COURTSHIP AND MATING 97 



in the abdomen. From the two ovaries come off oviducts that join 

 to form a single tube, the uterus, which opens externally through 

 a transverse slit at the middle of the epigastric furrow near the base 

 of the abdomen. It is probable that in ancient spiders the sperma- 

 tophore of the male was pressed into the small opening, or into a 

 pouch formed in front where it was to be stored. There are still 

 many spiders having only simple paired receptacles just in front of 

 the genital furrow, and no external evidence of the organ we know 

 as the epigynum. All the tarantulas, and the many true spiders with 

 generalized palpi (the Haplogynae), belong in this series, possessing 

 relatively simple male and female genitalia. The males introduce 

 their palpi, frequently simultaneously, into the transverse genital 

 opening and into the receptacles known as spermathecae which 

 serve as storage vessels for the sperms. Only the terminal part of 

 the organ, the embolus, is pressed into the spermathecae. 



The epigynum of the female is composed of two essentially sym- 

 metrical independent units, each of which serves as a sheath for 

 the male embolus of its particular side. There is a very close corre- 

 spondence between the physical proportions of the duct leading to 

 the female spermatheca, and the embolus of the male, a natural 

 result of parallel evolution. In all the higher spiders there exists a 

 pair of outside openings into which each male embolus can be in- 

 serted without gaining entrance through the medial genital pore 

 within the body itself. Much of the perfection and elaboration in 

 the palpi and epigyna must be attributed to this new position of the 

 orifices, which makes possible the adoption of a different mode of 

 pairing, in which the male is less vulnerable. 



The external epigynum has become specialized in various ways. 

 In many spiders there is an atrium, surrounded by a distinct rim, 

 within which lie the orifices. These latter are separated by median 

 ridges that guide the embolus into its particular channel. At the 

 front or behind may be a hood or tubercle articulating with an 

 apophysis of the palpus. A conspicuous finger often overhangs the 

 atrium and serves to fix the palpus in just the right position to make 

 the pairing possible. It should be noted that not every apophysis of 

 the male palpus has been developed to fit a corresponding depression 

 in the female epigynum. As in all structures of animals, the ortho- 

 genetic tendency to become more elaborate may go far beyond the 

 needs of the animal. Many of the spurs and strange projections in 

 complicated palpi may prove only useless luxuries that contribute 

 nothing to mating. 



