Honey-bee  drones  (males)  develop  from  unfertilized  eggs,
and  the  workers  (undeveloped  females)  and  "  queens "
(mature  females)  are  from  fertilized  eggs.  Thus  mating  a  black
drone  and  a  yellow  "  queen  "  will  result  in  half-blood  or
hybrid  workers  and  young  "  queens,''  but  yellow  drones.
Other  cases  of  parthenogenesis  are  found  among  insects  and


THE  ABTHBOPODS


387


in  some  low  crustaceans.  In  some  species  of  gall-insects  and
scale-bugs  males  have  not  been  found ;  and  permanent  par-
thenogenesis is  supposed  to  be  the  rule.  This  is  not  known  to
occur  in  any  other  group  of  animals ;  but  there  are  well-known
cases  of  flowers  whose  egg-cells  always  develop  without
fertilization.  Parthenogenesis  of  both  plants  and  animals  is
interesting  because  sexual  reproduction  is  usually  provided
for  in  all  types  of  organisms.
326.  Classification  of  Insects.  —  The  group  Insecta  is  a
clcLSS,  and  it  is  divided  into  about  twenty  orders.  Most  of
these,  however,  are  not  represented  by  many  common  insects,
and  so  will  be  omitted  from  the  following  account.
In  classifying  insects,  as  all  other  animals,  it  is  necessary
to  consider  all  points  of  external  and  in-
ternal stucture  in  order  to  determine  which
species  are  most  alike ;  but  it  is  a  fortunate
fact  that  it  is  often  possible  to  identify  by
means  of  some  one  structure.  In  the  case
of  many  insects,  the  wings  happen  to  be  the
convenient  parts  for  general  classification ;
but  similar  wings  should  not  be  taken  as
indicating  close  relationship  if  other  organs
are  not  homologous  (i.e.,  of  corresponding
structure).
The  most  common  insects  in  United
States  are  conveniently  grouped  in  the
following  orders:  —

Aptera.  —  Name  means  without  wings.
Example:  the  "silver-moth"  or  '*  silver-
•fish''  (Fig.  136)  and  the  "spring-tails."
These  are  the  oldest  insects  now  living.
