Savory - Arachnida 1977

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308 V. HETEROGRAPHIA
use of a computer in analysis of the many measurements
that
recorded.
The following is a selection from among the many conclusions that
have been reached in attempts to improve our understanding of the art
of web-spinning as practised by Araneidae.
The electron microscope shows that the spiral thread of an uloborid
web is composed of two strands, each 0·5 !Lm in diameter. There is, in
orb-spinners only, a well-defined valve, furnished with muscles, which
determines the diameter of a thread. There is close resemblance between
this apparatus in Uloboridae and Araneidae, but detailed differences
show that all the muscles cannot be homologous. Convergent evolution
is suggested. There is definite regulation of silk synthesis within the
ampullaceal glands, and the amount of silk remaining in the lumen
of the gland, and therefore available for immediate use, affects the type
of web produced, probably by a feedback mechanism.
There are such differences between the webs of individual spiders
that an experienced observer can recognize the webs of a particular
animal, or of one from a particular cocoon, as well as one of a particular
species. The webs of young spiders differ from those of mature ones. If a
young nymph is taken from the cocoon or egg-sac immediately after it
has hatched, it will at first produce only an irregular tangle of threads
but if allowed to remain in the egg-sac until the normal time of its
emergence "definite maturing
have occurred, and the first
webs are of regular form though smaller in size (H.M. Peters).
The effects of a laser beam on the central nervous ganglion showed by
the spinning of webs with fewer radii or fewer spirals. The effects of
many drugs, taken orally when the spider drank sugar solution, have
been described. For example, amphetamine caused irregular spacing of
radii and spiral; scopolamine produced a wider deviation in spiralling,
and LSD resulted in unusually irregular webs. Other drugs affected the
size of the catching area and the length and thickness of the threads.
::V:Iany experiments by P. J. Peters in transferring spiders from one web
to another during the act of spinning showed the importance of external
stimuli from existing threads. Similar effects have been seen to follow
from such stimuli as the tensions of various threads and the angle between
adjacent radii.
Since work on the problems of orb-web construction is likely to
continue, an agreed nomenclature for its different parts is clearly desirable.
In Fig. 107 the terms suggested by Jackson (1973) and Savory
( 1952) are combined.
37
Courtship in Arachnida
The .existence of ~ourtship among invertebrates is a phenomenon that
reqmres explanatiOn. Among Arachnida it is widespread and is often
complex, so that there must be some essential which established the
habit in the first place, and some advantage which has favoured
retention and development.
It. is well known that the courtship of spiders was first observed in
detail by Dr and Mrs Peckham of Milwaukee as long ago as 1889.
The spid:rs they were watching were Salticidae, a specialized family
charactcnzed by eyes, keen vision and bright colours. The vivid
description which these authors wrote attracted attention; their conclusion,
which was based on the theory of sexual selection was unfortunate,
if, at that time, inevitable. After a number of m~re or less
isol~ted de~criptions of courtship by various writers, the subject was
agam studied by Montgomery in Philadelphia, by Gerhardt in
Germany, and by Bristowe and Locket in Britain. ·
The general conclusion that followed from the observations of most
zo?logists :vas that .courtship was a method of conveying to the female
spider the mformatwn that a male was in her neighbourhood, with the
corollary that he was not to be treated as an intruder, killed and eaten.
As lonl? ~s the courtsl:ip of spiders only is considered, the theory of sex
re.cogmtwn c~n be mam~a~ned, but when other Arachnida are compared
:vith th_em, d1ffer~nt opmwns are possible and the courtship of spiders
Is seen m a new hght.
By starting with spiders, the theoretical study of the behaviour gets
off on the wrong foot. There are four reasons for this.
1. The ~bserver ~f spiders is watching the phenomenon at the top of
the arachmd e:olut~onary sca~e. Historically, it was an unlucky chance
that sex behavwur IS very easily seen in spiders and it differs considerably
from that of the other orders.
2. The interpretation has been disproportionately influenced bv a
belief that the female "always" kills and eats th~ male, who ~as