THE LIFE OF THE SPIDER 47 



limited to young spiders that have not stopped molting, or to those 

 few primitive spiders which molt after sexual maturity. 



It is now known that autotomy in the strictest sense that is, 

 the act of reflex self-mutilationdoes not occur in the Arachnida. 

 An appendage is dropped only after a visible effort on the part of 

 the spider, which struggles with such violence that the tension on 

 the member snaps it off at its weakest point. This action was termed 

 "autospasy" by Pieron in 1907; it involves the breaking of the 

 appendage at a predetermined locus of weakness when pulled by 

 an outside force. This locus is between the coxa and the trochanter 

 in the legs in most spiders: a point found by Wood to resist only 

 7 per cent of the stress that the next weakest juncture, that between 

 the metatarsus and the tarsus, could withstand. In harvestmen, the 

 weakest point is between the trochanter and the femur, and in other 

 animals the break may occur in quite different locations. 



The reaction of the spider to the loss of appendages varies con- 

 siderably. The loss of one, two, or even three legs in some of the 

 active crab spiders seems to result in little inconvenience to the 

 animal, which runs away without crippling effects. Stocky crab 

 spiders that have lost the first two pairs of legs take up a position 

 in which the short third legs are directed forward as in normal 

 posture, and are able to move about with relative ease. The stout 

 front legs of the crab spiders are at the same time organs of touch 

 and offensive weapons, and when they are lost, the ability to cap- 

 ture flies is seriously impaired. Mature males that have lost some 

 of their long front legs are at a distinct disadvantage during court- 

 ship, and fall easy prey to females not willing to meet them. 



Autotomy is easy to observe. If a spider is grasped by one of 

 the legs and the animal has a good hold on the substratum, the leg 

 will break loose at the usual locus between coxa and trochanter. On 

 the other hand, if the spider is held in the air and is unable to exert 

 some countering force by grasping an object, it is unable to drop 

 a leg. When held in a pair of forceps, the animal usually twists 

 around, grasps the forceps, and literally pulls its body loose from 

 the leg. The speed with which this is accomplished varies with the 

 species and with the thickness of its appendages, but it is practically 

 instantaneous once the spider begins to effect an escape. If two legs 

 are held firmly, some spiders break both of them easily, but some 

 of the stocky crab spiders are unable to exert enough force to free 

 themselves. 



Sometimes the spider is seized by a predator that is able only 



