410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



TEMPERAMENT 



Dining on the crumbs from the widow's table is a tiny, long-legged 

 commensal, Psilochorus utahensis (Chamberlin), identified for us by 

 Dr. W. J. Gertsch of the American Museum of Natural History, which 

 is commonly found in black widow webs in this locality, often half a 

 dozen or so, males and females, to a web, the females carrying their eggs 

 in the chelicerae. With this exception the widow is generally found 

 alone, since her fierce and predaceous habits cause her to kill any insect 

 invader or be killed. The "killer instinct" is apparent in the follow- 

 ing description of a typical conflict observed in a web located in a 

 well-lighted hole, about eye level, in a brickyard bank. The occupant 

 was a large female with two egg sacs in the upper part of the web. 

 She was hanging, back down, on one of the sacs — a characteristic 

 position. A small mirror was used to shine light into the hole so that 

 everything might be clearly seen. The observer placed another widow, 

 previously collected, into the outer part of the web. There was no 

 movement for a moment; then the alien oriented herself and moved 

 a short distance into the web. The occupant countered by leaving 

 the egg sac and advancing a spider's length toward the interloper. 

 Both advanced and stopped alternately until they approached each 

 other in the center of the web. All their movements were deliberate 

 and cautious. There was a moment of absolute stillness, then, with 

 a motion too quick for the eye, the occupant threw a shower of thick, 

 sticky silk, which spurted from the spinnerets at the tip of the abdomen 

 and was flung over the invader by a skillful manipulation of the hind 

 legs. The victim was rapidly rolled over and over in a net until it 

 was completely covered like a mummy. It was only then, when the 

 risk was past and all secure, that the poison bite was administered. 



Contests staged in the laboratory established the fact that a bite 

 upon another spider causes paralysis immediately, but does not 

 necessarily produce death. Victims removed from the web after they 

 were helpless and all prepared for the victor to begin feeding were 

 found to recover their powers of motion after several hours, and to be 

 apparently normal the next day. In contrast to the fierce nature of 

 the spider toward others of her kind is the extreme timidity which 

 she exhibits toward unnatural disturbances of her web, such as might 

 be caused by a collector. At the first molestation of the web, she 

 retreats with incredible speed into the central nest, and does not 

 venture out again for hours. We have never seen the slightest at- 

 tempt at defense, to say nothing of aggression, on her part under such 

 circumstances. 



MEANS OF DISPERSAL 



Spiders show two definite tropisms. When they are young they are 

 negatively geotropic, tending to climb upward; at all stages they are 



