Savory - Arachnida 1977

82 II. DE ARACH::-.;IDIS
declined to zero just before dawn. The content of 5-HT showed the
same peak at 2.00 a.m. The distinctive feature, which agrees with that
found in other animals, is that the occurrence, rather than the duration
of activity, is constant.
Spiders and harvestmen are not exceptional among arachnids in this
respect, for a similar rhythm is detectable in most, if not in all the other
orders. As an example, one may quote the observation of \Veygoldt
( 1971) that the Amblypygi in his laboratory showed three peaks of
activity during the day, the first and most pronounced just after sunset,
a second before dawn, and the third and weakest at about noon.
Similarly, Cloudsley-Thompson ( 1973) has described the activity of the
scorpion Butlwtus minax, which rises to two maxima during each day,
by reacting to the change from light to dark and, in daylight, to rising
temperatures.
The activity of many animals is similarly regular, but the mechanism
that controls them is often unknown. The rather abrupt changes that
are to be observed suggest that a series of neurophysiological events
must be involved. In any case the similarities between vertebrate and
invertebrate behaviour, taking the form of response to the internal
environment, is extremely interesting.
9
Zoogeography: Distribution
The distribution on the earth of any group of animals is the result of a
combination of the place of origin, the means of travel and the nature
of the obstacles encountered en route. The zoogeography of Arachnida
is no less interesting than is that of any other class.
Early considerations suggest that four categories can be recognized.
(i) Arachnida which, though confined to the hot tropical or very
warm sub-tropical belt are, however, more or less continuously
represented therein-scorpions, theraphosid spiders, Amblypygi and
Uropygi.
(ii) Arachnida which are also tropical or sub-tropical, but which
are sporadically and discontinuously distributed therein-Palpigradi,
Ricinulei, Schizomida and liphistiid spiders.
(iii) Arachnida which spread to the limits of the temperate zones­
Opiliones, Pseudoscorpiones, Acari and most spiders.
(iv) Arachnida which do, in fact, spread into the polar regionscertain
spiders, Opiliones and mites.
These may be taken in turn.
It is, in fact, very obvious that in the tropics the Arachnida are
represented by some of their largest members. The scorpions, Solifugae
and theraphosid spiders are groups which average more than 2 cm in
body length, a great contrast to all the other orders, in which the body
length averages less than a centimetre. This generalization is in striking
contrast with that usually known as Alien's rule, which states that in
any one group the polar members are larger than the tropical ones.
This rule, however, was based on observations on warm-blooded
mammals. The ratio area : volume is necessarily smaller for a large
animal than for a small one, so that the former loses heat less rapidly
than the latter. It is clear that for cold-blooded animals like Arachnida
the rule cannot be accepted.
The most remarkable of the tropical Arachnida are those that are
able to survive in the hostile conditions of the deserts that cover so