On the Trichobothria ("auditory hairs") in 



Arachnida, Myriopoda, and Insecta, with 



a summary of the external sensory 



organs in Arachnida. 



By 

 H. J. Hansen. 



I. Introductory Remarks. 



During the study in earlier years of various orders or 

 families of terrestrial Arthropoda I endeavoured to discover 

 as many external sensory organs as possible, and most of 

 the results may be found in my papers on orders or tribes 

 of Arachnida and Myriopoda. A treatise published by Prof. 

 Dr. F. Daiil in 191 1: Die Hdrhactrc (TricJiobothricn) und 

 das System der Spimienthiere, which contains some curious 

 statements on the »auditory hairs» and on the classification 

 of the Arachnida, has induced me to take up this matter and 

 allied topics. It is, in my opinion, a very interesting and 

 hitherto unnoticed fact, that sensor}/ hairs of a special kind 

 named by Dahl >Hörhaare» or »trichobothria» and by W. 

 Wagner and myself in earlier publications »tactile hairs», 

 are found not only in several orders of Arachnida, but besi- 

 des in two orders of Myriopoda and in groups of lower In- 

 sects, thus in representatives of all three classes of terrestrial 

 Arthropoda. At the same time I intend to put together an 

 abstract of our knowledge of external sensory organs in 

 Arachnida. It may be practical first to deal with the organs in 

 question in each of the three classes, then to mention the main 

 literature on the trichobothria and their probable function. 



