Suggestion
After clicking on one of the following sections,
once the images are downloaded (which make take 2 or 3 minutes),
it is necessary to zoom in strongly (Control+wheel of the mouse)
on each image
for otherwise the individual organisms will
hardly be visible.
Clustering consists in the fact that,
without any external incentive,
scattered individuals gather closely together
thereby forming an entity of much higher density.
Among collective motions clustering
is probably the simplest and arguably the most important. Why?
A. Bees
B. Small ants
Small ants (2 observations in parallel)
Small ants (4 observations in parallel)
Small ants (8 observations in parallel)
C. Big ants
D. Drosophilas
Drosophilas (1st experiment)
Drosophilas (2nd experiment,
2 observations in parallel)
We would like to express our gratitude to Pr. Dou Fei
and Dr. Liu Xia from the Genetic Research Institute
of Beijing Normal University for their kind help in this experiment.
E. Beetles
We would like to express our gratitude to Pr. N.J. Suematsu
of the Department of ``Mathematics and Applications'' of
Meiji University in Tokyo, to Pr. Dou Fei
and Dr. Liu Xia from the Genetic Research Institute
of Beijing Normal University for their kind help in this experiment.
FILM 1: Observation of Euglenas outside of a
clustering area, magnification is x90 (format: mov)
FILM 2: Observation of Euglenas outside of a
clustering area, magnification is x225 (format: mov)
For his kind help
we would like to express our gratitude to Pr. Michel Gho
of the ``Cell cycle and cell determination'' group
(Laboratory of Development biology, University of Paris,
Pierre and Marie Curie).
* Clustering can occur only if there is an attraction
between individuals. If this attraction increases
for shorter distances then, by coming
closer together, the separate
organisms are able to strengthen the interaction between
them. This self-amplifying process is subject to the sole
limitation that individual organisms must remain separate.
As can be seen in the pictures given below,
the resulting cluster is a stable and mostly static system.
In short, clustering is a direct consequence of
inter-individual attraction and,
contrary to many other collective motions, it
does not involve elaborate dynamic behavior.
* The fact that the density of the cluster is much higher
(typically hundreds of times higher) than
the density of the initial
population often gives it special qualitative properties.
As a simple illustration one can mention the ``clustering''
of the molecules of a gas into droplets of water. Clearly,
the physical properties of water greatly differ from those
of a gas. The same is true for many other clusters. Cities
have characteristics which do not exist in low density countryside
habitat.
These big ants (Camponotus Japonicus) walk much faster
than the smaller ants used in the previous experiments. For that
reason one would not expect them to form a cluster so
easily. This is indeed confirmed
by observation although eventually they did
form a cluster. Actually, their behavior is fairly similar
to the one described by Lecomte (see the papers above) for
the case of bees: (i) clustering will only occur
above a density threshold (ii) clustering will take
longer than for small ants.
The question is: do drosophila form clusters?
The two experiments which follow try to provide an answer.
No clustering was observed but
this experiment had to be cancelled because when it was finished
we found out that the source of light located under
the container raised its temperature by some 10 degres.
Thus the drosophilas were not in normal conditions
(they moved much faster than at room temperature).
The experiment was repeated (see below) with a ``cold''
light-source.
It turns out that the drosophila used in this experiment
do not form
any cluster even for densities much higher than
those used previously for ants. In other words,
it seems that drosophila are solitary insects.
Whether or not this can be due to the fact that,
generations after generations,
they were raised in laboratory conditions
(as opposed to natural conditions) remains
an open question.
FILM: Clustering of beetles
The small beetles are ``Alphitobius diaperinus'',
whereas the larger are ``Tenebrio molitor''. Both species
are worms in their pre-adult stage and are sold in stores
particularly to feed fishes and birds.
The worm of the small beetle
is also called ``buffalo worm'' or ``lesser mealworm''.
The present experiment lasted 5mn and a picture was
taken every 5 seconds. The diameter of the cup was 15.5 cm.
The experiment was performed in the evening (19:45) under
fairly uniform artificial light.
As can be seen, the beetles begin to move toward the edge
of the cup. The same reaction is also observed for
ants. Once the beetles are along the edge, their movements become
more or less one-dimensional.
The experiment was performed on 5 October 2012 at the
Bee Institute in Kunming (Yunnan province, China). Many thanks to
Prof. Ken Tan and to his group.
F. Euglenas
The Euglenas are unicellular protists which swim in water.
Their length is about 50 micrometers.
Similarly to plants,
they contain pigments of chlorophyll.
Highly photo-sensitive, they are attracted by low intensity
light but above a given intensity
threshold light has a repulsive
effect.
When a solution containing Euglenas is subject to
strong light, a clustering process occurs within a
few minutes which produces a pattern of grey lines.
Microscopic observation shows a high density of Euglenas
around these grey lines (see the photos).
Observation of clustering Euglenas
(pictures)
FILM 1: Same film as above (format: gif)
FILM 2: Same film as above (format: gif)