ON THE LAND CRAB, CAllDLSOMA ARMATUM. 361 



20. Observations on the Land Crab, Cardisoma armatum, 

 with especiiil regard to the Sense Organs. By Miss L. 

 E. Cheesman, F.E.S., (.urator of Insects to the Society. 



[Received March 18, 1922 : Read April 25, 1922.] 



As little has been written on the habits of this group of 

 Crustaceans, the following observations on nine specimens from 

 the River Gambia, although made under the artificial conditions 

 of the Caird House for Insects, appeared worth recording. 



Sight. — The eyes on their pedicels only partly fill the large 

 sockets. They function ineffectually in daylight, when the crab 

 appears to rely for guidance solely on the setae with which the 

 whole body is amply provided ; but in twilight the crab is able to 

 focus on a point with tolerable accuracy. 



The third pair of maxillipeds are lined at the apex with veiy 

 soft hairs which are kept moistened ; this organ is supposed to 

 cleanse the eyes from particles of grit etc., but is also used directly 

 they are exposed to a strong light. Miss Rathbun * mentions, in 

 reference to this species, that direct exposure to the sun is fatal to 

 them. This, of course, refers to the tropical sun of West Africa. 

 When taken out into the sunlight from the subdued light of the 

 Caird House in July the crabs used their maxillipeds with great 

 energy during the first quarter of an hour, they then submei-ged 

 in a tank of water — doubtless to moisten the respiratory surface 

 of the branchial cavity — and afterwards seemed indifierent to the 

 light, although exposed to it for more than an hour. Nor could 

 thev be induced to use their maxillipeds again on that occasion, 

 even when the eyes were purposely sprinkled with water and Avith 

 dust. 



An experiment with flashlight was carried out in the Caird 

 House. The crab which was exposed to the sudden strong light 

 was sitting in the water in front of the bay, but left the tank 

 with unusual speed and precipitated himself to the far end of the 

 bay, where he assiduously brushed his eyes with both maxillipeds 

 at once for several minutes. Later he was brought back for a 

 repetition of the same experiment ; he submerged at once in the 

 tank, and remained below water with only the eyes showing until 

 the flash when he scrambled out and repeated the former 

 manceuvres precisely. The sudden transition from a dim to a 

 very strong light appeared to occasion him real distress ; but the 

 switching on of the electric lights in the bay after dark, though 

 it will arrest the crabs' movements temporarily, does not 

 necessitate the use of the maxillipeds. 



* Bull. Amer. Mns. Nat. Hist, xliii. 1921, p. 459. 



