Savory - Arachnida 1977

276 IV. DE ARACHNOLOGIA
Thomisidae, which he describes as the ideal laboratory spiders, capable
of living in anything. Bonnet's great work on Dolomedes is a model of
its kind, unsurpassed in extent and fullness of detail, while in his study
of the life-history ofNephila, the same author has succeeded in hastening
the development of the females and retarding that of the males, so
that members of the same cocoon reached maturity simultaneously,
a striking modification of natural events.
In addition to this it is a fact not sufficiently recognized that the
Arachnida in general and perhaps spiders in particular provide very
suitable subjects for the study of animal behaviour. They are essentially
animals of the small-brained type, richly endowed by heredity with
instinctive modes of behaviour but without the power to learn that is
possessed by big-brained creatures. It is often unnecessary to postulate
the existence of a thinking or conscious mind in interpreting their
actions, so that Arachnida may successfully be used by mechanistic
biologists as examples of automata or organic machines.
It is probably for this reason that they are so ready to live an undisturbed
life in our observation cages. On occasions in the past,
experiments on insect behaviour have yielded untrustworthy results
because the insect did not exhibit its natural reactions when in captivity,
and Acari, Solifugae and, sometimes, Opiliones have been found to
give untrustworthy results due to behavioural changes when in captivity.
But these are on the whole exceptional difficulties. The circumstances
of captivity seldom disturb the placid and amenable arachnid, its
mechanical responses are called forth and its behaviour can be measured
and often predicted, as if one were working with a galvanometer or a
spectroscope.
An important feature of the recording of observations made of an
active animal is due to the physical limitations of the human observer.
The forgetting of a detail may be avoided by immediate note-taking,
but it may well happen that by the time the note is made the animal
has proceeded to the next stage of its operations, and this may be
completed before the hurried observer looks at it again. This can be
avoided by those who possess the necessary equipment by making
an immediate report of everything seen into a tape-recorder, from which
it can be recovered and repeated as often as necessary.
REARING
Bonnet's work on Nephila, mentioned above, as well as that of other
competent technicians, affords proof that the life history of an Arachnid
can be accurately determined by raising specimens from birth to
maturity. The eggs of spiders, harvestmen and false scorpions usually
32. PRACTICAL ARACHNOLOGY 277
hatch in normal laboratory conditions without difficulty. They run
opposing risks from desiccation, which effectively kills them, and damp,
which encourages the growth of equally fatal moulds. The general
advice to "Keep ova not too warm and not too wet" can only be
successfully followed after trial and possible error. Where conditions
are controllable a temperature of 20°C and a relative humidity of near
40% are usually near the optima.
After their escapE from the egg membrane young spiders must be
kept apart in separate vessels; other young arachnids are seldom equally
cannibalistic. They need food, and as a rule will accept most of the
small invertebrates that normally accompany them in lEaf litter. In
many laboratories the culture of Drosophila is a continuing operation
and provides a readily available source of nourishment. It must be
remembered that Arachnida on the whole feed occasionally and digest
slowly; also that they usually cease to feed just before ecdysis. Those
who deal with exotica, such as scorpions and Solifugae, can replace
Drosophila by locusts, which are as easily cultured.
The chief features to be determined are the number of ecdyses that
take place between birth and maturity, the length of the intervals
between successive moultings, and hence the total life span. These
differ from order to order and even from family to family.
An unexpected difficulty is encountered in the rearing ofharvestmen,
and may, perhaps turn up with other orders. Harvestmen possess the
advantage that they do not need to be given living prey, but they
counter this by dying an early death after the third or fourth ecdysis.
Klee has given both explanation and cure. The cause is the constancy of
humidity in the cages, which prevents complete hardening of the new
cuticle after moulting. The conditions of nature are more closely
followed if the lid of the cage is opened daily to allow the humidity to
fall from 100%) to about 40%.
Rearing in the laboratory and the consequent production of a complete
timetable for the life of a species is one of promising lines of
straightforward arachnology at the present time. The cast-off exoskeleton
deserves the closest scrutiny, as one by one they display the
stages of growth. The admirable work of P. D. Gabbutt on false
scorpions and of .Max V achon on spiders of the genus Coelotes are
examples of this. The knowledge they have provided often makes it
possible to determine the instar of a given specimen from details of its
leg joints or its setae.