Savory - Arachnida 1977

50 11. DE ARACHNIDIS
(iii) Inversion Phase: the curvature of the embryo is reversed and a
typical arachnid form is assumed.
Larval:
(i) Pre-larval Phase: the embryo is immobile, the metameres all
but invisible, the sense organs absent.
(ii) Larval Phase: spines, setae and tarsal claws appear.
Xympho-imaginal:
(i) Nymphal Stadia: l to x. The stadia are separated by ecdyses,
the number of these determined by the adult size.
(ii) Adult Stadium: sex organs mature and reproduction is possible.
An additional stadium occurs in species that undergo
a post-nuptial moult.
In an examination of this summary, the introduction of the term
larva is to be noted, for the arachnid larva differs from the larvae of
Insecta and Crustacea in that it does not lead an active life, feeding
itself and dispersing the species before undergoing a metamorphosis.
The larval spider is the object that breaks through the blastodermic
cuticle. Although it looks like a spider, its prosoma and opisthosoma are
at right angles to one another, it has no sense organs, and it is unable to
move, feed or spin. This condition may persist through two moults, so
that a distinction has been drawn between the "pre-larva" and the
larva. The third moult brings about so many changes that it almost
justifies the description of a metamorphosis. It produces a nymph
which can move, feed and spin. The nymphal condition lasts through a
number of stadia, which varies with the size to which the spider will
grow. There may be 12 nymphal instars, the last of which may be
visibly distinguished, and is always distinguished in males, as the subadult
instar. Nymphs resemble adults in all essentials except size, the
possession of secondary sexual characters and the ability to reproduce.
In general, it may therefore be said that the so-called larval period
is a stage of transition between embryo and nymph, during which the
organ systems are perfected. It is more in keeping with general
nomenclature to call it a prolongation of the embryonic state.
Araneists may well be advised to abandon their traditional habit of
neglecting or even throwing away the immature specimens that they
find, for these have much to teach us, and even the cast-off exoskeletons
left after moulting may be profitably examined. The successive nymphal
stadia of any one species are not readily distinguishable, but one
example of the gradual bodily changes is given by Vachon's investigation
of the regeneration of autotomized legs.
6. OKTOGEKY: GROWTH 51
He was able to distinguish six types of legs, differing in their trichobothria,
in the number of segments, and in the existence of smooth or
toothed claws. These are shovvn at Fig. 19. He has called this arthrogenesis,
and it is obviously an aspect of growth to which more attention
might wdl be given.
The above considerations are applicable to the growth stages of
pseudoscorpions, as described by Gabbutt and Vachon (1967, 1968). As
was described in the previous chapter, the false scorpion still in the
brood-sac is fed by its mother until it moults, and at this stage almost
justifies the description of larva given it by Vachon. Then it moults
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FIG. 19. "\rthrogerwsis of a spider's leg .. \fter \'achon. (I and II) Pre-larval stage;
( III) Larval stage; (IV, V and VIi Lan al stage.
and escapes into an independent existence as a protonymph; and by
successive ecdyses deutonymph, tritonymph and adult follow. In the
few species that have been inlcnsively studied these four instars can be
distinguished in a number of ways, such as micro-measurement of their
p1 oportional dimensions.
Features which, among others, provide the most readily perceived
distinctions between the stadia are the numbers of setae on the chelicerae
and carapace. On the chelicerae the fixed and movable fingers
are taken separately; on the carapace four rows of setae are named
anterior, ocular, median and postnior. The authors named have given
the following figures for two common European species: