BEHAVIOR OF SPIDERS AND OTHER INSECTS 417 



Oenothera, the evening primrose, has its pollen grains connected 

 by threads. Anthedon compta, an oligotrope of this plant, un- 

 like its nearest relatives, has scopae of long single bristles. The 

 bee goes to the stem and turns head downward, so as to work 

 the cob-webby pollen into the scopae. (3) The anthers of Ver- 

 bena are included in a slender tube and above them is a circle 

 of hairs which impresses one with the thought that they were 

 intended to prevent the extraction of the pollen. Verbenapsis 

 verbenae, the oligotrope of this flower, has her front tarsi pro- 

 vided with curled bristles. The bee thrusts both front feet into 

 the corolla and drags out the pollen with her front tarsi. 



According to Robertson, there are 223 indigenous nest-making 

 bees. One species flying the entire season, and fitted about as 

 Apis, might collect nearly as much pollen and support nearly 

 as many individuals. It would be to its advantage to be as 

 poly tropic as possible. ; The ecological specialization exhibited 

 by Anthedon, verbenapsis and other oligotropes is a fairly cer- 

 tain indication of the pressure of competition." The long- 

 tongued pygidial bees were developed as competitors of the 

 bumble bees, the first on the ground and the most poly tropic 

 of all bees. This explains their short and rapid flight and their 

 oligotropic habits. Likewise the Andrenidae, the Panurgidae 

 and related groups, which are often oligotropic, were probably 

 preceded by the Halictidae, the most polytropic of all short- 

 tongued bees. There are forty species of Halictidae flying through- 

 out the season. There are ninety-four other short-tongued bees 

 occupying the same region. It would be a hard matter for all 

 to fly throughout the season and compete with the Halictidae. 

 They have short times of flight, so distributed that not more 

 than fifty-two are flying in any month and these only in the 

 spring, when the Halictidae are the least abundant. All these 

 bees are least abundant when the Halictidae are most abundant. 

 " The early maximum flight, the non-competitive phenological 

 distribution, and the frequent oligotropic habits indicate that 

 these bees have managed to hold their own only by dividing 

 up the remaining field and occupying the most favorable corners 

 left by their polytropic competitors." 



Lovell's views upon the origin of oligotropism are diametri- 

 cally opposed to those of Robertson. In his recent reply (52) 

 he makes the following criticisms: — 



