BEHAVIOR OF SPIDERS AND OTHER INSECTS 419 



oligotropic habit would not be desirable. It is not an advan- 

 tage for a bee to confine its food to one kind of plant unless it 

 is always certain to obtain the supply it needs. By overstock- 

 ing a locality oligotropic bees would disappear or become poly- 

 tropic. 



5. The genus Perdita contains about 150 species, practically 

 all of which are oligotropic. An examination of the habits and 

 characteristics of the genus should throw some light upon the 

 origin of oligotropism. The facts are these: (a) the species 

 are mostly small; (b) they do not take long flights; (c) a part 

 of the species are vernal, but the majority fly in late summer 

 and autumn; (d) many visit the Compositae; (e) oligotropism 

 is as pronounced where there is only one or a few species as 

 where there are many; (f) many flowers are visited by more 

 than one species of Perdita; (g) the length of the tongues of 

 bees limit them to certain flowers, " thus it is the tube-length 

 of the flower, not competition, which is the factor limiting the 

 visits of many species of Perdita; (h) female inquiline bees do 

 not gather pollen and nectar for brood-raising and require only 

 nectar for themselves; nevertheless, many such bees, with short 

 terms of flight, visit only the Compositae. 



Lovell concludes: "According to the theory proposed by the 

 writer certain bees have become oligotropic because of the 

 direct advantage gained, combined with the fact that their 

 flight was synchronous, or nearly so, with the period of in- 

 florescence of the plant to which they restricted their visits. 

 This theory offers an explanation of the rise of oligotropism by 

 the observation of existing conditions. There may be, and often 

 are, accessory factors, but they are of secondary importance. 

 * * * Robertson concedes all that is required when he says, 

 ' The average flight is shorter and there are more of them with 



a short flight.' " 



AUDITORY SENSATIONS 



Hitherto the contributions to the experimental study of the 

 sense of hearing of butterflies and moths have been fragmentary. 

 As far as the moths are concerned, Turner and Schwarz (89) 

 and Turner (86) have attempted to remedy this defect. In 

 their joint paper (89) these investigators report the results of 

 laboratory experiments with Catocala unijuga and field experi- 

 ments with C. flebelis, C. habilis, C. neogama, C. piatrix, C. 



