Savory - Arachnida 1977

56 II. DE ARACHNIDIS
usually moult less often; underfed spiders moult more often. l'v1oulting
does not always cease when a spider is mature; post-nuptial moults
occur annually or more often among spiders with long lives.
After an ecdysis has been successfully carried through, a spider may
often be seen to stretch, bend and straighten its legs continuously for
some time. It is during these minutes that the new exoskeleton hardens,
and these exercises prevent the hardening process from spreading across
the joints and rendering the limbs rigid. The preening, which takes place
intermittently at the same time arranges the spines and setae and causes
them to set in the correct directions.
Less is known about the number of ecdyses among Arachnida of the
other orders. The scorpion Buthus occitanus has been known to moult
seven times, and Guenthal ( 1944) has recorded eight moults for the
harvestmen Phalangium opilio. This was accomplished in 30 days at a
temperature of 30°C, which considerably hastens the rate of development.
At ordinary temperatures about l 0 days pass between one moult
and the next.
False scorpions have been found by Vachon (1933) to undergo
ecdysis four times. The first moult of the species Chelifer cancroides occurs
in the incubation chamber, when the egg hatches into the "larva", and
this as mentioned above proceeds in turn to a protonymph, deutonymph,
tritonymph and adult.
Muma ( 1966) has recorded eight or nine ecdyses in the life of the
solifuge Eremobates.
A most important aspect of this process of ecdysis is the regeneration,
or reproduction oflost limbs, that accompanies it. Nearly all Arachnida
have the power to cast an appendage if it is injured or seized by a
predator, but the loss is only a temporary one. A new limb forms
beneath the exoskeleton, and comes into use when the next ecdysis
occurs. This power of regeneration is not shown by harvestmen, which,
in fact, drop their legs with great ease, but it is known to exist among
scorpions, false scorpions and spiders, and has been exhaustively
studied by Bonnet ( 1930), working with the spider Dolomedes plantarius.
His conclusions were that lost limbs require three moults before they
attain their normal size, a fact which explains the asymmetry often to
be seen in spider's legs. More than one leg can be regenerated at a
time; in fact Bonnet pushed his investigations to the limit by keeping
several spiders which had lost all eight legs. Their almost immobile
bodies were carefully tended and supplied with water to drink. They
accepted flies offered to them in forceps, and ate as much as normal
specimens. When the time of ecdysis arrived, all eight legs reappeared.
The character of the new limbs has been studied in great detail by
Vachon (1941). As was mentioned earlier in this chapter, he distin-
6. ONTOGENY: GROWTH 57
quishes betw~e~ em?r~onic, pre-larval, larval and nymphal instars, and
among the dist~ngmshmg characters are the leg spines and the claws.
Thus Vachon discovered that if a spider loses a leg just after a moult and
later loses the corresponding leg just before the next moult, there will
appear at that moulting for the first loss a larval leg and for the second
an em~ryonic leg. At later ecdyses, the regenerated legs go through the
recogm~ed st~ges of development, but at an accelerated speed, one
stage bemg missed out; and it is also found that in the same regenerated
leg the segments arc not necessarily all at the same stage the distal
ones being "older".
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Length of life is ~,cery variable among Arachnida. Fundamentally
four types of longeVity can be recognized.
:rhe commonest is the life cycle of nearly all the small species, and
occupies rather less than a year. Eggs are laid in the autumn the winter
is passe:J in development, and the young hatch in the spring.,They grow
up durmg .the summer, are mature in early autumn, mate, lay their
eggs, and die before the winter.
(ii) . A _life cycle of more than a year is shown by Arachnida whose eggs
are laid m. the ~umr_ner an:J are hatched in a few weeks. The nymphs
p~ss . wmte~ m h1bernatwn; they mature in the spring, and, having
laid their eggs m the summer, they die in the autumn.
(iii) Sor;te Arachnida, including many of the largest, live for 2 to 5
years, havmg taken from 1 to 3 years to reach maturity. It will be understood
that most of the records of such lives as these have come from
specimens kept in .t~e safety of the laboratory, where they found equable
temperatures, sufficient food, and freedom from their enemies.
.(iv) Las:ly, there are some, chiefly among the great mygalomorph
spiders, which cast their skins annuallv and in favourable circumstances
may live for 15, 20 or even 25 vears. '
Size,. however,. is not a determining factor. Juberthie (1967a) has
shown ti:at the hfe cycle among the Cyphophthalmi may last for as
long as mne years.
O~e characteristic of: the life span of an arachnid is its dependence
on orcumstances. Specimens that are kept cool live longer than those
that are kept warn:; and specimens that are underfed live longer than
those whose food Is plentifuL It seems clear that a greater metabolic
rate wears out the body to a significant extent. and brings its life to a
quicker ending. ,
Abno~malities among Arachnida are no less frequent than among
other ammals. A two-tailed scorpion was described in 1825 and examples
of tl;is cont~nue to be reported. In 1917 Brauer exa~ined 5,000
embryomc scorpwns, and found evidence of double tails in 13 of them
as well as duplication of the posterior mesomatic somites.
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