CHAPTER XVII.

ARACHNIDA, MYRIAPODA, AND INSECTA.

CLASS ARACHNIDA.

THE Arachnida are Articulate animals, in which the respiratory organs, when present, are in the form of pulmonary chambers or sacs, or of ramified tubes ("trachea") formed by an involution of the integument and fitted for breathing air directly; or both these organs are combined. In no case are the breathing-organs in the form of gills. There are four pairs of locomotive limbs, and there are no limbs attached to the segments of the abdomen. There is only one pair of antenna, and these are not present as antennæ, but are converted into jaws or pincers. The head is amalgamated with the thorax to form a cephalothorax, the eyes are sessile, and the-integuments are more or less chitinous.

The Arachnida are mainly distinguished from the Crustacea* by the absence of gills, and the general presence of organs adapted for breathing air directly. They are distinguished from the Insects by the possession of four pairs of legs, by never possessing wings, and by having simple eyes, whilst the head is amalgamated with the thorax. From the Myriapods they are distinguished by the fact that the legs of the latter are

Fig. 124.-A recent Scorpion (reduced). The great nipping-claws of the Scorpion are not legs, but are a development of organs belonging to the mouth.

never less than nine pairs in number, whilst the segments of the thorax are distinct from one another and from the head,

* Van Beneden would refer the Trilobites, King-crabs, and Eurypterids to the Arachnida, but such a radical change must be supported by overwhelming evidence before it can be accepted.

and the segments of the abdomen carry legs. As is the case with all the air-breathing Articulates, the remains of Arachnida, though of considerable theoretical interest, are of very rare Occurrence as fossils. They will therefore be very briefly noticed here. Of the groups of the Arachnida, the Mites (Acarida), the Harvest-spiders (Phalangida), the Book-scorpions (Pseudoscorpionida), the Scorpions (Pedipalpi), and the true Spiders (Araneida), have all been detected in a fossil condition. The three first groups require no consideration here, being almost unknown except as occurring in amber, which is a fossil resin of late Tertiary age. The Scorpions and Spiders both appear to have come into existence in the Carboniferous period, and the forms which then existed do not appear to have been strikingly different from living types.

ORDER PEDIPALPI.-The typical members of this order are the Scorpions (Scorpionida), in which the abdomen is distinctly

segmented, and not separated from the thorax by any marked constriction. The respiratory organs are in the form of pulmonary sacs opening on the under surface of the abdomen by distinct apertures or "stigmata." The jaws (maxilla) carry an enor

mously-developed pair of nipping-claws (fig. 124), and the antennæ are also converted into chela. The head carries six, eight, or twelve simple eyes, and the last joint of the abdomen (telson), terminates in a hooked claw, perforated for the transmission of the duct of a poison-gland.

As regards their distribution in time, the Scorpions commence in the Carboniferous period, where they are represented by the genera Eoscorpius and Cyclophthalmus. The most celebrated fossil Scorpion is the Cyclophthalmus senior (fig. 125) of the Bohemian Coal-measures. This remarkable form resembles the living Androctonus in having twelve eyes, but these are disposed in a circle, whereas in the latter there are six eyes on each side of the head.

ORDER ARANEIDA. This order includes the true Spiders, which are characterised by the amalgamation of the head and thorax into a single mass, to which the generally soft and unsegmented abdomen is attached by a constricted portion or peduncle. Respiration is effected by pulmonary sacs in combination with air-tubes (trachea). The head bears from six to eight simple

eyes.

The oldest-known Spiders occur in the Carboniferous Rocks. In the Coal-measures of Upper Silesia, Römer has described a Spider, which is allied to the living Lycosa, and which he has termed Protolycosa anthracophila. Other fossil Spiders have been described from the Lithographic Slates of Solenhofen (Middle Oolite), and from the Tertiary Rocks, and a good many species occur preserved in amber.

CLASS MYRIAPODA.

