Savory - Arachnida 1977
Savory - Arachnida 1977 Savory - Arachnida 1977
124 IlL PROLES ARACHNES these absurdities, for to the zoologist the scorpion presents more than its fair share of problems. Its courtship, for long very imperfectly known, is peculiar, its embryonic development is remarkable, and its apparent rate of multiplication is so small that its survival is almost miraculous. The proverbial "chastisement with scorpions" is able, if nothing else, to reduce the zoologist to a state of fitting humility. 14 The Order Palpigradi [Microthelyphonida Grassi, 1885; Palpigradi Thorell, 1888; Palpigradida Pearse, 1936] (Micro whip-scorpions) Arachnida of small size in which the pmsoma is covered by a carapace of three pieces, and the opisthosoma is of 11 somites, the last three narrowed and bearing a long, jointed telson. There are no eyes. The chelicerae are of three segments and are chelate. The pedipalpi are of six segments and are leg-like, with terminal claws. The legs are variously sub-segmented and end in paired claws. There are no gnathobases on any of the coxae, and the mouth is characteristically placed on a short projecting rostrum. The sternum is of four or five sternites. The prosoma of Palpigradi is protected by three chitinous shields. The foremost of these, the propeltidium, is the largest; it is an oval plate which covers four somites, extending from the chelicerae to the second pair of legs. It is followed by the small mesopeltidium, belonging to the fifth somite, and a somewhat larger metapeltidium, lying dorsally on the sixth somite. In this respect it will be seen that the Palpigradi support Hansen's conception of the primitive arachnid, possessing a "head" of four segments and a thorax of two, and show relations to Solifugae, in which three fused somites are followed by three free ones (Fig. 33). There are no eyes on the carapace, but two seta-like structures exist near its anterior margin and are usually described as sensory patches. The chelicerae superficially resemole those of Opiliones. They are composed of three The first is a straight rounded piece, projecting forward from beneath the anterior end of the cephalic shield. The second is at right angles to the first and therefore points downward; it has a pointed and toothed process extending downward on its inner edge. Against this works the third segment, which is also pointed and toothed and joins the second outside its process. Thus the
126 III. PROLES ARACHNES 14. THE ORDER PALPIGRADI 127 FIG. 34. Komenia lateral aspect. After Kraepelin. Fm. 33. Palpigradi; dorsal aspect. Species, Koenenia mirabilis. are chelate, the pincers working laterally. They are the only chelate limbs of Palpigradi and their only weapons; they are consequently capable of very free movement, and, with their long proximal have a comparatively wide range (Fig. 34). The pedipalpi are simple pedal structures, not specialized for any particular function and used in so leg-like a manner that, if the first pair of legs were used normally, Palpigradi might be said to possess five pairs of walking legs. The pedipalpi consist of nine podomeres, the tibia being followed by a basitarsus of two parts, while the tarsus is divided into three. There is a pair of terminal cla,vs. The first pair of true legs are by far the longest of the appendages, with a basitarsus (metatarsus) in four parts and a tarsus in three, it totals 12 pieces. The last six of these carry long sensory setae. The limb is not used for walking, but is carried stretched out ahead of the animal, in the same way as are the forelegs of some of the spiders and mites. It is clear that these legs are the chief sense organs of the animal. ·The second and third legs have the normal complement of seven segments each, but the fourth, owing to the of a divided tarsus, number eight segments. All the legs have two claws on the tarsi. The mouth of Palpigradi is unique. Belo·w the proximal end of the chelicerae is a soft egg-shaped prominence, pointing forward and composed of epistome and hypostome. The mouth appears as a transverse slit at the end of this oral process, lying as it were between the two lips. There is no other arachnid in which the mouth is so far forward as to lie between the basal joints of the chelicerae, and it may be that this is a primitive position, from which the mouth has not migrated as far back as it has in the allied orders. The sternum is also very remarkable, differing from that of all other Arachnida and probably representing a much more primith·e There are four pro somatic sternitcs (Fig. 35). The anterior of these is the largest and consists of the fused sternites of the segments the pedipalpi and the first pair oflegs; the three succeeding ones arc smaller and lie directly between their corresponding conical coxae. Thus if the hypostome be taken as representing sternite 1, the large anterior plate represents sternites 2 and 3, and the remainder sternitcs 4, 5 and6. This is a full complement and there is no other arachnid prosoma with so well-defined a segmentation on its ventral surface. A soft weakly-sclerotizcd pedicel unites the two parts of the body, and is followed by an opisthosoma of 11 more somites. Tergites and stcrnites, though present, are weak and transparent; they are visible in ordinary circumstances, but can with some difficulty be seen with the help of a microscope. The genital orifice lies on
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124 IlL PROLES ARACHNES<br />
these absurdities, for to the zoologist the scorpion presents more than its<br />
fair share of problems. Its courtship, for long very imperfectly known,<br />
is peculiar, its embryonic development is remarkable, and its apparent<br />
rate of multiplication is so small that its survival is almost miraculous.<br />
The proverbial "chastisement with scorpions" is able, if nothing else,<br />
to reduce the zoologist to a state of fitting humility. 14<br />
The Order Palpigradi<br />
[Microthelyphonida Grassi, 1885; Palpigradi Thorell, 1888;<br />
Palpigradida Pearse, 1936]<br />
(Micro whip-scorpions)<br />
<strong>Arachnida</strong> of small size in which the pmsoma is covered by a carapace of<br />
three pieces, and the opisthosoma is of 11 somites, the last three narrowed<br />
and bearing a long, jointed telson. There are no eyes. The chelicerae are of<br />
three segments and are chelate. The pedipalpi are of six segments and are<br />
leg-like, with terminal claws. The legs are variously sub-segmented and<br />
end in paired claws. There are no gnathobases on any of the coxae, and the<br />
mouth is characteristically placed on a short projecting rostrum. The sternum<br />
is of four or five sternites.<br />
The prosoma of Palpigradi is protected by three chitinous shields. The<br />
foremost of these, the propeltidium, is the largest; it is an oval plate<br />
which covers four somites, extending from the chelicerae to the second<br />
pair of legs. It is followed by the small mesopeltidium, belonging to the<br />
fifth somite, and a somewhat larger metapeltidium, lying dorsally on<br />
the sixth somite. In this respect it will be seen that the Palpigradi support<br />
Hansen's conception of the primitive arachnid, possessing a "head"<br />
of four segments and a thorax of two, and show relations to Solifugae,<br />
in which three fused somites are followed by three free ones (Fig. 33).<br />
There are no eyes on the carapace, but two seta-like structures exist<br />
near its anterior margin and are usually described as sensory patches.<br />
The chelicerae superficially resemole those of Opiliones. They are<br />
composed of three<br />
The first is a straight rounded piece,<br />
projecting forward from beneath the anterior end of the cephalic<br />
shield. The second is at right angles to the first and therefore points<br />
downward; it has a pointed and toothed process extending downward<br />
on its inner edge. Against this works the third segment, which is also<br />
pointed and toothed and joins the second outside its process. Thus the
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