Savory - Arachnida 1977

80 11. DE ARACHNIDIS
results from sheltering behind words. Some advance has now been made
from that position, so that we now look upon instinctive actions as being
reflex acts, following one another in predetermined succession, coming
often under the influence of external circumstances, so that a symmetry
of activity is produced, and often also under the influence of
internal conditions, so that new types of activity constantly appear.
This union of external and internal factors and the realization of the
nature of the latter is important. Instincts are something more than
reflexes, but this "something" is of a material nature, secreted by glands;
it acts as a chemical compound distributed by the blood. Thus it becomes
at once subject to direct study, and mere difficulties of technique
are the obstacles to a full knowledge of the nature of every hormone, of
the way in which it is produced and the physicochemical nature of its
mode of action.
An exceptionally fine analysis of an instinctive action in this way has
been made by Peters ( 1931-3), in his study of the capture of insects by
the common spider Araneus diadematus. He concludes that a series of
stimuli, each followed by a characteristic reaction, produces the habit,
and he has broken down a process, which many have watched and
which appears to be a continuous operation, into the following steps:
STIMULUS RECEIVED
I. Vibration of threads of web
2. Struggling of prey
3. Contact with prey
4. Contact with silk wrapping
S. Chemical stimulus of short bite
REFLEX RESPONSE
Movement to centre of disturbance
Long bite
\'\Trapping or enshrouding
Short bite
Wrapped prey is carried off
An analysis, similar to this in every essential, has been made by
Homann ( 1928), who studied the approach of the jumping spider
Evarcha blancardi towards its victim. Here again the outward appearance
is one of a steady stalking, and here again the separate steps have been
recognized and isolated:
STIMULUS RECEIVED
I. Image in posterior lateral eye
2. Image in anterior lateral eye
3. Images in both anterior lateral
eyes
4. Images in anterior median eyes
REFLEX RESPONSE
Turning of spider's body towards
object
Turning of spider's body continues
Spider begins to move forwards
Spider begins to creep towards
victim, or to court a female
8. ETHOLOGY: BEHAVIOUR 81
Finally, an aspect of arachnid behaviour which appears as a characteristic
feature in the lives of many is due to the rhythmical nature of
individual activity. It has been said above that an arachnid spends a
large proportion of its life waiting, and doing nothing; to this there
must be added a recognition of the fact that idleness is often interrupted
at regular intervals.
Rhythmic behaviour, which is by no means limited to the Arachnida,
is of two kinds. There arc exogenous rhythms, which are responses to
regular recurring changes in the environment; and these are to be
contrasted with endogenous rhythms, consequent upon internal changes
within the organism. The former tend to disappear if the particular
stimuli are removed; the latter persist even when the animal is kept
under constant conditions, artificially maintained.
Probably the most familiar example of rhythmic behaviour among
Arachnida is the simple observation that the spiders in our gardens are
usually seen to be spinning their webs in the evenings, while there are
other species that are equally accustomed to spin th~ir ~ebs just befo~e
the dawn. An example of precise observations of th1s kmd of rhythmic
behaviour is that described by Nlarples ( 1971) concerning the common
species ,Zygiella atrica. The habit of this spider is to spend the day _in
shelter, though in communication with its web, and to take up 1ts
position in the centre of its web during the night.
Hourly observations of 127 webs gave the following result:
5.30 a.m.-1 0.00 p.m.
10.00 p.m.-10.30 p.m.
10.30 p.m.-11.00 p.m.
4.00 a.m.- 5.30 a.m.
No spiders in the webs
Spiders beginning to occupy hub
All spiders occupying hubs
Spiders spinning webs
Behaviour of this kind, clearly associated with the intensity of the
light, is common enough among Arachnida of many orders. It is
particularly obvious among Opiliones, as may be seen by anyone who
keeps harvestmen in cages. Their rhythms were examined by ~dgar
and Yuan ( 1968), working on Phalangium opilio and seven speCies of
Leiobunum, with the use of a kymograph. Their results showed that
under all ordinary conditions 90% of their activity occurred between
6.00 p.m. and 6.00 a.m. .
Fowler and Goodnight ( 1966), working with Leiobunum longzpes,
compared its activity with the secretion of 5-hydroxytryptamine. This
compound is widespread in animals and plants, its presence can be
readily shown and the amount measured.
The results obtained showed that activity began about two hours
before sunset rose to a maximum at about 2.00 a.m., and then rapidly
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