Savory - Arachnida 1977

54 II. DE ARACHNIDIS
FIG. 21. Newly hatched, or post-embryo, solifuge.
( 1972) of the species Cryptocellus pelaezi. From the egg there emerges a
larva, characterized by the possession of only three pairs of legs. The
tergites of its opisthosoma are well separated and the tubercles on the
carapace and elsewhere are scattered without arrangement. The larva
is followed by three nymphal stages, the first of which differs from the
larva mainly in the appearance of the fourth pair of legs. The second
and later nymphs bear a longitudinal furrow on the carapace, and the
tubercles begin to take on a definite arrangement. The third nymph
continues this arranging, while the longitudinal furrows on carapace
and abdomen become more marked. All stages up to this are creamcoloured
but the adults are reddish-brown.
The fi~e stages differ in the number of segments into which the tarsi
of the legs are subdivided. These are as follows:
Leg I Leg 2 Leg 3 Leg 4
Larva 2 2 2
First nymph 4 3 2
Second nymph 5 4 4
Third nymph 5 4 5
Adult 5 4 5
6. ONTOGENY: GROWTH ss
No arachnid life history is more remarkable than that of the Cyphophthalmi,
which lasts for nine years. This is described in Chapter 25.
The period of actual growth in the true sense of increase in size is
confined to a short interval, probably not more than a couple of hours.
following the moulting or casting of the exoskeleton. The ecdysis, as it
is properly called, is not difficult to forecast in spiders, for their legs
change colour a day or two before the moult, darkening until they are
almost black. The spider also ceases to feed and its silk glands are
affected, so that the fluid silk within becomes more viscous and the
spider scarcely spins a thread.
Among harvestmen there are in general no such warnings, but
false scorpions show a change of behaviour. They wander about, as if
they were looking for a suitable place, and when they have found one
they build a protective cocoon or moulting chamber.
In all the orders the operation is the same in its essentials. A moulting
fluid appears to be secreted between the old exoskeleton and the new
one, acting in part as a lubricant and in part as the cause of the pressure
which produces the first split. This generally runs round the sides of the
prosoma and along the middle of the lower surface of the opisthosoma.
The carapace easily falls off and the cuticle of the abdomen shrivels.
The difficult part of the business in all orders is the extraction of the
eight legs from their old cases. For a spider this may take anything up to
half-an-hour or more, as the legs are slowly pulled out by a series of
rhythmical heaves. Among harvestmen, whose legs are longer, the extraction
is assisted by the chelicerae, which grasp the legs and pull them.
Araneomorph spiders normally hang themselves up on silk threads
from the spinnerets during the process of ecdysis. Harvestmen do not
produce silk of their own, but they usually moult in a suspended
position, with the legs of the fourth pair hooked on to some suitable
support. Their skeletons may sometimes be found on walls, hanging on
a thread of spider's silk; but in the absence of such means of support
they are able to moult, like false scorpions, standing up.
In any account of ecdysis the obvious questions which suggest themselves
are: how long passes between one moult and the next, and how
many times does an arachnid moult?
Bonnet ( 1930) has shown that the number of ecdyses that a spider
makes depends on the size that it is destined to reach. There may be
only four moults for a spider under S mm long, seven or eight for one
twice as long, and ten or 12 for a larger one still. At the extremes stand
Gertsch's observation that a male Mastophora cornigera moults twice
only, and Baerg's record of 22 for Eurypelma californica.
Further, the number is not constant, even within a species, for it is
affected both by sex and by food supply. Male spiders, being smaller,