Savory - Arachnida 1977

306 V. HETEROGRAPHIA
a number ofvertical threads, which run downwards to the ground and
at their lower ends have viscid drops that delay crawling insects.
These viscid drops place the theridiid web in the position of being the
precursor of the orb-web. \V ere the leading threads to be increased in
number until they surrounded the refuge, and were they then joined by
intercommunicating threads, the basic design of the orb-web has sprung
into existence.
\!Vhether this is so or not, the fact that Latrodectus webs spun in the
gra~s are based on the same principle brings into consideration two other
examples ofweb design in the Agelenidae and Uroboridae.
Anyone who has kept our familiar Tegenaria in a box knows that the
spider first establishes itself in one corner of its cage, and during the
first evening draws out a small number (four or six) of single, unconnected,
radiating threads. These are attached to distant spots on the box
sides. Later, treading on these threads, the spider connects them with
cross-threads, using the swaying, side-to-side movements so characteristic
of the family. As this behaviour is nightly repeated the web grows
in thickness and strength though not appreciably in area.
Lamoral ( 1970 in litteris) has drawn my attention to an Uloborus found
in South Africa which spins a calamistrated, funnel-shaped orb-web.
This is shown diagrammatically in Fig. 107.
This, we believe, represents the primitive type of orb-web. Its relation
to more advanced webs is shown by the fact that though in these
webs the shelter or refuge is some distance from the web itself, the
signal thread is still connected to the central hub. A point of some importance
is that the principle is demonstrated equally by calamistrated
and non-calamistrated webs, and so strengthens the modern belief
that the presence or absence of a cribellum is an almost insignificant
detail.
An overall view of these four webs, namely those of Latrodectus,
Titanoeca, Tegenaria and Uloborus, give strong support to the hypothesis
that all webs grow from the same fundamental course of movements.
Further consideration should be given to the application of this
hypothesis to other web patterns, for example those of Hyptiotes and
Alloepeira.
The orb-web will always be the one that provokes the most bafliing
problems in web study. Its beauty is not a biological matter, but its
symmetry is, for in its apparent symmetry and its deviations from perfect
regularity must lie the clues to its construction.
There may be recognized certain stages of web-spinning, which
merge into a continuous process, and we must say that instinctive
actions are responsible for their orderly succession. We believe that to
some extent the directions in which the spider moves are determined
36. THE SPIDER'S WEB 307
FIG. 107. 0Jomenclature of the orb web.
A, Bridge thread; B, frame thread; C, radius; D, divided radius; E, mooring thread;
F, hub; G, strengthening zone; H, free zone; K, capturing zone; l\f, viscid spiral;
:\, sector.
by the tensions of the threads on which it treads, and evidence of this
kind builds up to the conception of the web as a gradually de\·eloping
field of forces, to which the instinctively driven spider is compulsorily
obedient. One thing appears to be certain; that when once the spinning
has begun it continues in a self-determining manner. Tilquin ( 1942)
suggested that the form of any one orb-web is dependent on a few
initial proportions. He depicts the web as a dynamic whole, in whose
growth physical influences play a dominant part in guiding the responding
spider.
Tilquin's contribution has been followed by much careful investigation
of web problems, carried out by \Vitt, Reed, Peakall and others.
The Annual ~Jeeting of the American Association for the Advancement
of Science included a symposium on web-building spiders; this work
has continued and results have owed much to improved photographic