HANSEN: EXTERNAL SENSORY ORGANS IN ARACHNIDA. 255 



so-called auditory hairs might serve as a kind of organs for 

 possible alterations in the weather. 



In my paper from 1893 I discarded Dahl's view of the 

 trichobothria which I named tactile hairs, and pointed out the 

 main features of their occurrence in various orders of Arach- 

 nids; in the papers on Palpigradi (H. J. H., b) and Tartari 

 des (H. J. H., & W. S., c) additional observations are given. 



Such was the state of things when F. Dahl in 191 1 

 published his treatise: Die Hdrhaarc (Trichobothrien) ïiiid das 

 System der Spiinienthiere (Dahl, c). The name »tricho- 

 bothria» is very good, but I do not know if it is used by 

 Dahl in 191 1 for the first time or perhaps invented before, 

 possibly by another author. Dahl critisized VVagner's paper 

 in a way which is very far from convincing, and maintained his 

 opinion on the auditory function. He mentioned my discov- 

 ery in 1893 of very small clavate tactile hairs on the tarsi 

 in the Amblypygi, but his critical remarks show only that 

 he had looked on my description (in English) in the most 

 superficial way. He enumerated six reasons which should 

 make it necessarj^ to adopt his opinion on the function of 

 the trichobothria; among these reasons only three need quo- 

 tation, as the three others are absolutely insignificant. The 

 first is that tones from an instrument make them vibrate. 

 The second is that Araneae really can hear — a fact also 

 mentioned by Wagner — and that in Aranese no other or- 

 gan has been found, which can be interpreted as having an 

 auditory function. (This question shall be dealt with later on.) 

 The third reason (the sixth of Dahl) is the fact that the 

 trichobothria can be sharply distinguished from hairs which 

 certainly are tactile. Dahl does not seem to have known 

 that trichobothria quite similar to those in Arachnida had 

 been found respectively on the antennae or on the cerci in 

 various Thysanura, and that trichobothria of a somewhat 

 aberrant type were known in two orders of Myriopoda. But 

 evidently he laid much stress on their occurrence or absence 

 in the orders of Arachnida, as he gives an analytical key 

 of the classification of this class, and in that key uses the 

 »auditory hairs» as one of the main characters, one may 

 almost say as the most important character. Though the 



