HANSEN.* EXTERNAL SENSORY ORGANS IN ARACHNIDA. 24 1 



II. Arachnida. 

 A. The trichobothria. 



The name trichobothriuin is well composed, as it signi- 

 fies a hair in a pit, and these sensory hairs always originate 

 from the bottom of calicles and do not fill their inner cavity. 

 The most convenient object to examine is a large, dark- 

 coloured Scorpion, f. inst. Pandinus cyancns. As in all 

 Scorpions the trichobothria are only found on some joints of 

 the palps and always on the hand and the fixed finger of 

 their chelae. By aid of a pocket lens one observes on this 

 part of the chela a number of round holes; through each 

 hole projects a long and very fine hair which is very far 

 from filling the hole, so that the hair is freely movable. In 

 a transverse section of the wall of the chela the hole is seen 

 to be the opening of a very much wider cavity, on the 

 bottom of which the hair is inserted. In Pandinus the inner 

 wall of the cavity is adorned in a peculiar way as if covered 

 with a stratum of prismatic cells much reminding one of the 

 cells in a honeycomb (H. J. Hansen, a, p. 147 — 48), but such 

 adornment is of secondary interest, and is f inst. not found 

 in the calicles of Araneae. It may be added that these holes 

 with their hairs have been known during more than half a 

 century. In Pandinus cyancns I found in 1893 (1. c.) 14 such 

 calicles on the immovable finger and 12 on the hand of the 

 chela, thus in all 26 hairs on this joint, the fifth of the 

 appendage, when the mouth-part itself — by W, SöRENSEN 

 and myself later interpreted as the mandible — is counted 

 as first joint. On the fourth joint I found 17, and on the 

 third joint 3 calicles, but they do not occur on the more 

 proximal joints or on the movable finger. In smaller forms, 

 f. inst. Androctonus australis L., the number of calicles is 

 lower, and there is probably some difference in number and 

 arrangement according to families or genera. In all Scorpi- 

 ons trichobothria are wanting on all other appendages and 

 on the body. 



In the Cheloncthi (Pseudoscorpions) trichobothria are 



