Sensory Discrimination: The Chemical Sense 89 



time to dry. Evidences of irritation by the neighborhood of 

 asafcetida were observed also by Graber in Pagurus (268). 

 In some Crustacea the sense of smell is possibly con- 

 cerned in guiding the male to the female. Certain cope- 

 pods which daily migrate from near the surface of the water 

 to greater depths and back again have had this behavior 

 explained as a result of the reactions of the females to light, 

 plus the tendency of the males to follow the females. That 

 the latter is an affair of chemical stimulation is indicated 

 by the fact that the females were sought even when con- 

 cealed in tubes (534). In the case of some other Crustacea, 

 however, the sexes do not seem to be aware of each other's 

 neighborhood until they come into actual contact (331, 

 333)- 



22. The Chemical Sense in Arachnida 



The two most important divisions of the phylum Arthrop- 

 oda, besides the Crustacea, are those of the Arachnida and 

 Insecta. Spiders, as is well known, have highly developed 

 responses to mechanical stimulation; the web-making 

 species in particular are sensitive to very slight web vibra- 

 tions. The food reactions of spiders have never, so far 

 as the writer knows, been tested, but various observers 

 report sensitiveness to chemical stimulations, such as those 

 produced by odorous oils, not in contact with the body. 

 Spiders of the family Attidag would react to glass rods 

 dipped in such oils and brought close behind them, but 

 would not react to clean glass rods when similarly placed 

 (570). The reactions seem to be of a negative character 

 (618), and, of course, in all such cases it remains uncertain 

 whether the possible conscious accompaniment is a specif- 

 ically olfactory unpleasantness or an unpleasant irritation 

 of the body surface. Pritchett found that irritating and 



