Savory - Arachnida 1977

158 Ill. PROLES ARACHNES
(vi) Lobed glands, peculiar to the Theridiidae.
(vii) Flagelliform glands, which supply the core of the viscid spiral
threads.
(viii) Cribellum glands, which feed the cribellum.
Attention may be directed to the flagelliform glands, not mentioned
by Apstein or Warburton. They were discovered by Sekiguichi in 1952
and their existence was later denied. However, Anderson ( 1971) reaffirmed
their existence, adding that they were "not very conspicuous".
He also distinguished between large and small ampullaceal glands.
An investigation of these glands has been made by Millot ( 1929-3la).
He found it impossible to retain this classification, for the glands
are of many kinds and the types represented in one family are only
rarely similar to those found in others. There is indeed a close similarity
between all silk glands in histological structure and produce, but the
form and disposition of the glands have been evolved independently in
each group. There is no standard arrangement that can reasonably be
described as the general type.
The nature of the silk glands and their relation to the poison glands
is first suggested in the fact that the two kinds of gland occupy the same
position in false scorpions and spiders. Their homology was proved by
Millot's work ( 1929) on the spider Scytodes. Here the cephalothoracic
gland is found not to be homogeneous throughout its length, but to
consist of two kinds of cells. Those in the posterior lobe produce a silklike
glutinous substance, which the spider spits out at its insect prey,
a method of attack which is unique among Arachnida. The cells in
the anterior part are typical poison-producing cells, such as occur in the
venom glands of all spiders. Individuals differ from one another in the
proportion of the two kinds of cell present, with the result that the bites
of some do not kill their victims, while the bites of others are quickly
fatal.
This homology between silk and venom is maintained when we turn
to the silk glands of Pseudoscorpiones. In this order the silk glands lie
in the forepart of the prosoma, their ducts pass along the chelicerae and
open at the so-called galea at the tip of the movable finger. Thus gland,
duct and orifice of the silk-secreting apparatus of Pseudoscorpiones are
completely homologous with the gland, duct and orifice of the poisonproducing
apparatus of Araneae.
Pseudoscorpiones use their silk solely for their own protection. They
close the entrance to their hiding-places with a curtain and they build
small nests of solid particles such as grains of sand, brushed over with
layers of silk. In this nest they moult, incubate their eggs and hibernate.
The spinning activities of Acari have been summarized by Andre
18. THE ORDER ARANEAE 159
( 1932). The chief spinners are the Tetranychidae, which often live in
colonies on trees and plants, and sometimes smother both sides of the
leaves with layers of silk. Under this silk sheet they find shelter for themselves,
their eggs and larvae .. Many species are known from all parts of
the world, and a single species does not habitually spin on the same
kind of tree, but may be found on different species.
The silk of mites is so fine that a single thread is invisible to the naked
eye. It is secreted by prosomatic glands, comparable to the silk glands of
Pseudoscorpiones and the mixed silk and venom glands of the sicariid
spiders. The ducts of the silk glands open inside the mouth, and by
observation from beneath the liquid silk can be seen issuing therefrom.
It is drawn out by the chelicerae, whose stylets or movable fingers manipulate
it, sometimes assisted by the pedipalpi. The silk is never viscid.
In other families, a number of isolated examples of silk production
have been reported, some of them with doubtful accuracy. Almost
always the structure produced is a sheet-like shelter under which the
mite moults or deposits its eggs. As examples there may be mentioned
the species Analges passerinus, which spins nests on the bodies of birds
under the feathers, where the nests may be found containing eggs and
young; and Oribata castanea, which spins colonial tents under stones,
sheltering 40 or more individuals.
DISTRIBUTION
The Liphistiomorphae are composed of two extinct families and one
recent one. This includes but ten species, interesting because they are
the most primitive living spiders (Fig. 56). Spiders of this sub-order
were dominant in Palaeozoic times, but their survivors are limited to a
relatively small area in eastern Asia (Fig. 57), where they live, some in
caves, some on hills and some in the jungle. They make silk-lined burrows,
closed with a trap-door.
The Theraphosomorphae include the true "trap-door spiders" or
FIG. 56. Liphistius batuensis.