434 Thos. H. Montgomery, Jr. 



with the temperature as well as with the individual spiderling. 

 Most of those cocoons laid from the middle of June on hatched 

 in nineteen or twenty days, and two in as short a time as seventeen 

 days; longer intervals are characteristic of eggs laid earlier in the 

 year, and the earliest cocoon always takes the longest time to 

 hatch; this is readily seen on comparing successive cocoons in 

 Table I, and shows that the rate of development depends directly 

 upon the temperature. 



The total number of cocoons raised by those seventeen spiders 

 that furnished series of them was 187, an average of eleven to each 

 spider. One individual formed eight cocoons, one formed nine, 

 four formed ten each, four formed eleven each, five formed twelve 

 each, while two formed thirteen each, the range thus extending 

 from eight to thirteen. The time interval between successive 

 cocoons varies with the month, so probably with the amount of 

 nourishment, it being shortest in July and August; such intervals 

 may be easily computed from the data given in Table I. 



Sexual Dimorphism and the Sex Ratio 



On the North American continent there are two good species 

 of the genus Latrodectus Walck., L. mactans Fabr. and L. geo- 

 metricus Keys., as I have convinced myself by a study of the 

 material in the United States National Museum; for the oppor- 

 tunity of examining this collection my thanks are due to the cour- 

 tesy of Mr. Nathan Banks. This collection contains specimens 

 of mactans from Cahfornia (San Bernardino, Tulare county, 

 Clemente Island), Texas, New Mexico, Nebraska, District of 

 Columbia, Colorado, Georgia, North Carolina and Oregon; 

 while Marx^ states that it occurs also in Pennsylvania, Ohio and 

 Utah. Though mactans shows this wide distribution and is every- 

 where of rather confined local occurrence it does not appear to 

 have split into geographical races. 



The sexes of mactans show a marked dimorphism both in size 

 and color, as seen in the following comparison. 



3 Catalogue of the described Aranea? of temperate North America. Proc. U. S. Nat. Mus., xii, 1890 



