THE SEX RATIO AND COCOONING HABITS OF AN 

 ARANEAD AND THE GENESIS OF SEX RATIOS^ 



THOS. H. MONTGOMERY, Jr. 



With Two Figures 



This communication presents a study of the numerical propor- 

 tions of the sexes in Latrodectus determined for 41,749 newly 

 hatched young, with briefer observations on such proportions in 

 other spiders; then an account of the general cocooning habits; 

 next an attempt to show that different species of organisms prob- 

 ably have different sex ratios, with an explanation of the origin 

 of such differences. 



I LATRODECTUS MACTANS EABR. 



This is the largest North American Theridiid and it was selected 

 partly because of the ease with which it may be kept, but more 

 particularly on account of the great degree of sexual dimorphism: 

 with the adult males and females so different in form and size 

 it was anticipated that the sexes might be distinguished at the time 

 of hatching, and this hope was reahzed. 



At Austin, Texas, where I have been observing these spiders, 

 the web of this species is found usually on the ground beneath 

 a stone or log, sometimes several feet up within a crevice of a rock 

 wall. The female remains at the upper portion of the web, and 

 uses a niche or cranny as a retreat. Her web is composed of 

 unusually powerful threads, capable of holding the strongest 

 beetles and even of sustaining small stones; indeed I allowed my 

 captives to fasten down with it the glass plates serving as covers 

 for their cages, and this they did so firmly that the glass would 

 not fall when the cages were inverted. Adult males I have found 



' Contributions from the Zoological Laboratory of the University of Texas. No. 89. 

 The Journal of Experimental Zoology, vol. v, no. 3 



