SPIDER-CRAB 



SPIDERS 



631 



Vol. IV. p. 782 ; and the work on the battle by 

 Major Henderson (1892). 



Spider-crab. See CRAB. 



Spider-fly (Ornithamyia), a genus of dipter- 

 ous insects closely allied to the Forest-fly, but 

 parasitic on birds, not on quadrupeds. Thus, 0. 

 avicularia frequently infests the common fowl, the 

 blackcock, and other birds in Britain. It is green- 

 ish yellow, with smoke-coloured wings. 



Spider-monkey, a name given to an Ameri- 

 can genus of monkeys, Ateles. These monkeys 

 have no thumb, or it is rudimentary. They occur 

 only in South America, Mexico, and Central 

 America. The popular name has been given to 

 them on account of their long, slender limbs. Like 



Spider-monkey (Atelet Bartlettii). 



other New- World monkeys ( Platyrrhini ), they 

 have a prehensile tail, and are naturally purely 

 arboreal in habit. Although denizens of the trop- 

 ical forests of the New World, they can, like the 

 tropical apes of the Old World, endure a very con- 

 siderable amount of cold. One species, A. vellerosut, 

 lives in large companies in forests at a height of 

 7000 feet in Mexico. The Coaita is the A. paniscus. 

 About fourteen species of the genus are known. 



Spiders (Araneidte) form an order within the 

 heterogeneous class Arachnida. The name, which 

 seems to be a corruption of s/tinther, refers to the 

 well-known spinning powers which these animals 

 so cleverly exercise. They are found almost every- 

 where upon the earth, especially in warm coun- 

 tries, and are of much importance in checking the 

 multiplication of in.sects. A few tropical forms, 

 notably the bird-catching spiders, exceed two inches 

 in length of body, but the majority measure only 

 some fraction of an inch. 



Structure and Functions. The body is divided 

 into two distinct parts an nnsegmented cephalo- 

 thorax, tearing six pairs of appendages, and a soft 

 nnseginented abdomen, at the end of which are the 

 spinnerets. On the skin there are hairs of various 

 kinds, some l>eing specialised as sensitive struc- 

 tures. The colours, which are often very brilliant, 

 are due in part to pigments occurring in the tissues, 

 or diffused on the skin, or concentrated in special 

 chromatophores, in part also to the way in which 

 the light is reflected from the hairs and skin. 

 They vary with age and sex, with food and en- 

 vironment, and, as we shall see, are often of 

 importance in courtship and in protective adapta- 

 tions. Soon after the young spider is hatched it 

 caste its cuticle, and this moulting is repeated at 

 intervals until the full size is reached. It is 



Fig. 1. Foot of 

 Garden Spider. 



probable that the Attidfe moult from seven to 

 eleven times before reaching maturity. As this 

 moulting proceeds, the colour, which in young 

 spiders is generally bright yellow or green, whitish 

 or livid, gradually becomes in tints and markings 

 that which is characteristic of the species. 



There are six pairs of appendages : ( 1 ) the 

 clawed chelicerse or falces, of which the last joint 

 works against the second last and contains a 

 poison-gland; (2) the leg-like pedipalps, the ter- 

 minal joint of which is modified in the male for 

 copulatory purposes; (3-6) four pairs of walking- 

 legs, of which the foremost pair are much used as 

 feelers. The embryo has four pairs of abdominal 

 legs which abort. Near the anus are situated two, 

 three, or four pairs of closely approximated spin- 

 nerets, and on each of these 

 there are numerous ' spinning- 

 spools," out of which oozes the 

 viscid fluid which hardens into 

 the silken thread. A figure is 

 given of the foot of the hind- 

 most leg in the Garden Spider, 

 showing the claws and spines, 

 which are of use in draw- 

 ing out the silken secretion. 

 Among other external charac- 

 ters are the simple eyes, of 

 which a variable number, most 

 frequently eight, occur on the 

 head ; the openings or stigmata of the respiratory 

 sacs or tubes ; the opening of the reproductive ducts 

 between the anterior stigmata ; antt (in the female) 

 the adjacent but separate aperture or apertures of 

 the receptacula seminis. 



The nervous system conforms to the usual 

 Arthropod type, consisting of a dorsal brain, a 

 ring round the gullet, and a ventral nerve-cord ; 

 but the ventral ganglia are concentrated in a single 

 ganglionic mass in the thorax, giving off nerves to 

 the limbs and other parts. 



As regards the senses of spiders, it seems that 

 few have much power of precise vision. Some 

 discern rapid movements of objects, but seem 

 unable to see their cocoon though it be but a few 

 inches off. The hunting Saltigrades, however, 

 have been observed to stalk prey from a distance 

 of 10 inches, but that this is regarded as excep- 

 tional shows how limited the ordinary power of 

 vision is believed to be. In many cases it lias been 

 experimentally proved that spiders exhibit an 

 apparent preference (perhaps dependent on sexual 

 associations) for certain colours, and it is certain 

 that the males of some kinds (e.g^ Attida") display 

 their bright colours l>efore their desired mates. 

 The limited nature of visual power is in great part 

 compensated for by the exquisite delicacy of the 

 sense of touch, for the lurking spinner feeis rather 

 than sees the insect tangled in its snare or web. 

 Vibrations such as those caused by the whizzing of 

 insects' wings or by a tuning-fork are propagated 

 along the taut lines of the web and interpreted 

 by the spider. According to some observers, the 

 courtship is sometimes conducted in this telephonic 

 fashion. This tactile sensitiveness seems to be in 

 great part diffused over the body, but the hairs 

 towards the ends of the legs are specially sensitive. 

 We know little with regard to the sense of hearing 

 in spiders. That some males, e.g. of Theridium and 

 Mygale, are able to produce a stridulating noise, 

 suggests that their mates can hear, but the evidence 

 is not conclusive. Nor are the numerous accounts 

 of spiders which descend chandeliers and the like 

 to listen to music, for spiders often do this when 

 there is no music, and are especially likely to do 

 this of an evening, and it is difficult to abstract 

 the influence of vibrations other than those of 

 sound-waves. Many, however, believe that spidern 



