﻿404 
  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  Most 
  insect 
  species 
  are 
  adjusted 
  to 
  a 
  definite 
  optimum 
  temperature 
  

   within 
  which 
  they 
  do 
  their 
  best 
  work. 
  

  

  REACTION 
  TO 
  HUMIDITY 
  ( 
  HYGROTROPISM 
  ) 
  

  

  Insects 
  are 
  highly 
  sensitive 
  to 
  the 
  degree 
  of 
  moisture 
  in 
  the 
  air, 
  and 
  

   it 
  is 
  probable 
  that 
  humidity 
  sensitiveness 
  is 
  an 
  important 
  factor 
  in 
  

   determining 
  the 
  geographical 
  distribution 
  of 
  many 
  species. 
  Reac- 
  

   tions 
  to 
  humidity 
  must 
  come 
  from 
  the 
  general 
  physiological 
  effect 
  of 
  

   moisture 
  on 
  tlie 
  insects' 
  tissues, 
  and 
  loss 
  of 
  body 
  moisture 
  by 
  evapo- 
  

   ration 
  will 
  follow 
  the 
  same 
  rule 
  as 
  radiation 
  of 
  heat, 
  being 
  propor- 
  

   tioned 
  to 
  the 
  surface, 
  and 
  therefore 
  greatest 
  in 
  small 
  animals, 
  unless 
  

   the 
  latter 
  are 
  protected 
  against 
  it, 
  as 
  are 
  insects 
  with 
  their 
  hard, 
  

   impervious 
  body 
  covering. 
  Many 
  insects, 
  however, 
  that 
  do 
  not 
  take 
  

   liquids 
  as 
  a 
  part 
  of 
  their 
  regular 
  diet 
  eagerly 
  drink 
  water 
  when 
  

   they 
  can 
  get 
  it. 
  

  

  REACTIONS 
  TO 
  SMELL 
  AND 
  TASTE 
  ( 
  CHEMOTROPISM 
  ) 
  

  

  Smell 
  and 
  taste, 
  for 
  the 
  most 
  part, 
  are 
  quite 
  different 
  sensations 
  

   with 
  us, 
  but 
  their 
  stimuli 
  would 
  appear 
  to 
  be 
  in 
  many 
  cases 
  the 
  same 
  

   if 
  they 
  consist 
  of 
  disengaged 
  molecules 
  or 
  other 
  particles 
  (ions) 
  of 
  

   the 
  substances 
  perceived. 
  The 
  faculties 
  of 
  gustation 
  and 
  olfaction 
  

   are 
  commonly 
  called 
  the 
  chemical 
  senses, 
  and 
  their 
  receptive 
  organs 
  

   are 
  termed 
  chemoreceptors, 
  implying 
  a 
  sensitiveness 
  to 
  the 
  chemical 
  

   properties 
  of 
  the 
  stimulating 
  substances. 
  The 
  terras 
  are 
  to 
  be 
  recom- 
  

   mended, 
  however, 
  chiefly 
  for 
  their 
  convenience, 
  since 
  they 
  assume 
  a 
  

   greater 
  knowledge 
  than 
  we 
  yet 
  have 
  concerning 
  the 
  action 
  of 
  odor 
  

   stimuli, 
  and 
  they 
  would 
  imply 
  that 
  odor 
  and 
  taste 
  stimuli 
  alone 
  have 
  

   a 
  " 
  chemical 
  " 
  effect 
  on 
  the 
  receptor 
  cells. 
  

  

  In 
  vertebrates, 
  the 
  receptive 
  organs 
  of 
  taste 
  and 
  smell 
  always 
  occur 
  

   on 
  moist 
  membranes 
  in 
  the 
  mouth 
  and 
  nasal 
  passages, 
  and 
  it 
  is 
  there- 
  

   fore 
  commonly 
  assumed 
  that 
  moisture 
  is 
  a 
  necessary 
  condition 
  for 
  the 
  

   action 
  of 
  smell 
  and 
  taste 
  stimuli. 
  There 
  can 
  be 
  no 
  question 
  that 
  

   insects 
  are 
  highly 
  sensitive 
  to 
  odors, 
  but 
  all 
  their 
  sense 
  organs, 
  except 
  

   those 
  of 
  the 
  preoral 
  cavity 
  and 
  the 
  alimentary 
  canal, 
  are 
  situated 
  

   on 
  dry 
  external 
  surfaces. 
  The 
  results, 
  or 
  interpretations, 
  of 
  experi- 
  

   ments 
  made 
  by 
  different 
  workers 
  trying 
  to 
  determine 
  the 
  location 
  of 
  

   the 
  organs 
  of 
  smell 
  in 
  insects 
  vary 
  so 
  much 
  that 
  at 
  present 
  we 
  can 
  not 
  

   accept 
  any 
  particular 
  generalization 
  as 
  fully 
  expressing 
  the 
  truth. 
  

   It 
  seems 
  probable 
  that 
  the 
  olfactory 
  organs 
  are 
  not 
  confined 
  to 
  any 
  

   one 
  part 
  of 
  the 
  insect, 
  but 
  occur 
  in 
  various 
  places. 
  So 
  many 
  different 
  

   kinds 
  of 
  sense 
  organs 
  are 
  distributed 
  over 
  the 
  body 
  and 
  the 
  append- 
  

   ages 
  of 
  insects, 
  however, 
  that 
  it 
  is 
  a 
  difficult 
  matter 
  to 
  isolate 
  any 
  

   group 
  for 
  experimental 
  purposes 
  and 
  determine 
  thereby 
  just 
  what 
  

   organs 
  serve 
  as 
  the 
  olfactory 
  receptors. 
  Since 
  none 
  of 
  the 
  suspected 
  

  

  