404 



ARACHNIDA 



predacious or parasitic animals, but a certain number of them, including 

 many mites, feed on plants. All of the Linguatulida and about half of 

 the species of mites are parasitic. Most arachnids are terrestrial but 

 the Tardigrada, Pygnogonida and two families of the Acarina are 

 aquatic. 



History.— The name Arachnida originated with Lamarck in 1801, 

 wlio at that time separated these animals from the Insecta aptera of Lin- 

 naBus and his immediate followers. The subclass contains about 20,000 

 species grouped in 11 orders. 



Key to the orders of Arachnida: 



Cj Abdomen distinctly segmented. 

 bi Animals not parasitic. 



Ci Long segmented postabdomen or segmented caudal filament (except the 

 Tarantididac) present. 



<fi Postabdomen with caudal sting present 1. Scobpionida 



^2 Caudal filament (except the Tarantulidae) . 

 e-L Animals minute; caudal filament with segmental bristles. .2. Falpigradi 

 €2 Animals larger; caudal filament, when present, smooth.... 3. Pedipalpi 

 C2 No postabdomen or caudal filament. 

 dx Head distinct from thorax, bearing first 3 pairs of 



appendages 4. Solpugida 



dn Head not distinct. 

 Ti Pedipalps chelate and very long. 



5. PSEUDOSCORPIONIDA 



€2 Pedipalps not chelate ; legs very long and slender. 



6. Phalangiida 

 1)2 Animals worm-like and internal parasites in 



vertebrates 9. Linguatulida 



Cj Abdomen not segmented. 

 &i Animals usually terrestrial. 

 Ci Cephalothorax distinctly separated from abdomen. 



7. Araneae 

 C2 Cephalothorax and abdomen not distinct. . . .8. Acarina 

 &o Animals aquatic. 



Ci Animals microscopic 10. Tardigradi 



C2 Animals marine and not microscopic ; legs very 



long and slender 11. Pycnogontda 



Fig. 640— Dia- 

 gram of the dor- 

 sal aspect of Cen- 

 trurus (Banks). 



1, cephalothorax ; 



2, pre-abdomen ; l^, 

 postabdomen ; 4, 

 mandibles; 5, 

 pedipalps ; 6, lat- 

 eral eyes; 7, 

 median eyes ; 8, 

 spine ; 9, sting. 



(Fit 



Order 1. SCORPIONIDA.* 

 640.) Elongated arachnids with a short, un- 



segmented cephalothorax and a long abdomen consisting 

 of 13 segments, of which the anterior 7 form the pre- 

 abdomen and are about of the same diameter as the 

 cephalothorax, and the posterior G form the much narrower, tail-like post- 

 abdomen, at the end of which is a poisonous sting; 3 to 6 pairs of eyes 

 usually present; mandibles short and chelate; pedipalps veiy long and 



• See "Scorpioncs und Pedipalpi," by K. Kraepolin, Das Tierreich, 1899. "Synop- 

 sis of the North American Scorpions, Solpugids, and Pedipalpi," by Nathan Banks, 

 Am. Nat., Vol. 34, p. 421, 1900. 



