ECONOMIC AND MEDICAL IMPORTANCE 247 



more than trivial symptoms in man, indeed little more than the 

 mechanical injury of breaking the skin. Considerable pain often 

 accompanies the bite, it is true, but this ordinarily lasts less than an 

 hour. Our tarantulas live such secretive lives that opportunity to 

 bite man does not often present itself. Only in the fall of the year 

 are they to be seen in numbers, and those seen are males wandering 

 about in search of the females. The males are not very belligerent 

 and are rather easy to tame. 



The effects of the bites of common North American tarantulas 

 on laboratory animals vary considerably. In some instances the bites 

 seem to have no noticeable effect on white rats and guinea pigs. 

 In other cases these small mammals die quickly. The common taran- 

 tula of the Canal Zone and the lowlands of Central America, Sen- 

 copelma rubronitens, kills guinea pigs in half an hour, and causes 

 painful symptoms in man that persist for several hours. Species of 

 Dugesiella and Eurypelma of similar or even larger size produce no 

 symptoms of importance on experimental animals or on man. 

 Pachylomerus audouini, our largest eastern trap-door spider, was 

 allowed to bite man experimentally. Although this spider is as 

 large as many of the tarantulas, its venom was seen to be of less 

 potency than that of many of the common true spiders. Indeed, 

 there is little reason to believe that any of the mygalomorph spiders 

 from the United States are dangerous to man. 



However, since the large tarantula group is composed of quite 

 diverse elements, we cannot label them all harmless without much 

 more data on their venoms. The poison of the largest spiders in 

 the world, immense creatures of the genera Lasiodora and Gram- 

 mostola from Brazil, is highly toxic to cold-blooded animals, but is 

 very nearly ineffective on warm-blooded animals and man. The 

 mere mechanical hurt from the fangs of such large spiders is, of 

 course, considerable; and this, along with local complications not 

 definitely due to the venom, has given them a reputation that they 

 do not entirely merit. On the other hand, the venom of Trechona 

 venosa, a large funnel-web tarantula but vastly inferior in size to 

 many true tarantulas, belongs to the neurotoxic type and was found 

 to be dangerous when injected into human beings. 



In the United States the only spiders known to have a venom 

 producing neurotoxic symptoms are those belonging to the genus 

 Latrodectus, a name derived from the Greek and meaning a "robber- 

 biter." They occur around the world in the tropics, and extend 

 far into the northern and southern temperate zones. The genus 



