Savory - Arachnida 1977

118 Ill. PROLES ARACHNES
13. THE ORDER SCORPIONES
119
FrG. 31. Sternum and pectine of Euscorpius.
(iiil sternite;
Fw. 30. ~\1outh parts of scorpion, showing the coxal processes on I and 2.
mouth is furnished with four blades from pedal coxae, which slovllly
triturate the food as it is held in the chelicerae.
The third coxae resemble the second in shape, but they have no
apophysis and are about twice as large. The fourth are even longer:
they do not broaden so much towards their distal ends, which reach past
the.centre of the first abdominal sternite.
The remaining segments of the legs are normal in form: their homologics
were discussed above. The tarsi bear two large curved claws
without teeth and below and between them is a third median smaller
claw. Some ~f the segments before the last may bear spurs at their
distal ends.
The sternum of all scorpions is a very small plate between the third
and the fourth coxae. In some genera it is only a narrow trans\·erse
strip of chitin, in others it is a small triangular plate and in the rest it is
pentagonal, as in all young ones.
Close behind the sternum (Fig. 31) is the plate-like genital operculum,
simple and inconspicuous, and immediately behind this, lying close to
the fourth coxae, are the pectines. These are peculiar appendages, quite
characteristic of the scorpions which have derived them from the first
book-gills of the Xiphosura. Their use has never been quite satisfactorily
described, and many different suggestions have been put forward. In
the early literature of scorpion biology at least half-a-dozen ideas as to
their function may be found:
(l) They detect the presence of food.
(2) They are used in mating to hold the male and female close
together.
(3) They clean the body and limbs.
( 4) are accessory respiratory organs.
(5) They act as fans, driving air to the lungs.
(6) They are secondary sex organs.
Most of these are fanciful imaginations. 1\lore recent opinions are
based on the fact that the pectines are plentifully supplied with nerves,
indicating that they may be some kind of sense organ. Experiment
seems to show that they have several functions:
( 7) They react to the dryness or moistness of the air.
(8) They detect vibrations of the ground, giving warning of the
approach of enemies or prey.
(9) They determine whether the ground is smooth enough or hard
enough for the depositing of the spermatophore.
The back of the pectine is made of three pieces, the proximal part
the longest and the middle one the shortest. The number of teeth is
different in different and varies from four to over 30.
The opisthosoma is manifestly divisible into a mesosoma and a
metasoma, of seven and five somites respectively. The of the
first six mesosomatic somites are of gradually increasing length, tht:
seventh is characteristically trapezoid in shape. The pleura between
them become much stretched during pregnancy. Only five sternites are
visible, the first four eat:h a pair of slit-like openings of the
book-lungs.
The somites of the metasoma or "tail" are subcylindrical, the tergite
and sternite of each being fused to form a ring of chitin. The upper side
has a median groove, while the sides and lower surface bear a variable
number of parallel longitudinal ridges of small spines. The last
bears the telson, a bulb-like reservoir which contains the poison gland
and which is produced into the sharp curved point of the sting A,
Fig. 28).