EVOLUTION AND ADAPTATION IN THE PALPUS OF MALE 



SPIDERS. 



Jas. a. Nelson. 



The following conclusions are the results of study of 85 repre- 

 sentatives of 2 1 families of American spiders, taken from the col- 

 lection in Cornell University, through the courtesy of Professor 

 Comstock. 



The simplest form — structurally — of the male palpus is that 

 found in the sub-order Tetrapneumones, which, as is well known, 

 is recognized as including those members of the Araneida which 

 are primative in respect to other structural characters. It will 

 therefore be justifiable on two grounds, namely, the structure of 

 the palpus itself, and the general structure of the members of this 

 group — to regard this form of male palpus as the most primitive 

 existing type, thus enabling us to form some conception of its 

 ancient condition. It accordingly serves to indicate the point of 

 departure from which the other more highly modified types have 

 been developed. This simpler type of male palpus is fairly well de- 

 fined, although varying in many particulars in the different mem- 

 bers of the sub-order. Fig. i is a schematized representation of this 

 type. The essential portion of the palpal organ, the bulb (6), is 

 placed at the end or on the ventral surface of the sixth, tarsal, or 

 terminal segment, if we consider the female palpus as represent- 

 ing the original form of the second pair of appendages. The bulb 

 ^•aries from long pyrif orm to spheroidal in shape ; it is articulated 

 at its base to the tarsal segment by a narrow zone of unchitinized 

 integument, and bears at its summit a tapering process, or style 

 (5) . Within the bulb is a tubular canal, coiled in a helicoid curve. 

 This canal is wider at its base and narrows gradually toward its 

 ])eripheral end, which passes into the style and opens at its apex 

 by a narrow aperture. Its function is to contain the sperm and 

 it is therefore termed the semeniferous canal (represented in 

 dotted outline) . It is important to note the position of the heli- 

 coid curve just mentioned. It is placed in such a manner that 

 its central axis, the helical line (represented in the figures by an 

 arrow), passes through the base of the style at the distal end of 

 the bulb, and the base of the bulb itself at its central end, in this 



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