412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



Field mice and related mammals probably take the spider when they 

 find her, since we have never observed one under a rock that shelters 

 rodents. Possibly some of the hunting spiders can overcome the 

 widow, although in laboratory observations she has always prevailed 

 over any other spider put in with her. 



In all the thousands of spiders that we have handled we have never 

 noticed a single case of a disease or parasite that appeared to be 

 destructive of the mature spider. In a small percentage of cases 

 (3 out of 207 in one series) we have found the egg sacs parasitized 

 by a small ichneumon which the late Professor Crosby, of Cornell, 

 identified for us as Gelis sp., relatives of which are widely known as 

 parasites of spider cocoons. The flies oviposit on or in the egg sac, 

 the larvae consume the contents of the sac, pupate inside the sac, 

 and the flies emerge. We have also found, in the debris of the spider's 

 web, in the empty egg sacs and in various other similar places, the 

 active and omniverous larvae of one of the Dermestid beetles, the 

 common museum pests. Whether these grubs are only scavengers 

 cleaning up the waste from the spider's table, or are also capable of 

 destroying the living eggs, we have not been able to determine as 

 yet. But we suspect that both are true. 



There can be no question, however, but that the blue mud dauber 

 (Chalybion cyaneum Klug) is a predator on the black widow. We 

 have collected many mud dauber nests and have frequently found 

 immature black widows stored as food for the larvae. Recently 

 Irving and Hinman (1935) have published similar findings. In 

 Lamar, Colo., where black widows have been extremely prevalent the 

 past 2 years, residents have told us that the mud daubers seem un- 

 usually scarce. It may be that in nature the mud dauber is one of 

 the chief factors in holding the black widow in check. 



EXPERIMENTAL 

 STUDIES ON TOXICITY 



The first problem was to study the toxicity of the venom and to 

 develop a reliable means of assay. Most experimental studies pre- 

 viously reported fall under two heads as far as the source of the 

 material used is concerned. Some workers have proceeded by placing 

 the spider upon a shaved area of the animal used and pinching it with 

 forceps until it bit. Others have used the macerated heads of the 

 spiders as their experimental material. Early in our work we tried 

 both methods, but soon learned that they were unreliable and that 

 consistent results could not be obtained. If one employs the first 

 technic, he encounters several factors which are difficult or impossible 

 to control. The actual amount of venom introduced will obviously 

 be dependent upon the size of the spider, its past history — that is, 



