it  soon  attains  a speed  which  is  absolutely  prodigious ; 
so  that  the  pest  appears  to  have  arrived  suddenly  on  a crop 
where  its  first  occurrence  did  not  attract  attention.  The  attack 
may,  of  course,  be  sudden  in  reality  if  swarms  of  the  winged 
females  alight  on  a crop  which  was  previously  free  from  the 
pest.  This  often  occurs  in  the  still,  heavy  atmosphere  which 
precedes  a thunderstorm  in  the  middle  of  summer,  and  the 
air  is  sometimes  full  of  the  winged  insects,  which  do  not  fly 
actively  but  drift  along  on  the  slight  currents,  most  of  them 
doomed  to  perish  when  the  storm  breaks,  while  the  lucky 
ones  which  have  succeeded  in  finding  their  appropriate  plants 
attain  a new  feeding  ground  and  spread  the  attack.  This  is, 
no  doubt,  what  is  meant  when  a certain  atmospheric  condition 
is  si)oken  of  as  a “ blight.”  Cases  are  sometimes  reportetl 
where  rain  does  not  seem  to  have  been  i)articularly  beneficial 
to  diseased  crops,  but  these  are  certainly  exceptional.  The 
delicate  insects,  whether  in  the  air  or  on  the  plant,  are  battered 
down  by  myriads  and  destroyed  by  the  heavy  rain,  while  the 
plants  are  at  the  same  time  invigorated,  and  the  sudden  arrival 
of  Avet  weather  often  entirely  cures  a badly  blighted  crop.  On 
the  other  hand  the  insects  are  almost  alwaj'S  on  the  under  side 
of  the  leaves  and  protected  by  the  Avrinkling  they  have  caused 
as  well  as  by  the  various  materials  they  exude,  so  that  the 
disease  may  recur  as  actively  as  ever  after  the  rains. 
Again,  if  the  weather  is  merely  warm  and  damp  many 
sjDecies  of  aphis  flourish,  but  none  of  them  can  fail  to  suffer 
from  downright  heavy  rain. 
Among  the  influences  which  prevent  plant  lice  from  eA^er 
Fig.  1.— Seven-spot  ladybird  and  its  clusiA^ely  Oil  these  pests,  and  three 
larva,  enlarged  (Natural  size  indicated).  them  are  SO  COllimon  and  SO 
effective  in  destroying  green  fly  that  they  deserve  to  be  well 
knoAvn  and  recognised  as  friends  by  every  farmer.  They  are 
the  ladybird,  the  lace-wing  fly,  and  the  hover  fly.  Most  people, 
no  doubt,  are  Avell  acquainted  Avith  the  ladybirti,  but  astonish- 
ingly feAV  recognise  its  grub,  Avhich,  after  all,  is  the  chief 
agent  in  destroying  green  fly,  and  it  is  very  often  mistakenly 
complained  of  as  causing  injury,  being  a great  deal  more 
conspicuous  than  the  real  culprit.  It  is  a slaty-grey  grub 
increasing  to  more  than  a very 
small  fraction  of  the  numbers 
Avhich  they  would  be  capable  of 
reaching  under  favourable  cir- 
cumstances, rain  is  certainly  one 
of  the  chief;  but  fortunately  there 
are  many  insects  Avhich  prey  ex- 
