SUPER-ORDER CAULOGASTRA. Distinguished from the rest of the Epectinate Arachnida, as well as from the Scorpiones, by the deep constriction of the retained pregenital somite to form a slender, stalk-like waist between the cephalothorax and abdomen and by the presence of a pair of spiracles opening alongside the genital orifice on the first abdominal somite.
Order Pedipalpi. Abdomen composed of II somites with dis tinct tergal and sternal plates, without spinning appendages. Mandibles chelate or hemichelate, without poison-glands. Palpi very large and prehensile, their basal segments meeting in the middle line. Legs of first pair long slender clawless and tactile; see PEDIPALPI.
Sub-order Uropygi. Abdomen with a postanal sclerite, which, with the narrowed last three somites, forms a tail-like termination to the body. The last somite with a pair of defensive, acid secreting glands. The cephalothorax long and narrow. Basal seg ments of the palpi almost immovably fused to form a trough be neath the mouth. Legs of the first pair with only the terminal segments many-jointed.
Tribe Urotricha (Oxopaei). Postanal sclerite forming many jointed flagellum. Two pairs of lung-sacs. Carapace unsegmented with median and lateral eyes. Palpi chelate, folding almost in a horizontal plane.
The single family Thelyphonidae contains many genera (Thely plionus, Hypoctonus, Mastigoproctus, etc.), occurring in the tropics of Asia and America. A genus (Geralinura) has been found in the Coal Measures.
Tribe Tartarides. The postanal sclerite short, unjointed. Only one pair of lung-sacs retained. Cephalothorax segmented, its last two somites free. A mere trace of eyes at most remaining. Palpi not chelate, folding in a vertical plane and armed with a terminal piercing claw.
The single family Hubbardiidae (Hubbardia, Schizomus, Tri thyreus) is distributed in the warmer parts of Asia, Africa and America, and contains very small, less highly organized form than the Thelyphonidae.
Sub-order Amblypygi. Differs from the Uropygi in the absence of a caudal termination to the body and of acid-glands, in having a broad cephalothorax with its appendages radiating round a broad sternal area, the palpi more freely movable and armed with many spines, and the legs of the first pair long and antenniform.
The numerous genera (Phrymus, Tarantula, Charis, etc.), be longing to the group, which is found in the warmer parts of Europe, Asia, Africa and America, are referred to three families. The genus Graeophorus occurs in the Coal Measures.
Order Araneae (Spiders). Distinguishable from the Pedipalpi mainly by the following characters : The abdomen is provided with two pairs of appendages, which belong morphologically to the third and fourth somites and are the manipulators of the silk secreted in the abdominal glands, the ducts of which open upon them. (2) The mandibles are provided with the poison-gland opening close to the tip of the second segment which is trans formed into a piercing fan. (3) The palpi are small, pectiform, not prehensile ; their basal segments are not united but are separated by the labial sternal plate which forms a lower lip to the mouth. In the males the terminal segment carries the intromittent organ. (4) The first pair of legs are locomotor, like the other pairs.
That the Araneae are tolerably closely related to the Ambly pygi is forcibly attested by the structure of the coxal glands, the endosternite and the alimentary canal.
Sub-order Mesothelae. Abdomen segmented, with II tergal plates and two sternal plates covering the lung-sacs and genital orifice, followed by the two pairs of biramous spinning appendages which retain their embryonic position in the middle of the lower BY COURTESY OF POCOCK AND PICKARD-CAMBRIDGE FIG. 9.-KOENENIA, THE ONLY GENUS OF THE PALPIGRADIFig. 9.-KOENENIA, THE ONLY GENUS OF THE PALPIGRADI surface of the abdomen, traces of seven sterna between these appendages and the anus.
The single genus Liphistrus of the family Liphistiidae, restricted to the East Indies, is the most primitive spider known. Appar ently related is Arthrogiosa from the Coal Measures.
Sub-order Opisthatheae. Abdomen at most indistinctly seg mented posteriorly, without tergal plates and the sternal plates represented only by the pulmonary opercula. The spinning append ages at the posterior end of the body close to the anus.
All existing spiders, except the Lithistiidae, belong to this sub order which is divisible into two tribes.
Tribe Mygalomorphae. The plane of the articulation of the mandibles to the cephalothorax is vertical, distal segment or fang closing backwards in a line sub-parallel to the long axis of the body. Coxal glands opening on the fifth cephalothoracic somite. Two pairs of lung-sacs always present.
This tribe, confined to tropical and temperate regions of the world, contains the bird-eating spiders (Avicularia, Thevaphosa, etc.), often called tarantulas, the trap-door spiders (Cteniza, Nem esia, etc.), the purse—web spiders (Atypus, etc.) and others. They are referred to several families, the Theraphosidae, Ctenizidae, Atypidae, etc., the Theraphosidae being the largest spiders known.
Tribe Arachnomorpha (Araneae verae). The plane of the ar ticulation of the mandibles horizontal, the fangs closing almost transversely inwards; the coxal glands opening on the third somite of the cephalothorax.
This tribe, the most specialized section of the order in struc ture and instincts, contains most of the living species of spiders. They exhibit great variation in the structure and position of their breathing order, in the formation of the mouth parts, the num ber and distribution of the eyes, the construction of their spin ning appendages and of their external genital organs. By these variations they have been classified into a number of families, Araneidae (Aranae, Nephila), Lycosidae (Lycosa, Ocyale), Atti Sub-order Panctenadactyli. The serrula, on the movable digit of the mandible fixed throughout its length, an accessory process on the immovable digit. Carapace narrow in front, mandibles small. Families Cheliferidae (Chelifer), Garypidae (Garypas).
Sub-order Hemictenodactyli. The serrula, free at its distal end; no accessory process; carapace wide, mandibles large. Families Obisidae (Obisium), Chthoniidae (Chthonius).
The arachnids of this group are all of small size, their large pincer-like palpi giving them a superficial resemblance to scorpions. Their old-fashioned popular name, book-scorpion, was due to the discovery of specimens in libraries. Most of the species, however, live under stones or bark where they feed upon mites, etc.