﻿408 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  192T 
  

  

  here 
  the 
  perception 
  of 
  currents 
  in 
  a 
  liquid 
  medium 
  by 
  the 
  lateral 
  

   line 
  organs; 
  and 
  many 
  fishes, 
  Parker 
  says, 
  have 
  the 
  complete 
  series 
  

   of 
  sense 
  organs 
  leading 
  from 
  touch 
  to 
  hearing. 
  

  

  REACTION 
  TO 
  CURRENTS 
  ( 
  RHEOTROPISM 
  ) 
  

  

  Many 
  aquatic 
  insects 
  are 
  able 
  to 
  orient 
  themselves 
  according 
  to 
  the 
  

   direction 
  of 
  the 
  flow 
  of 
  the 
  water, 
  and 
  to 
  take 
  consistently 
  a 
  posi- 
  

   tion 
  headed 
  either 
  upstream 
  or 
  downstream. 
  Likewise, 
  flying 
  insects 
  

   are 
  responsive 
  to 
  the 
  direction 
  of 
  air 
  currents. 
  Sensitive 
  reactions 
  to 
  

   air 
  movements 
  are 
  sometimes 
  distinguished 
  as 
  anemotropism. 
  It 
  is 
  

   not 
  known 
  that 
  insects 
  possess 
  special 
  rheostatic 
  organs. 
  

  

  REACTION 
  TO 
  PRESSURE 
  ( 
  THIGMOTROPISM 
  ) 
  

  

  Under 
  this 
  head 
  is 
  included 
  general 
  sensitivity 
  to 
  contact 
  with 
  

   external 
  objects. 
  All 
  insects 
  are 
  responsive 
  to 
  touch, 
  the 
  stimulus 
  

   being 
  perceived 
  either 
  through 
  the 
  general 
  body 
  covering 
  or 
  by 
  the 
  

   motion 
  imparted 
  to 
  the 
  movable, 
  innervated 
  hairs 
  of 
  the 
  cuticula. 
  

   In 
  general, 
  reactions 
  to 
  contact 
  are 
  negative 
  in 
  that 
  they 
  consist 
  of 
  

   efforts 
  to 
  avoid 
  the 
  touching 
  object; 
  but 
  many 
  insects 
  have 
  a 
  pro- 
  

   pensity 
  for 
  getting 
  beneath 
  things 
  or 
  into 
  tight-fitting 
  places, 
  as 
  into 
  

   crevices 
  in 
  the 
  bark 
  of 
  trees 
  or 
  under 
  stones 
  and 
  boards 
  lying 
  on 
  the 
  

   ground, 
  and 
  this 
  habit 
  is 
  regarded 
  as 
  a 
  positive 
  contact 
  reflex. 
  Sen- 
  

   sitivity 
  to 
  touch 
  is 
  unquestionably 
  a 
  primitive 
  sense. 
  In 
  soft-bodied 
  

   insects 
  the 
  general 
  surface 
  of 
  the 
  skin 
  is 
  sensitive 
  to 
  pressure, 
  but 
  

   nearly 
  all 
  insects 
  have 
  specific 
  organs 
  of 
  touch 
  in 
  the 
  tactile 
  hairs 
  

   distributed 
  everywhere 
  over 
  the 
  body 
  and 
  the 
  appendages. 
  

  

  Fabre 
  believed 
  that 
  certain 
  delicately 
  lined, 
  eversible 
  pouches 
  on 
  

   the 
  back 
  of 
  the 
  pine 
  processionary 
  caterpillar 
  of 
  Europe 
  are 
  sen- 
  

   sitive 
  to 
  atmospheric 
  pressure 
  and 
  serve 
  the 
  insects 
  as 
  barometric 
  

   organs 
  that 
  enable 
  them 
  to 
  forecast 
  changes 
  in 
  the 
  weather. 
  

  

  UNKNOWN 
  SENSORY 
  REACTIONS 
  

  

  It 
  would 
  be 
  impossible 
  to 
  include 
  in 
  any 
  classification 
  all 
  the 
  pos- 
  

   sible 
  sense 
  reactions 
  of 
  insects, 
  because 
  insects 
  probably 
  have 
  many 
  

   senses, 
  such 
  as 
  hunger, 
  thirst, 
  pain, 
  of 
  which 
  we 
  know 
  nothing. 
  The 
  

   idea 
  of 
  an 
  unknown 
  sense, 
  " 
  something 
  of 
  which 
  we 
  have 
  no 
  con- 
  

   ception," 
  however, 
  though 
  it 
  always 
  appeals 
  to 
  the 
  imagination, 
  has 
  

   little 
  to 
  support 
  it, 
  for 
  when 
  it 
  is 
  considered 
  that 
  sense 
  activities 
  

   depend 
  upon 
  activities 
  of 
  the 
  environment, 
  an 
  " 
  unknown 
  sense 
  " 
  

   remains 
  a 
  very 
  improbable 
  speculation 
  until 
  we 
  discover 
  a 
  stimulus 
  

   for 
  it. 
  The 
  physicists, 
  by 
  means 
  of 
  instruments 
  or 
  observations 
  of 
  

   effects, 
  have 
  discovered 
  no 
  force 
  commonly 
  present 
  in 
  nature, 
  except 
  

   radioactivity, 
  that 
  we 
  have 
  not 
  long 
  known 
  by 
  means 
  of 
  our 
  sense 
  

  

  