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TEXT-BOOK OF ZOOLOGY 



threads with equal ease. They are also used for fixing, smoothing out, 

 and tearing off the threads. 



In front of and between these pectinated claws is found a smaller, 

 much-curved ambulatory daw, which probably protects the former from 

 wear when the spider is running on hard surfaces. This claw, being 

 provided with teeth at its base, is evidently also employed in spinning. 

 The same applies to the toothed bristles placed immediately in front 

 of it. 



(/) The four pairs of long legs arise, as in insects, from the thorax, 



which, however, is fused with the head in 

 a single portion, the cephalo-thorax. This 

 division of the body encloses the muscles 

 which move the mouth parts (see Sec- 

 tion 3) and legs, and accordingly, unlike 

 the .softer abdomen (see Section C), is 

 surrounded by a strong chitinous envelope 

 (see p. 311). The two divisions of the 

 body are united merely by a thin stalk. 



2. Organs of Sense. When the snare 

 is complete, the spider takes up a position 

 in the middle of it or hides in some con- 

 venient recess, maintaining communica- 

 tion with the web, however, by means of 

 a " signal line," which it draws after 

 it. When a victim has got into the net 

 and become adherent to the viscid drops 

 of the spiral thread, the spider is forth- 

 with made aware of it by the agitation of 

 the " signal line," which is always kept 

 taut, and at once pounces forth upon its 

 victim. On the other hand, if the web is 

 merely shaken by the wind, the spider 



does not stir from its hiding-place, which shows a wonderfully acute and 

 discriminating sense of touch. The sight of the spider is, however, very 

 feeble, and that in spite of its eight eyes at the anterior margin of the 

 cephalo-thorax. (Indicate their position.) The spider is also said to be 

 possessed of the senses of hearing and smell. 



3. Masticatory Organs. The spider kills its victim almost instan- 

 taneously by a poisonous bite of the finely-pointed, claw-like terminal 

 joints of its mandibles or chelicerce. These claws are traversed by a canal 

 through which the poison, generated in a special poison gland, is con- 

 ducted into the wound of the victim. During repose these claws are 



LAST TARSAL JOINT OF GARDEN 

 SPIDER. (Much enlarged.) 



F. , The pectinate tarsal claws ; G. , 

 ambulatory claw ; g.B., toothed 

 bristle. 



