Savory - Arachnida 1977

5
Embryology: Development
In all Arachnida the male and female sexes are recognizable and ova
normally develop only after fertilization by a spermatozoon. The course
of gametogenesis has been followed in only some of the orders.
Oogenesis produces both ova and yolk, the former being produced
after the casting out of t>vo polar bodies from the secondary oocytes.
l\Iuch of the cytoplasm in an ovum is converted into yolk, which appears
first in the form of droplets. These collect in lines radiating from the
egg-nucleus, which is drawing nourishment from the surrounding
lymph.
Spermatogenesis often includes a proportion of amitotic division,
resulting in the formation of two types of spermatozoa, both of which
have been seen in the spermathecae of spiders. This amitosis was
described by ·warren in 1925, and a parallel formation of two kinds of
spermatozoa has been demonstrated by Juberthie (1964) in Cyphophthalmi.
The phenomenon is also known among insects and molluscs.
The diploid number is not the same in all orders, nor even in the
same order. In the scorpion Buthus it is 20 to in Opisthocanthus it
is 80 to 100. Yaginuma (1964) recorded for three species of scorpion
the haploid numbers of 27, 12 and 11. In a table summarizing results
from other species this number varied from 3 to "about 60".
In both Cropygi and Amblypygi 24 autosomes have been counted,
together with one X-chromosome. Among spiders, counts have given
18 for Anyphaena and 48 for Dugesiella. The chromosomes of 57 species
of spiders from 17 families have been described by Suzuki ( 1954). He
found that the haploid number varied from 4 to 24, except in
Heptathele, where it was 48. He further noted that the number was
higher among the more primitive families of the Theraphosomorphae
than the more specialized families, where 15, 13 and 12 were by far the
commonest counts. Generally the spermatocytes carry two X-chromosomes,
X 1 and X 2 , which are together present in half the spermatozoa,
but in a few species three or even four X-chromosomes were detected.
5. EMBRYOLOGY: DEVELOPMENT 45
In Opiliones the diploid number varies from 32 to 16, and there may
be one X-chromosome.
For many years there have been reports of apparent parthenogenesis
among Arachnida. Camp bell ( 1884) wrote of a Tegenaria parietina
which spent much of its life in captivity and can have had no chance of
meeting a male. From a cocoon that she made t\vo young spiders
emerged. Damin (1894) told of a Filistata testacea which moulted twice
in captivity and laid a number of eggs, of which 67 produced normal
nymphs.
There have been several records of incomplete development of
unfertilized ova and a most interesting account by Machado (1964)
of the spiders Theotima. No males and no spermatozoa in the
spermathccae of three species of this genus have ever been seen, and
after laboratory investigation ::\Iachado is confident that this is a case
of parthenogenesis.
Relatin scarcity of males is well known in the harvestman Jlegabunus
diadema. Phillipson ( 1939) took one male and 406 females of this
species and kept ten of the latter under observation. Thirteen batches of
were produced and young nymphs hatched from them. Equally
convincing reports of parthenogenesis or partial development of
unfertilized eggs have referred to Phalangium opilio.
All reliable investigations of the proportions of the sexes in the
recently hatched young agree that their numbers are approximately
equal, and it may therefore be assumed that the sex of the individual is
determined in the usual way by the chromosomes of the male gametes.
Fertilization occurs during the process of egg-laying, when the sperm
are released from the spermathecae, enabling one of them to enter each
egg before the hardening of the chorion and vitelline membrane.
The development of the zygote which now follows cannot be
adequately described in general terms. The most detailed
have all been devoted to the eggs of spiders, but at almost eYery
the process is different in one of the other orders, and the embryology
of some of these is at present but partially or scarcely known. A selection
of topics may well give a better idea of the diversity of arachnid embryology
than any attempt to construct a continuous
bristling
with exceptions.
The early divisions serve to underline this statement, since at least
four kinds of segmentation have been described. Total segmentation is
confined to a few mites and scorpions; among spiders and false scorpions
segmentation is at first total, but is soon replaced by merely superficial
divisions, and this superficial division is found in most of the other
orders, as it is among insects. In some scorpions segmentation is
discoidal.