ECONOMIC AND MEDICAL IMPORTANCE 241 



ligerent. However, one wonders whether these spiders are equipped 

 with eyesight sufficiently keen even to see man at a distance of 

 several feet. All spiders react to the presence of prey in an ex- 

 tremely swift and efficient manner, and in running through the 

 stereotyped actions of capturing and killing, they will give the im- 

 pression of viciousness. Even their defensive attitudes can be in- 

 terpreted as indicative of belligerence. 



By far the majority of spiders are relatively helpless creatures, 

 always willing to scurry out of the way, never attempting to bite 

 without the greatest provocation. Indeed, many of them must be 

 forced by extreme means to bite when their venom is requried for 

 experiment. The large spiders alone are capable of breaking the 

 tough skin of a human being; the smaller ones can inflict mere 

 superficial scratches, which, in some cases, reach the small capil- 

 laries and draw a touch of blood. Ordinarily, there is no reaction 

 beyond the slight mechanical laceration of the skin, and the sensa- 

 tion of pain is comparable to the jab of a pin. Only when the bite 

 is inflicted by a large spider armed with very strong weapons will 

 there be any considerable injury to the skin. 



The biting apparatus of the spider consists of the two chelicerae, 

 and the venom sacs in which the poison is produced. Each chelicera 

 has a stout basal segment, broadly articulated to the head, and a 

 movable fang. When the spider bites, it presses the sharp, spinelike 

 fangs into its victim's body and makes two separate punctures; at 

 the same time, muscles squeeze the glands, forcing their poison into 

 these wounds. The venom is usually a colorless liquid having the 

 consistency of a light oil; it is said to have a bitter taste. The 

 amount injected into the prey appears to be extremely variable- 

 dependent on the available supply at the moment, the age and con- 

 dition of the spider, and the degree of excitation produced by the 

 prey. There is reason to believe that its release is to a large extent 

 controlled by the spider, and that in many instances the spider re- 

 frains from using poison on prey easily held in its grasp and not 

 capable of strong resistance. Repeated biting exhausts the venom 

 supply; the bites become progressively less poisonous. 



The size of the spider does not give a clear index to the size of 

 the chelicerae, the volume of the venom glands, or the character of 

 the venom. In two families of distantly related spiders (Uloboridae 

 and Heptathelidae) the glands have been nearly or completely lost; 

 in some others of closer kinship (Scytodidae and Filistatidae) they 

 are partially modified for other purposes and have become tremen- 



