  For any system, whether physical or non-physical, knowledge of the form and
strength of inter-individual interactions is a key-information. In an approach
based on statistical physics one needs to know the interaction Hamiltonian. For
non-physical systems, based on qualitative arguments similar to those used in
physical chemistry, interaction strength gives useful clues about the
macroscopic properties of the system. Even though our ultimate objective is the
understanding of social phenomena, we found that systems composed of insects
(or other living organisms) are of great convenience for investigating group
effects. In this paper we show how to design experiments that enable us to
estimate the strength of interaction in groups of insects. By repeating the
same experiments with increasing numbers of insects, ranging from less than 10
to several hundreds, one is able to explore key-properties of the interaction.
The data turn out to be consistent with a global correlation that is
independent of distance (at least within a range of a few centimetres).
Estimates of this average cross-correlation will be given for ants, beetles and
fruit flies. The experimental results clearly exclude an Ising-like
interaction, that is to say one that would be restricted to nearest neighbours.
In the case of fruit flies the average cross-correlation appears to be negative
which means that instead of an inter-individual attraction there is a (weak)
repulsive effect. In our conclusion we insist on the fact that such
"physics-like experiments" on insect populations provide a valuable alternative
to computer simulations. When testable group effects are predicted by a model,
the required experiments can be set up, thus permitting to confirm or disprove
the model.
