CHAMBERS'S INFORMATION FOR THE PEOPLE. 



SUB-DIVISION (A). Trachearia (trachea, the wind- 

 pipe) includes such spiders as breathe by tracheae. 

 These tracheae are tubes which ramify through 

 the tissues of the body, and open on the surface 

 by distinct apertures called stigmata. The tubes 

 are kept pervious by a spiral elastic filament, 

 which is coiled up within them. The Trachearia 

 have never more than four eyes. 



ORDER i. Podostomata,T\&?} are all marine, 

 and are commonly known as ' Sea-spiders.' Some 

 of them, as Nympha, are found amongst the 

 stones and weeds on the sea-shore, while others, 

 such as Pycruogonum are parasitic upon fish and 

 other marine animals. 



ORDER 2. Acaritta, or Monomerosomata (monos, 

 one, tntros, a part, and soma, the body) in- 

 cluding the Mites and Ticks. The body con- 

 sists of a roundish mass, exhibiting no trace 

 of segmentation. They breathe by tracheae. 

 The family of the Linguatulina is remarkable 

 in that, in their adult condition, the characters 

 of the order are not easily traced, but the young, 

 when still in the egg, are furnished with four 

 pairs of jointed legs. The Linguatulina are 

 found in the lungs of some of the mammalia. 

 The family of the Macrobiotida are microscopic, 

 and are known by the name of Sloth or Bear 

 Animalcules. Their favourite habitat is under the 

 gutters of houses. They resemble the Rotiferae 

 in so far as, though dried for a considerable time, 

 they return to life the moment they are moistened. 

 But the most familiar of all is the family of the 

 Acarida, including the Cheese- 

 mite, which has four pairs of 

 legs, and uses them for walk- 

 ing. The Sarcoptes scabiei 

 produces, by burrowing under 

 the skin, the disease called 

 ' itch.' The Hydrachnidce, or 

 water -mites, have their legs 

 fringed, and swim about in water. The Ticks 

 (Ixodes) have a ' rostrum,' which enables them to 

 penetrate and attach themselves to the skin of 

 their host, generally one of the beetles. 



ORDER 3. Adelarthrosomata. The abdomen is 

 united to the cephalo-thorax, but it is always more 

 or less distinctly segmented. The mouth is armed 

 with jaws, and the tracheae open by two or four 

 openings on the under surface of the body. To 

 this order belong the family of the Phalangidce, 

 or Harvest-spiders, characterised by the enormous 

 length of their legs, which resemble stilts. The 

 family of the Cheliferidce includes the Book-scor- 

 pion, whose palpi are long, and terminated by 

 long nippers like those of the scorpion. Their 

 favourite habitat is amongst old books. 



SUB-DIVISION (B). Pulmonaria. These breathe 

 by pulmonary sacs, which are involutions of the 

 integument, opening on the surface of the body 

 by stigmata, which lead into a blind chamber. 

 They have six or more simple eyes. The Pulmo- 

 naria comprise two orders. 



ORDER i. Pedipalpi in which the abdomen is 

 distinctly segmented, and attached to the cephalo- 

 thorax. This order includes the family of the 

 Scorpionidce which are amongst the largest 

 and best known of the Arachnida. They have an 

 elongated abdomen, armed at its extremity with a 

 short curved claw. It is with this organ that the 

 Scorpion inflicts its sting, which is doubtless very 



144 



Acarus (Mite). 



disagreeable, and often attended with troublesome 

 symptoms, though its danger, to man at least, 

 seems to have been overrated. The maxillary 

 palpi are very large, and form prehensile organs 

 like the claws of a lobster. With these they seize 

 their prey, strike it with the sting, and then devour 

 it. They have four. pairs of stigmata opening on 

 the under side of the abdomen. It is in the tropics 

 that the Scorpions attain their largest develop- 

 ment 



ORDER 2. Dimerosomata (Gr. di, two, meres, 

 a part, and soma, a body) includes the true 

 Spiders. The head and thorax are united into a 

 single mass, and the abdomen is soft and unseg- 

 mented, being separated from the cephalo-thorax 

 by a slight constriction. The head carries six or 

 eight simple eyes. The mandibles are large, and 

 perforated by the duct of a gland, which secretes a 

 poisonous fluid. The palpi are never chelate 

 or clawed. The pulmonary sacs are two or four 

 in number, and open on the under surface of 

 the abdomen. Tracheae may also be present. 

 But by far the most remarkable organs are the 

 spinnerets, by means of which these animals spin 

 their curious and beautiful webs for intercepting 

 the prey upon which they expect to feed, or for 

 lining their abodes. The spinnerets are teat-like 



Spider, with thread-making organ magnified. 



organs, four or six in number ; they are per- 

 forated at the apex by minute openings placed 

 on the under surface of the abdomen. The sub- 

 stance composing the web is secreted by glands 

 placed close to the spinnerets. It is viscid at first, 

 and passes through the spinnerets, which give it 

 its proper thread-like shape, becoming hard on 

 exposure to the air. The Argyroneta aquatica, 

 or Diving-spider, weaves for itself a bell-shaped 

 cell, at the bottom of the water, to which it retires 

 to devour its prey. It carries down air by entang- 

 ling it amongst the hairs which cover its body. 

 All spiders are predaceous, but some do not 

 construct a web for the capture of their prey, 

 such as the Tarantula (Lycosa tarantula) of 

 Southern Europe, which springs suddenly upon 

 its unwary victim. The Mygale, the most power- 

 ful of the spider order, when at rest, covers a 

 space of six or seven inches in diameter. Other 

 species are called Mining-spiders, because they 

 construct their habitations in the ground, lining 

 them with their silk-like secretion. The Aranea 

 domestica (Common House-spider) is well known. 

 The maxillary palpi of the male spider are speci- 

 ally constructed for bringing the male into contact 





