The External Anatomy of Spiders 



The terminal tenent hairs. — The tarsi of spiders are armed, 

 in many species, with hairs of the type known as tenent hairs, 

 i. e., hairs that are dilated at the extremity (Fig. 123), and which 

 serve to aid the animal in clinging to smooth surfaces, probably 

 as in insects by means of an adhesive fluid excreted through 

 the cavity of the hair. A bundle of such hairs which exists in 

 certain spiders at the tip of the tarsus just below the claws may 

 be designated as the terminal tenent hairs (Fig. 124). This 

 bundle is often divided into two by a smooth line; these bundles 

 are the fasciculi unguiculares or claw-tufts of certain writers. 

 A small sclerite upon which the bundle or bundles of terminal 

 tenent hairs are borne is sometimes referred to as the hypopodium; 

 this is sometimes covered by the empodium. 



The scapula (scop'u-lae). — The lower surface of the tarsus 

 and the metatarsus are often armed with tenent hairs. A brush 

 of such hairs is termed a scapula (pi. scopulcc). The scopula 



Fig. 



CALAMISTRUM OF ULOHORUS GEXKT'LATUS 



and the terminal tenent hairs are frequently both present. A 

 thick clothing of ordinary hairs in this position is not a scopula. 



The calamistrum (cal-a-mis'trum). — Those spiders that pos- 

 sess the peculiar spinning organ known as the cribellum have also 

 on the upper margin of the metatarsus of the hind legs one or two 

 rows of curved spines (Fig. 123); these constitute the calamistrum. 

 The calamistrum plays the part of a hackle in the formation of 

 the hackled band, characteristic of the webs of these spiders. 



Male spiders on reaching the adult stage either lose the cribel- 

 lum and calamistrum or retain them in a vestigial condition; 

 but such males can be recognized by the wide separation of the 

 fore spinnerets. 



The lyriform argans. — Near the distal extremity of each 

 segment of the leg except the tarsus there are one or more minute 



123 



