Savory - Arachnida 1977

116 Ill. PROLES ARACHNES
13. THE ORDER SCORPIONES 117
Fm. 29. Chelicera of Buthus, dravm from inside.
FIG. 28. A scorpion; dorsal aspect.
Buthus occitanus.
described as diplostichous; while the lateral eyes are monostichous and
more closely resemble the eyes of Limulus.
The chelicerae (Fig. 29) are of three segments. The first of these is
ring-like and is concealed by the edge of the carapace; the second is
somewhat longer, convex above and outside, coated with setae on its
inner surface and produced on this side into a pointed and toothed
process. The third segment is the movable portion and is articulated to
the second outside this process. Like the fixed process it is curved and
toothed, but it is rather longer and ends in two points between which the
tip of the fixed process rests. The teeth with which these parts are provided
are much used in classification.
The pedipalpi are of six segments and are large efficient weapons,
very characteristic of the order.
The coxa is almost cubical and possesses no maxillary process; the
trochanter is also quite short. The third segment is long, and when at
rest lies directed backward, parallel with the side of the carapace. The
fourth segment lies at right to the third, pointing outward, and
the fifth making another right points forward. This penultimate
segment is often very large; it is continued on its inner side into a
pointed toothed process, which the freely moving sixth segment
closes, repeating the plan of the chelicerae on a much larger scale. The
edges of the forceps are provided with pointed tubercles, \Yhich may be
regular or irregular in size and disposition.
The chelicerae arc, however, much more than mere weapons with
which the scorpion attacks its prey or defends itself in combat. They
are carriers of systematically trichobothria, sense organs that
have been exhaustively studied by Vachon ( 1973). His work, mentioned
in Chapter 3, must be recognized as marking the birth of trichobothriotaxy;
and well illustrates the manner in which modern systematic
arachnology progresses as more and more precise attention is given to
matters of apparent detail.
The legs are of seven segments: the first pair are the shortest and the
fourth the longest. The coxae are very large and in such close contact
with one another that they form practically the whole of the lower
surface of the prosoma. The two posterior pairs are immovable, but
the first two pairs are movable and being provided with manducatory
lobes form accessory mouth parts. The
here is unique.
The second coxa is the larger and more conspicuous; it is subtriangular
in form, the base of the triangle lying to the outside, somewhat ind~nted
where the trochanter articulates with it. On the fore-edge of the coxa
near the apex of the triangle is a strong forwardly-directed blade or
apophysis which, at its distal end, meets its fellow in the middle line.
The first coxa is the smallest. It lies in the angle between the second
coxa and its process, and has a similar smaller process of its own which
lies outside and close against that of the second (Fig. 30). Thus the