Savory - Arachnida 1977

4:8
II. DE ARACHNIDIS
Fw. 18.
m!"chanism of embryo scorpion. After :\Iathew. (i) ovarian tubule;
embryo; (iii) appendix.
6
Ontogeny: Growth
mouth of the embryo. The embryonic chelicerae are modified to grasp
this teat between chitinized grooves in the specialized third somite.
of the embryonic pharynx shows that the nourishment is
actually sucked in.
The. development of a pseudoscorpion is characteristic. Two prelarval
stadia are distinguishable. At an early stage a pumping organ is
formed in connection with the pharynx of the embryo. This is a
muscular sac which forces nutritive fluid into the embryonic gut during
the whole of the gestation period. At hatching it becomes converted
into the mouth parts of the protonymph, which is retained in the
brood-sac or incubation chamber.
This is a peculiar feature of the order. Just before the fertilized ova
travel down the oviduct the accessory glands secrete through the
genital orifice a membrane which forms an incubation chamber. The
eggs are laid into this, in numbers which vary between two and 50 or
more. The incubation chamber remains in position below the opisthosoma
and is still in connection with the ovary, of which Vachon aptly
describes it as an external diverticulum.
Within this chamber or brood-sac the embryonic development of the
zygotes takes place. \Vhen the young are so complete as to be
able as false scorpions, they are still attached to the genital
the female, and are fed by the forceful actions of their mother on a
product of the ovary called "false scorpion milk". The little animals
swell as a result to three times their former volume and the incubation
chamber projects bulkily on each side of the mother's opisthosoma.
Ecdysis follows, producing the protonymph, which leaves its mother
and makes its way out of the breeding cocoon to begin life on its own.
The hatching of an arachnid from its egg consists of the breaking of the
chorion and vitelline membrane, and it may be that it is simply the
emerging from these coverings of the first instar. Alternatively, the little
arachnid may shed its cuticle once or even twice while still in the egg,
and emerge 'in its second or third instar. These differences follow
familial or generic taxa rather than specific or individual instances.
One of the characteristics of the class is that the first free-living form
is at once recognizable as an arachnid, and is not, in the generally
accepted sense of the word to be described as a larva, but rather as a
nymph. Invertebrates that share this resemblance between the adult
and newly hatched young are said to show "direct development".
Customarily the nymph is described as differing from the adult only in
its smaller size and its inability to reproduce, but among Arachnida
careful and detailed study of the younger stages has revealed organized,
steady progress in the course of growth. The young arachnid passes
through periods that can be precisely defined and recognized.
For Araneae, these have been named by Vachou and
as:
( 1) Embryonic Period: from fertilization of the ovum to the end of
inversion of the embryo.
Larval Period: from inversion to the appearance of an acti\·e
animal capable of an independent life.
(3) Nympho-imaginal Period: from leaving the cocoon until death.
Each of these. main periods may be further analysed:
Embryonic:
(i) Primordial Phase: from fertilization to the formation of the
germinal layers and germinal disc.
(ii) lV1etamerism Phase: from inversion to a segmented embryo, of
an elongated form and with appendages beginning to
appear.