The Myriapods are Articulate animals in which the head is distinct, and the remainder of the body is divided into nearly similar segments. There is no marked boundary-line between the thorax and abdomen, and the segments of the latter carry locomotive limbs. There is one pair of jointed antennæ, and the number of legs is always more than eight pairs. Respiration is effected by air-tubes (trachea).

The living Myriapods are divided into the three orders. Chilopoda, Chilognatha, and Pauropoda. In the Chilopoda are the Centipedes, characterised by their masticatory mouth, and carnivorous habits, by the possession of legs in single pairs (usually from fifteen to forty pairs), and by having antennæ of from fourteen to forty or more joints. In the Chilognatha are the Millipedes and Gallyworms, characterised by their vegetarian habits, by having the segments of the body so amal

gamated that the legs appear to be arranged in double pairs, and by having antennæ of six or seven joints. In the Pauropoda is the single genus Pauropus, characterised by having only nine pairs of legs, and the antennæ bifid, with three long multi-articulate appendages.

Fig. 126.-Millipede (lulus).

The oldest-known Myriapods occur in the Coal-measures, the two best-known genera being Xylobius and Archiulus. These genera belong to the order Chilognatha, and comprise, therefore, vegetable-feeders. In Xylobius (fig. 127) the seg

(After Dawson.)

a, Natural size; b, Anterior portion, enlarged; c, Posterior portion, enlarged.

ments are divided by cross sutures into numerous fragments, in a manner wholly unknown amongst recent forms. Several species of this genus are known, of which the one figured above derives its specific name from the fact that it is found in the hollow trunks of Sigillaria. It must have possessed, like the living Gallyworms, the power of rolling itself up into a ball (Dawson). In the allied genus Archiulus, the segments are not broken up into fragments, as they are in Xylobius. The characters of both these genera are so peculiar that they have been placed in a separate family under the name of Archiulida. Other Myriapods have been discovered in the Carboniferous Rocks of North America and Britain, and have been referred to the genus Euphoberia. The true place of

these is uncertain, and they seem to have possessed the anomalous character of a row of dorsal spines. In the Lithographic Slates of Solenhofen (Middle Oolites) occur the remains of an animal which is referred to the Myriapoda by Count Münster, under the name of Geophilus proavus. Other Myriapods have been described from Tertiary strata and from amber.

CLASS INSECTA.

The Insects are Articulate Animals, in which the head, thorax, and abdomen are distinct from one another. The thorax consists of three segments, each of which carries a pair of legs. Mostly there are two pairs of wings borne by the two hinder segments of the thorax. The abdomen never carries locomotive limbs, but the last abdominal segments may carry reproductive or sensory appen dages. A single pair of jointed antennæ is present, and the eyes are generally compound. Respiration is effected by air-tubes (trachea).

As regards the general distribution of the Insecta in time, the oldest-known forms are from the Devonian Rocks of North America. Here occur the remains of several insects which belong to the order of the Neuropterous Insects (or to the Pseudo-neuroptera). Amongst the most remarkable of these is the Platephemera antiqua of Mr Scudder (fig. 128). This species must have attained a large size-five inches in expanse of wing-and it is regarded by Mr Scudder as being referable to the Ephe merida (the May-flies). This eminent authority, however, regards it as a "synthetic type;" that is to say, as a form combining peculiarities of structure which are now only found in different groups.

Fig. 128.-Wing of Platephemera antiqua (after Dawson), Devonian.

Three other genera belonging to the Neuroptera have been. described from the Devonian Rocks of North America, under the names Homothetus, Lithentomum, and Xenoneura.

In the Carboniferous Rocks, the remains of Insects, as might have been expected, are comparatively more abundant, though still far from common. In the rocks of this period we have representatives of the orders Neuroptera, Orthoptera, and Coleoptera (Beetles). The Neuroptera are represented by a remarkable form, which has been referred to the Ephemerida under the name of Haplophlebium Barnesii (fig. 129). This insect