Abdomen segmented, with or without a "post-abdomen" Respiration by means of pulmonary sacs. In this order are the true Scorpions, together with certain other. animals which are in some respects intermediate between the Scorpions and the true Spiders. The members of this order are distinguished by the fact that the abdomen in all is distinctly segmented, but is not separated from the cephalothorax by a well-marked constriction. They agree in this character with the Adelarthrosomata; hence the two are sometimes united into a single order (Arthrogastra), but they are separated by the nature of the respiratory organs, the latter breathing by tracheae, and not by pulmonary sacs.

Family 1. Scorpionidae

The Scorpions are amongst the best known of the Arachnida, as well as being amongst the largest. They are distinguished by their long, distinctly segmented abdomen, terminating in a hooked claw (figs. 162, 167). This claw, which is really a modified "telson," is the chief offensive weapon of the Scorpion, and is perforated at its point by the duct of a poison-gland which is situated at its base. The abdomen is composed of twelve somites, not counting the telson, of which the last five constitute a true "tail" or "post-abdomen;" but there is no evident line of demarcation between this region and the cephalothorax. The second segment of the abdomen carries below two curious comb-like organs, of uncertain use, but probably connected with reproduction. The thoracic segments carry four pairs of ambulatory feet. There are six, eight, ten, or twelve simple eyes carried on the top of the head. The maxillary palpi are greatly developed, and constitute strong nipping-claws, or "chelae" (figs. 162, 167). The mandibles (antennae) also form claws, or "chelicerae." The respiratory organs are in the form of pulmonary sacs, four on each side, opening upon the under surface of the abdomen by as many stigmata, each of which is surrounded by a raised margin, or "peritrema" (fig. 162, 3).

Fig. 167.   Scorpion (reduced).

Fig. 167. - Scorpion (reduced).

The Scorpions are mostly inhabitants of warm regions, and their sting, though much exaggerated, is of a very severe nature. They live under stones or in dark crevices, and run swiftly, carrying the tail curved over the back. They feed on insects, which they hold in the chelate palpi, and sting to death. The largest forms, from Central Africa and South America, attain a length of nine or ten inches.

Family 2. Thelyphonidae

The members of this family in external appearance closely resemble the true Spiders, from which they are separated by the possession of a segmented abdomen, and long spinose palpi, and by the absence of spinnerets. They are distinguished from the Scorpionidae by the amalgamation of the head and thorax into a single mass, which is clearly separated from the abdomen by a constriction, as well as by the fact that the maxillary palpi terminate in movable claws instead of chelae. Further, the extremity of the abdomen is not furnished with a terminal hook or "sting."

In Thelyphonus (fig. 166, C) the abdomen terminates in three post-abdominal segments, to which a long many-jointed caudal appendage is attached; but in Phrynns the abdomen ends in a button-like segment. The first pair of legs is the longest (immensely so in Phrynus), the falces are not chelate; and the maxillary palpi, though of large size, and sometimes didactyle, do not form true chelae. The genus Thelyphonus is confined to the tropical parts of Asia, America, and Australia, and the genus Phrynns is also wholly tropical.