332 AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



parts of the abdomen are distended during gestation, and this distension 

 produces changes in the color of the animal, it may be by modifying in 

 some way the various secretions from the liver and other organs, and in 

 some cases, perhaps, widening the intervals between color centres and color 

 hairs, and breaking up groups of the same. 



The little pits or dark spots upon the dorsum of the abdomen, which 



mark the attachment of' the muscles within, seem to me to be centres for 



the aggregation of coloring material. At least the dorsal pat- 



jffects of terns appear to be grouped in some regular way around these 



muscular attachments. Thus the action of the muscles on the 

 Action. 



skin and chitinous shell or walls seems to compel certain aggre- 

 gations along the lines of use that form these colors and patterns. It 

 might be important in this connection to consider what is the ordinary 

 effect of muscular action upon the distribution of pigment and colored 

 hairs in vertebrate animals. 



The color rings or annuli around the joints of the legs of spiders may 

 be influenced by action of the muscles. The tendency of these darker and 

 more vivid colors is towards the ends of the joints, as though by the at- 

 tachments and prevailing outward action of the muscles the pigment were 

 forced mechanically or otherwise attracted toward these points. The foot 

 or terminal joint is usually dark, and often black. In the cephalothorax 

 may be noted the same tendency of color to group itself somewhat sym- 

 metrically around the points of muscular attachment, particularly the cen- 

 tral depression. 



IV. 



Color and markings are undoubtedly influenced by sex. Peckham, after 



summarizing the Attidse of France from the studies of M. Simon, finds 



that in thirty-nine species the male is plainly unlike the female, 



Inflt ence being in twenty-six instances much more conspicuous, while in 



o ex n fifty_ nve the sexes are similar, or, if they differ, the male is no 



more conspicuous than the female. These facts make it clear 



that the sexes commonly differ, the male being brighter than the female. 



Peckham considers it not too much to say that in the Attidse at least 



two-fifths of all the species have the male more conspicuous than the 



female. l 



Menge, in referring to the greater brilliancy of the male of Micromata 

 ornata, says that it only assumes its bright color as a bridal adornment, 

 and in this connection makes a statement that in the ThomisidaB and Sal- 

 tigrada? the males are generally more beautifully colored than the females. 2 



PhilaBus militaris, a common American Attoid, is another illustration. 

 In the male the cephalothorax and abdomen are bright bronze brown, the 



1 Sexual Selection, page 20. 2 Menge, Preussische Spinnen, II., page 396. 



