HANSEN: EXTERNAL SENSORY ORGANS IN ARACHNIDA. 257 



manifold way. Sometimes a pause is suddenly interrupted 

 by a single sharp tone instantly answered by a similar one 

 from another specimen or simultaneously from several speci- 

 mens, and one gets the impression of the different moods 

 agitating the animals during the pains of the work: eager- 

 ness, impatience, fretfulness, weariness, exultation: and some- 

 times the whole compan}^ mades a kind of chorus-cry for 

 the measured regulation of the work». — I think that at 

 least all stridulating Insects and larvae, and probably many 

 other Insects, can hear. And the so-called chordotonal or- 

 gans, which have been found in various Insects, are most 

 probably the auditory organs in this class. 



But to return to the Arachnida. That most or probably 

 all Araneae can hear has been stated by W. Wagner, who 

 has performed fine biological work on many animals of that 

 order. And it seems to me very improbable that all Opiliones 

 are deaf, but trichobothria are unknown in this order. Most 

 of the Solifugae have well developed stridulating organs on 

 the antennae in both sexes (H. J. H. a, p. 184—186), but 

 trichobothria could not be discovered on any part of the 

 animals, and it is extremely improbable that the sensory organs 

 at the end of the palps and first pair oflegs are auditory, while 

 in Araneae the same function has been taken over by the 

 trichobothria. Besides one is inclined to consider the tricho- 

 bothria on the fingers of the large chelae in the Chelonethi 

 and on the chelae and more distal joints of the palps in 

 Scorpions as tactile organs, especially when the movements 

 of the palps with the chelae in walking animals are taken into 

 account. And is the statement of Dahl really correct that 

 in the Arachnida no other organ than the trichobothria is 

 known which can be supposed to have an auditory function.' 



If the chordotonal organs in Insecta are to be consid- 

 ered as auditory, it is very probable that the above-mentioned 

 lyriform organs in Arachnida have the same function, an 

 opinion favoured by Bertkau, Schimkewit.SCH and VV. 

 Wagner. The assumption at least removes various difficul- 

 ties, but it will be difficult to prove it; I suppose that the 

 Solifugae may perhaps be the best objects for experiments, 

 as the lyriform organs in this order are found only on the 



