46 AMERICAN SPIDERS 



fifth molt, October 17; sixth molt, January 5; final molt, a time after 

 January 5 that was not indicated. 



Various morphological changes accompany molting; some of 

 them being very significant. The presence of a third claw on the 

 tarsi of very young spiders that are two-clawed as adults indicates 

 that the three-clawed condition is the primitive one. Young wolf 

 spiders have the eye formula of the Pisauridae, a fact which cor- 

 roborates our belief that the former were derived from an ancestor 

 very much like recent pisaurids. The young of Tibellus oblongus, 

 a greatly elongated species, have the general body form and the 

 eye relations of species of the more conservative Thanatus. 



Each molt represents a crisis in the life of the spider, and brings 

 with it dangers of many kinds. During the transformation the 

 spider is completely helpless, trussed up in old worn clothing and 

 exposed to attack from many enemies. Crickets, sowbugs, meal- 

 worms, and other omnivorous animals, not serious adversaries under 

 normal conditions, are liable to nibble and kill it; it lies vulnerable 

 to attack from the meanest foe. Normal enemies find it completely 

 unable to fight back. Furthermore, the mechanical difficulty of 

 extracting its appendages may prove insurmountable, and the im- 

 prisoned creature will perish, or so mutilate its legs that its chance 

 for life in a hostile world is much diminished. G. and E. Deevey 

 found that nearly half the deaths before maturity (thirteen out of 

 thirty-one) among the black widow spiders they reared were the 

 result of failure to complete a molt. One out of every twelve 

 spiders in the total of one hundred fifty-eight that were studied 

 from hatching to death died from this cause. 



AUTOTOMY, AUTOPHAGY, AND REGENERATION 



The spider shares with many other arthropods the ability to 

 drop an appendage without great inconvenience called "autotomy" 

 and the ability to replace it in a more or less perfect form by sub- 

 sequent regeneration. This latter power of replacing lost or muti- 

 lated organs is a very old one, and, most strongly developed in 

 lower animals, serves as a device of great importance from the 

 viewpoint of protection and survival. Often the spider is able to 

 escape the clutches of an enemy without greater loss than the 

 shedding of one or two of its appendages. Whereas autotomy oc- 

 curs in spiders of all ages, the regeneration of new appendages is 



