Sensory Discrimination: the Chemical Sense 85 



but an oil that belongs, for us, to the latter class might belong 

 to the former in the case of a spider. If the sensibility were 

 sharply localized, that fact would point in the direction of a 

 specific olfactory sensation ; but while some authorities think 

 the spider's feelers or palpi are smell organs (25), others 

 believe that sensibility to chemical stimulation is distributed 

 over the body (258, 351). Nagel finds no specific organ of 

 smell and little smell sensibility in spiders (292). 



A member of the Arachnida which presents but slight super- 

 ficial resemblance to the spiders is Limulus, the horseshoe 

 crab. Limulus shows taste reactions, but no response to 

 smell stimuli. If the mandibles at the base of the legs be 

 rubbed with inedible objects, there is no reaction. Similar 

 negative results are obtained by holding strong-smelling food 

 close to the mouth or jaws. But if an edible substance be 

 rubbed on the mandibles, strong chewing movements take 

 place. Ammonia or acid vapor will produce these same 

 chewing reflexes, but the claws make snapping movements 

 "as though to pick away some disagreeable object." If a wad 

 of blotting paper wet with ammonia or acid be laid on the 

 mandibles, the chewing movements are reversed and the object 

 is sometimes picked up by the claws and removed. Patten 

 found organs which he believed to be gustatory on both the 

 mandibles and the claws (315). Pearl observed no gustatory 

 reactions in the free-swimming embryo of Limulus (317). 



25. The Chemical Sense in Insects 



Throughout all the branches of the animal kingdom thus v 

 far mentioned, the chemical sense has functioned chiefly as 

 a food sense. There has been but little evidence jotJhe 

 development of qualitative discrimination within the sense 

 itself. Thatis, while in many cases an animal can apparently 

 distinguish the edible from the inedible, and gives negative 



