76 THE WEST AMERICAN SCIENTIST. 



ary sacks, are little cavities containing leaf-like plates over whir 

 the blood flows and is oxygenized by air that enters througii 

 small openings on the outside, called stigmata. These sacks 

 are on the underside of the alDdomen. Not far from them are 

 two tubes called trachea, which resemble the only breathing or- 

 gans of insects, except that they are much shorter. 



The principal nerve mass is in the lower part of the cephalo- 

 thorax and has the stomach and heart above it. The six or eight 

 eyes are on the top and front of the cephalo-thorax and are very 

 good for seeing objects above the animal, but of no use for tritles 

 under foot or at the side. 



Spiders are armed with two strong, sharp curved and per- 

 forated fangs, connected with poison glands, and these fangs 

 generally have a lateral motion. The mouth is eminently a suck- 

 ing organ, provided with powerful muscles, and one use of the 

 fangs is for holding the prey while the spider absorbs its life 

 blood. They also have a short jointed appendage, like a leg, on 

 each side the mouth, called maxillary palpus. The basal joints 

 of these organs are modified into hard jaws which serve for 

 crushing food, while the terminal joint has tactile function. 



The Trap-door spider and the Tarantula belong to the same 

 family — Mygalidae— and differ from other spiders in three im- 

 portant particulars. Their maxillary palpi are very long, so 

 long that they look like a fifth pair of legs; their fangs move 

 up and down, instead of having a side motion, and they have 

 four instead of two pulmonary sacs. The sacs are the yellow- 

 ish spots seen on the under side of the abdomen. The Myga- 

 lidae embraces the largest spiders. 



The large bird-catching spider of South America is first 

 cousin to what is called 'Tarantula' in California. Our species is 

 not a tarantula at all, and does not even belong to the same family 

 as the tarantula of south Europe. 



Both our species are called tarantulas, and there is consider- 

 able popular confusion about the animals and their habits. They 

 are easily told apart. The 'Tarantula' (Mygale Hentzii) is large 

 — when extended it often covers the space of four or more inches; 

 it is very hairy; black or dark brown in color, with sometimes an 

 ashy tinge; and has long legs which indicate a wanderer. 



The Trap-door spider (Cteniza California) is only half as- 

 large, is downy, pale brown, and has short legs. There is the 

 greatest possible difference of opinion in regard to the tube build- 

 ing habit of the two species. I have never found a 'Tarantula' 

 in a nest with a trap-door or a Trap-door spider in one with an 

 open mouth. I have put Mygales of both sexes in jars of earth,, 



