﻿The 
  British 
  Species 
  of 
  Andrena 
  and 
  Nomada. 
  235 
  

  

  ill 
  a 
  given 
  locality 
  an 
  Andrena 
  and 
  its 
  parasite 
  may 
  be 
  

   both 
  numerous, 
  and 
  the 
  next 
  year, 
  though 
  the 
  former 
  

   may 
  be 
  again 
  plentiful, 
  the 
  latter 
  may 
  be 
  very 
  scarce 
  or 
  

   even 
  not 
  found 
  at 
  all. 
  The 
  cause 
  of 
  this 
  is, 
  I 
  think, 
  

   entirely 
  due 
  to 
  the 
  difference 
  of 
  habits, 
  for 
  the 
  host 
  in 
  

   the 
  shelter 
  of 
  its 
  burrow 
  is 
  less 
  affected 
  by 
  unfavourable 
  

   weather 
  conditions 
  than 
  the 
  exposed 
  parasite. 
  A 
  day 
  

   or 
  two 
  with 
  continued 
  cold 
  rains 
  no 
  doubt 
  decimates 
  the 
  

   latter. 
  On 
  wet 
  days 
  we 
  have 
  often 
  found 
  M 
  electa, 
  Epeolus 
  

   and 
  Nomada 
  hanging 
  on 
  to 
  herbage 
  or 
  shrubs 
  in 
  the 
  manner 
  

   described, 
  soaked 
  with 
  the 
  rain 
  and 
  torpid 
  with 
  cold. 
  

   On 
  the 
  other 
  hand, 
  with 
  favourable 
  conditions 
  Nomada 
  is 
  

   capable 
  of 
  very 
  rapid 
  increase, 
  remarkably 
  so 
  in 
  the 
  case 
  

   of 
  some 
  species 
  that 
  attack 
  those 
  species 
  of 
  Andrena 
  

   that 
  form 
  large 
  colonies. 
  In 
  such 
  cases 
  the 
  parasite 
  can 
  

   enter 
  and 
  oviposit 
  in 
  a 
  number 
  of 
  different 
  cells 
  in 
  the 
  

   time 
  that 
  it 
  takes 
  to 
  store 
  a 
  single 
  one. 
  Consequently, 
  

   in 
  rare 
  instances 
  we 
  have 
  known 
  a 
  Nomada 
  even 
  to 
  out- 
  

   number 
  its 
  host 
  considerably, 
  though 
  such 
  a 
  success 
  is 
  

   not 
  often 
  likely 
  to 
  last 
  more 
  than 
  one 
  or 
  two 
  seasons. 
  

   This 
  phenomenon 
  is 
  not 
  at 
  all 
  confined 
  to 
  Nomada, 
  for 
  

   under 
  exceptional 
  circumstances 
  a 
  careful 
  examination 
  

   of 
  large 
  colonies 
  of 
  Anthophora 
  2nlipes 
  and 
  Collet 
  es 
  suc- 
  

   cincta 
  has 
  shown 
  an 
  enormous 
  preponderance 
  of 
  their 
  

   parasites, 
  M 
  electa 
  armata 
  and 
  Epeolus 
  cruciger 
  {rufipes), 
  in 
  

   some 
  seasons. 
  

  

  From 
  these 
  general 
  remarks 
  on 
  Nomada 
  and 
  other 
  para- 
  

   sitic 
  bees 
  it 
  will 
  be 
  seen 
  that 
  a 
  wide 
  field 
  is 
  here 
  presented 
  

   for 
  accurate 
  observations, 
  our 
  knowledge 
  on 
  many 
  points 
  

   being 
  of 
  the 
  most 
  slender 
  kind. 
  The 
  most 
  we 
  can 
  say 
  

   is 
  that 
  one 
  certain 
  host 
  at 
  least 
  is 
  now 
  known 
  for 
  every 
  

   species 
  of 
  Nomada 
  that 
  we 
  have 
  in 
  this 
  country, 
  excepting 
  

   only 
  N 
  . 
  flavopicta, 
  which 
  requires 
  further 
  investigation. 
  

   How 
  many 
  of 
  the 
  associations 
  recorded, 
  whether 
  by 
  British 
  

   or 
  Continental 
  authors, 
  are 
  correct, 
  when 
  they 
  do 
  not 
  agree 
  

   with 
  those 
  which 
  are 
  given 
  here 
  under 
  the 
  various 
  species 
  

   of 
  Andrena, 
  is 
  quite 
  uncertain, 
  but 
  it 
  may 
  be 
  said 
  with 
  

   certainty 
  that 
  some 
  are 
  absolutely 
  incorrect 
  and 
  impossible. 
  

   Having 
  found 
  myself 
  more 
  than 
  once 
  deceived 
  in 
  assigning 
  

   a 
  parasite 
  to 
  host, 
  even 
  after 
  having, 
  as 
  I 
  thought, 
  taken 
  

   particular 
  pains 
  to 
  be 
  correct, 
  I 
  feel 
  sure 
  that 
  it 
  often 
  needs 
  

   great 
  care 
  and 
  repeated 
  observations 
  to 
  arrive 
  at 
  correct 
  

   conclusions. 
  

  

  Not 
  many 
  of 
  our 
  British 
  Nomada 
  are 
  regularly 
  double- 
  

  

  