io8 AMERICAN SPIDERS 



paraxial as contrasted with the diaxial position of true spiders and 

 other details of the mouth parts. The chelicerae (Plate XV) are 

 robust and two-segmented, as usual, but with their long axis par- 

 allel to that of the body, and with movement in a vertical plane. 

 As befits these powerful spiders, the fang of the chelicera is a stout, 

 curved weapon. In order to drive the fang into the victim, the body 

 must be elevated. These creatures strike with great speed, but be- 

 cause of their poor eyesight and the necessity for waste motions, 

 their method is probably inferior to that of the true spiders. When 

 confronted by man or any creature outside its normal experience, 

 the tarantula throws itself back and maintains its body in a position 

 of readiness to strike. This is a defensive attitude, but also one fa- 

 vorable for attack. 



The venom glands of the mygalomorph are entirely contained 

 within the basal segment of the chelicera. Since its offensive needs 

 are met by a powerful body and robust jaws, the necessity for 

 great quantities of potent venom is minimized. In most tarantulas 

 the coxa of the palpi also lacks the endite or maxilla, an expansive 

 lobe used in crushing and cutting the prey. 



All the Mygalomorphae have two pairs of book lungs, clearly 

 visible on the ventral surface of the abdomen and notable for their 

 large size. Only one family of true spiders, the Hypochilidae, has 

 retained this four-lunged condition, and they are the most gen- 

 eralized of all true spiders in many other respects as well. 



A moderate number of mygalomorph spiders range up into the 

 temperate zones, but the group is essentially tropical and subtropical 

 in distribution, about fifteen hundred species being known from 

 these zones all around the world. During the Paleozoic Era, their 

 ancestors dwelt in the swampy, humid forests that became the coal 

 measures of the United States and Europe. No tarantulas are known 

 from the Mesozoic, but we can be sure that they were well rep- 

 resented, and perhaps at that time equaled the true spiders in num- 

 bers and variety. Because of their secretive habits, which have 

 resulted in a meager fossil record, few Cenozoic mygalomorphs are 

 known; small numbers have been found in the Baltic amber of the 

 Oligocene, and in the Oligocene shales of the Florissant formation. 

 At some time during the early history of the Mygalomorphae 

 the line split into two principal branches, which have descended to us 

 side by side as our modern fauna. On the one hand are the typical 

 tarantulas and the trap-door spiders; they represent the largest and 

 the best-known series. The second group is somewhat inferior in 



