Apldihi’. 
■277 
broods  consisting  entirely  of  female  insects,  some  wingless  and 
some  winged  ; but  when  autumn  comes  round  again  a certain 
number  of  the  young  plant  lice  develop  into  males,  and  the 
life-cycle  begins  anew.  Such  is  the  ordinarj*  type  of  life- 
history,  but  it  is  often  much  more  complicated,  as  in  the  case 
of  the  Phylloxera  of  the  grape  vine,  which  attacks  the  roots 
during  one  part  of  the  year,  while  later  it  produces  galls  on  the 
leaves,  the  rootr-feeding  and  leaf-feeding  forms  being  entirely 
different  in  appearance. 
If  the  under  surface  of  an  infested  cal)bage  leaf  be  examined 
in  the  summer,  several  different  kinds  of  plant  lice  will  seem 
to  be  huddled  together.  In  reality  there  will  only  be  winged 
and  wingless  females  and  their  young,  but  these  latter  differ 
from  the  adult  forms  in  colour  and  details  of  structure,  and 
moreover,  at  each  successive  moult  their  appearance  is  some- 
what changed. 
Nearly  all  plant  lice  are  furnished  with  a pair  of  tubes  or 
“ coniicles  ” at  the  end  of  their  body  from  which  the  fluid 
called  honey-dew  exudes.  This  substance,  which  is  sometimes 
so  abundant  as  absolutely  to  drip  from  a badly  infested  plant, 
greatly  adds  to  the  injury  caused  by  sucking  the  sajj,  for  it 
entirely  chokes  up  the  minute  pores  on  the  leaves  which  are 
unable  to  perform  their  functions.  It  is  this  honey-dew  which 
is  so  attractive  to  ants,  some  species  of  which  keep  aphids  in 
their  nests  as  men  keep  milch  cattle.  Those  plant  lice  which 
do  not  possess  cornicles  nevertheless  exude  mealy  or  waxy 
excretions  from  the  surface  of  the  body.  These  various  ex- 
creted substances  afford  protection  to  the  insects,  and  account 
has  to  be  taken  of  them  in  selecting  an  insecticide,  for  merely 
watery  fluids  are  readily  thrown  off,  and  soft  soap  or  some 
similar  material  must  be  added  to  an  aphis  wash  to  make  it 
adhere  to  the  leaves  till  it  has  done  its  work. 
The  rate  of  reproduction  varies  in  different  species  and 
under  diffei’ent  weather  conditions,  but  it  is  easy  to'  see  that 
the  peculiar  life-history  of  these  creatures  renders  an  enormous 
increase  of  plant  lice  possible.  It  is  no  unusual  state  of  things 
for  a wingless  female  to  produce  twenty  young  daily,  and  for 
these  to  attain  maturity  in  five  days  and  reproduce  in  their 
turn.  The  rose  aphis  increases  in  something  like  this  fashion, 
and  Huxley  calculated  that  taking  the  weight  of  a single  aphis 
as  the  millionth  of  a grain,  the  total  weight  of  the  tenth  brood 
alone,  if  all  survived,  would  exceed  the  weight  of  the  population 
of  China.  * Obviously  it  is  possible  for  vast  numbers  to  be 
destroyed  by  rain  and  by  insect  foes  as  they  invariably  are, 
and  yet  for  the  i-emnant  to  be  quite  sufficient  to  do  very  serious 
harm  to  the  crop  they  attack  ; and,  equally  obviously,  there  is 
no  pest  among  all  the  various  creatures  which  prey  upon  our 
