644 ARACHNIDA. 



insects. The only way to preserve them is to throw them into 

 alcohol ; when pinned the}- shrivel up and lose their colors, 

 which keep well in spirits. 



The colors of spiders vary much at different seasons of the 

 year, especially during the frosts of autumn, when the changes 

 produced are greatest. All spiders are directly beneficial 

 to agriculture by their carnivorous habits, as the}' all pre}- 

 upon insects, and do no harm to vegetation. Their instincts 

 are wonderful, and their habits and organization worthy of 

 more study than has yet been paid them in this country We 

 have no species poisonous to man, except when the state of 

 health renders the constitution open to receive injury from 

 their bite, just as mosquitoes and black flies often cause 

 serious harm to some persons. 



The Arachnids ai"e divided into three groups, or suborders, 

 the Araueina, the Pedipalpi, and the Acarina. 



ARANEINA. 



The Spiders are distinguished from other Arachnids by hav- 

 ing mandibles used exclusively for biting, a spherical, sac-like 

 abdomen, not divided into segments, and attached to the head- 

 thorax by a slender pedicel. The maxillre resemble the tho- 

 racic feet. They breathe both by lungs and tracheae, and do 

 not undergo a metamorphosis, the young on being hatched hav- 

 ing four pairs of legs. 



The mandibles (Plate 12, fig. 3, front view, Avith the eight 

 ocelli above) are vertical and end in a powerful hook, in the 

 end of which opens a duct (Plate 12, 3 a, b) connected with the 

 poison gland situated in the head. The maxillaj, represented 

 by the so called palpi, though in reality the maxillae themselves, 

 with a flattened coxal lobe at the base (Plate 12, fig. 2, &, palpi 

 of female ; fig. 8, do. of male) are simple in the female, but in 

 the male the terminal joint is enlarged and modified greatly as 

 an apcessory genital organ. The cephalothorax is not jointed, 

 and there are usually eight, rarely six, simple eyes (ocelli). 

 In the genus Nops from Cuba there are, however, only two, 

 while in certain cave-inhabiting species, according to Menge. 



