182 THE ESSEX NATURALIST
one unit called the cephalothorax. The first pair of limbs or
chelicerae are adapted for grasping (like a crab's claw), the
second pair are sometimes prehensile, but in some orders, as for
example the harvest-spiders, they are sensory or mainly so. All
have four pairs of walking legs. While these characters give the
impression of a fairly homogeneous class, examination of the
orders which comprise it shows that this is not so. The two
orders to which the harvest-spiders (Opiliones or Phalangida)
appear to be most closely related, namely the true spiders
(Araneida) and mites (Acarina) must briefly claim our attention.
Without going into any internal and more precise details a
harvest-spider can be recognised in the field by these characters
(Plate 13) :
(i) the cephalothorax and abdomen are completely united from side to
side
(ii) the second pair of legs is the longest
(iii) there are two eyes usually placed on an ocularium (a raised part of
the exoskeleton) on top of the cephalothorax
(iv) there are two odoriferous glands whose openings can be seen (rather
like eyes) one at each side and near the front edge of the cephalo-
thorax. In a few species these are difficult to see
(v) no silk is produced
(vi) the reproductive organs are distinctive and can be extruded by
gentle lateral pressure on the abdomen in the adult animal.
Harvest-spiders differ from mites by their larger size and in
not having the rostrum of the latter whose mouth parts are
biting, piercing or sucking. The anal aperture in mites is a vertical
slit ; in harvest-spiders it is horizontal.
Confusion with true spiders is more likely to arise. But the
following emphasise the differences:
(i) the cephalothorax and abdomen of a true spider is united by a
small waist
(ii) the second pair of legs is not the longest
(iii) eyes usually six or eight; no ocularium
(iv) no odoriferous glands
(v) all spiders can produce silk and possess abdominal spinnerets.
Though true spiders and harvest-spiders are superficially alike
the above characters do not suggest a very close relationship.
This is supported by the fact that fossil spiders and harvest-spiders
have both been found in rocks of the Carboniferous period.
The interesting origins of the names Phalangida and Opiliones
are discussed by Dr. Bristowe (ibid.). This author believes that
Phalangium has been derived from phalanx—a formidable
soldier, because the name originally referred to a venomous spider
which Linnaeus confused with a harvest-spider since both lived in
fields. I feel that an alternative suggestion is worth considering.
Might not the name Phalangium be derived from phalange—a