HOW TO CARRY OUT EXPERIMENTS WITH LIVING SYSTEMS: PRACTICAL TIPS

A. Dialogue with Nature

Back in the 18th and 19th century, physicists  were able to
set up new experiments within a few days. For instance in the month 
following the 
successful Foucault experiment related with the rotation of the
Earth around its axis, similar experiments were set up in Germany, Great
Britain and Italy. Moreover, within the same month,
a similar effect was demonstrated by Auguste Bravais by using a conical
pendulum.

The ability to test an idea quickly through new
experiments is probably a crucial requirement for a fruitful
``dialogue'' between Nature and theoretical
understanding. Unfortunately, with the increased cost and sophistication
of new physical devices (whether accelerators, 
telescopes or detectors of gravity waves) the dialogue between theory
and observation has become more difficult. For instance, one may recall
that the string theory of elementary particles started 
over 40 years ago but that its connections with observation still remain
elusive.

Trying a new experiment is asking Nature a question.
If the experiment has been well designed, Nature will provide a clear
answer. Thus gradually, this dialogue will lead to a better undestanding.

As far as ``physics-like'' 
experiments on living systems are concerned, there are two
possibilities.

 (i) The experiments can be done by entomologists willing to
cooperate with physicists. Thus, our group in Beijing has 
a fruitful collaboration with the team of Prof. Xu in Guangzhou. 
However it seems that such cooperations are fairly rare because of the
cultural divide between the two fields. 

 (ii) The second possibility is that physicists set up and
ran their own experiments. At first sight, the idea of physicists working
on populations of ants (or other insects) may seem weird. However, conducting
such experiments is not very different from conducting experiments in physics.
In a sense, it is even easier because it requires less sophisticated devices.
Perhaps the most difficult part is to know how to start. That is why below we provide
practical advice. Once physicists can do their own experiments
they will be able to establish a more direct dialogue with Nature and this
should make progress faster.

B. Practical tips

The first and probably the most important decision concerns the selection
of the living organisms. There are several possibilities. We discuss them below
by order of size of the organisms. In this respect, one must keep in mind that most 
collective properties are better displayed in large populations. This would suggest to
select rather small organisms. Another important property is of course whether
these organisms belong to a solitary, gregarious (living in groups but without
elaborate social organisation) or social insects (mostly ants, bees and termites). 

 (i) ``Dictyostelium discoideum'' is an amoeba about 5 micrometers in size. 
In some conditions these organisms adopt a gregarious behavior and form 
large colonies. This aggregation process 
has been actively studied in the 1970s and 1980s.
However, the small size of these organisms makes bare eyes observation impossible. 
``C. elegans'' is a worm about 1mm in size which has been widely used as a laboratory
model in genetic studies. Under starvation conditions it also exhibits an aggregation process.

 (ii) Drosophila are fruit flies which are about 3mm long. As they are much used 
in genetic research, it is easy to obtain them from biological laboratories. 
They are usually seen as solitary insects. As such, they are good models
for a comparison with social or gregarious insects.


 (iii) Ants exist in many sizes which, depending on the species, 
range from one or two millimeters 
to 15mm. In summer time it is fairly easy, at least for some species,
to collect them by putting a bait (e.g. some sweet food)
near the entrance of their colony. Because of their efficient
foraging and 
recruitment organization, the first ants may arrive some 5 minutes after the bait 
has been left and
after 45 minutes there may be about one hundred ants on the food.
However, this way of collecting ants has many disadvantages: it does not work
in rainy or cold weather, it is fairly difficult to determine the species of the 
ants, one cannot be sure that all the collected ants
come from the same colony,
for some species (especially big ants) this method
simply does not work because only a small number will gather around the food. 
A method that is much more convenient is to buy the ants. For instance,
on the Chinese website ``Taobao'' it is possible to buy entire colonies 
belonging to various species for a reasonable price. 

(iv) ``Tenebrio molitor'' is a species of beetle that is used in laboratory experiments.
It measures about 15mm and is considered a gregarious insect. Its larvae (i.e. 
the stage before the transformation into an adult) is a worm which is sold for the
purpose of being given to birds or fishes. Some related species are of smaller size and may
therefore be more convenient.

(v) Small fishes (less than 3cm in length) are sold fairly cheaply 
as fodder for big fishes.
Although more expensive, gold fishes (or similar species) can also be used.
As shown by the pictures on the first page of the webpage, they display clear
group behavior. 

Useful devices

Once one has got the insects the next problem is how to use them
in an experiment. To be specific let us assume that the insects are ants
and that one wants to dispose them on the nodes of a lattice in order
to study the range of their interaction. 

To do that it is very convenient to make the ants ``sleep'' by flooding them
with carbon dioxide. They will become motionless within a few seconds, and will
not wake up for several minutes which leaves enough time to put them
in the right places. Small bottles containing compressed carbon dioxide
can be bought fairly easily because they are sold for various purposes, such as 
making making spakling water or accelerating the growth of algae in an aquarium.

Needless to say, once the ants wake up one must prevent them from leaving the
container. There are at least two methods for
that. 
The first one is to cover the vertical sides of the container with a 
liquid called Fluon (basically it contains PTFE=Polytetrafluoroethylene, 
which is also called Teflon). Once it has dried this layer of Fluon will prevent the 
ants from climbing up the sides of the container because their feet will not
stick to the surface.
Of course, this method does not work with fruit flies. In this case one can
make a container of any desired shape by cutting out a piece from
a slab of flexible PVC of about 3mm in thickness and cover it with a glass plate.