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ARACHNIDA
which would be recognized at once as true scorpions by a child or a savage. The Silurian scorpion Palaeophonus, differs, so far as obvious points are concerned, from a modern scorpion only in the thickness of its legs and in their terminating in strong spike-like joints, instead of being slight and provided with a pair of terminal claws. The legs of the modern scorpion (fig. 10; fig. 51) are those of a terrestrial Arthropod, such as a beetle; whilst those of the Silurian scorpion are the legs of an aquatic Arthropod, such as a crab or lobster. It is probable that the Silurian scorpion was an aquatic animal, and that its respiratory lamellae were still projecting from the surface of the body to serve as branchiae. No trace of “stigmata,” the orifices of the lung-chambers of modern scorpions, can be found in the Scottish specimen of Palaeophonus, which presents the ventral surface of the animal to view. On the other hand, no trace of respiratory appendages excepting the pectens can be detected in the specimen (see fig. 49).
1911 Britannica-Arachnida-Comparison.png

Fig. 50.—Comparison of the sixth prosomatic limb of a recent scorpion (B), of Palaeophonus (C), and of Limulus (A), showing their agreement in the number of segments; in the existence of a movable spine, Sp, at the distal border of the fifth segment; in the correspondence of the two claws at the free end of the limb of Scorpio with two spines similarly placed in Limulus; and, lastly, in the correspondence of the three talon-like spines carried on the distal margin of segment six of recent scorpions with the four larger but similarly situated spines on the leg of Limulus; s, groove dividing the ankylosed segments 4 and 5 of the Limulus leg into two.

(After Pocock, Q. J. Mic. Sci., 1901.)


1911 Britannica-Arachnida-Buthusaustralis.png
From Lankester, Journ. Linn. Soc. Zool. vol. xvi., 1881.

Fig. 51.—Drawing from life of the desert scorpion, Buthusaustralis, Lin., from Biskra, N. Africa.

Fossil scorpions of the modern type are found in the Coal Measures. At the present day scorpions of various genera are found in all the warm regions of the world. In Europe they occur as far north as Bavaria and the south of France. The largest species measure 9 in. from the front of the head to the end of the sting, and occur in tropical India and Africa. Between 200 and 300 species are known. The scorpions use their large chelae for seizing prey and for fighting with one another. They never use the sting when (as frequently happens) they attack another scorpion, because, as was ascertained by A. G. Bourne (24), the poison exuded by the sting has no injurious effect on another scorpion nor on the scorpion itself. The stories of a scorpion stinging itself to death when placed in a circle of burning coals are due to erroneous observation. When placed in such a position the scorpion faints and becomes inert. It is found (Bourne, 24) that some species of scorpion faint at a temperature of 40° Cent. They recover on being removed to cooler conditions. A scorpion having seized its prey (usually a large insect, or small reptile or mammal) with the large chelae brings its tail over its head, and deliberately punctures the struggling victim twice with its sting (fig. 52). The poison of the sting is similar to snake-poison (Calmette), and rapidly paralyses animals which are not immune to it. It is probably only sickly adults or young children of the human race who can be actually killed by a scorpion’s sting. When the scorpion has paralysed its prey in this way, the two short chelicerae are brought into play (fig. 53). By the crushing action of their pincers, and an alternate backward and forward movement, they bring the soft blood-holding tissues of the victim close to the minute pin-hole aperture which is the scorpion’s mouth. The muscles acting on the bulb-like pharynx now set up a pumping action (see Huxley, 26); and the juices—but no solid matter, excepting such as is reduced to powder—are sucked into the scorpion’s alimentary canal. A scorpion appears to prefer for its food another scorpion, and will suck out the juices of an individual as large as itself. When this has taken place, the gorged scorpion becomes distended and tense in the mesosomatic region. It is certain that the absorbed juices do not occupy the alimentary canal alone, but pass also into its caecal off-sets which are the ducts of the gastric glands (see fig. 33).


1911 Britannica-Arachnida-Euscorpius italicus.png 1911 Britannica-Arachnida-Euscorpius italicus2.png

From Lankester, Journ. Linn. Soc.


Fig. 52.—Drawing from life of the Italian scorpion Euscorpius
italicus
, Herbst, holding a blue-bottle fly with its left chela,
and carefully piercing it between head and thorax with its sting.
Two insertions of the sting are effected and the fly is instantly
paralysed by the poison so introduced into its body.

From Lankester, Journ. Linn. Soc.


Fig. 53.—The same scorpion carrying the now
paralysed fly held in its chelicerae, the chelae
liberated for attack and defence. Drawn from
life.



All Arachnida, including Limulus, feed by suctorial action in essentially the same way as Scorpio.
Scorpions of various species have been observed to make a hissing noise when disturbed, or even when not disturbed. The sound is produced by stridulating organs developed on the basal joints of the limbs, which differ in position and character in different genera (see Pocock, 27). Scorpions copulate with the ventral surfaces in contact. The eggs are fertilized, practically in the ovary, and develop in situ. The young are born fully formed and are carried by the mother on her back. As many as thirty have been counted in a brood. For information as to the embryology of scorpions, the reader is referred to the works named in the bibliography below. Scorpions do not possess spinning organs nor form either snares or nests, so far as is known. But some species inhabiting sandy deserts form extensive burrows. The fifth pair of prosomatic appendages is used by these scorpions when burrowing, to kick back the sand as the burrow is excavated by the great chelae.
References to works dealing with the taxonomy and geographical distribution of scorpions are given at the end of this article (28).
Section β. Epectinata.—The primitive distinction between the mesosoma and the metasoma wholly or almost wholly obliterated, the two regions uniting to form an opisthosoma, which never consists of more than twelve somites and never bears appendages or breathing-organs behind the 4th somite. The breathing-organs of the opisthosoma, when present, represented by two pairs of stigmata, opening either upon the 1st and 2nd (Pedipalpi) or the 2nd and 3rd somites (Solifugae, Pseudo-scorpiones), or by a single pair upon the 3rd (? 2nd) somite (Opiliones) of the opisthosoma, there being rarely an additional stigma on the 4th (some Solifugae). The appendages of the 2nd somite of the opisthosoma absent, rarely minute and bud-like (some Amblypygi), never pectiniform. A prae-genital somite is often present either in a reduced condition forming a waist (Pedipalpi, Araneae, Palpigradi) or as a full-sized tergal plate (Pseudo-scorpiones); in some it is entirely atrophied (Solifugae, Holosomata, and Rhynchostomi). Lateral eyes when present diplostichous.
Remarks.—The Epectinate Arachnids do not stand so close to the aquatic ancestors of the Embolobranchia as do the Pectiniferous scorpions. At the same time we are not justified in supposing that the scorpions stand in any way as an intermediate grade between any of the existing Epectinata and the Delobranchia. It is probable that the Pedipalpi, Araneae, and Podogona have been separately evolved as distinct lines of descent from the ancient aquatic Arachnida. The Holosomata and Rhynchostomi are probably offshoots from the stem of the Araneae, and it is not unlikely (in view of the structure of the prosomatic somites of the Tartarides) that the Solifugae are connected in origin with the Pedipalpi. The appearance of tracheae in place of lung-sacs cannot be regarded as a starting-point for a new line of descent comprising all the tracheate forms;