Papilio machaon - osmetrium, 2nd

[Martin Dilling]

Hello Bill: Your theory of rare food competition is quite agreeable.

P. machaon is a low-density-species, especially here in central europe,
where suitable host plants can only be found at very special places
(special soil, warm micro climate etc.). Both, male and female, have to
fly to special mating places, typically mountains or high rocks, for
which with we have the nice word "hill topping". Then, the female
swallowtails have to spend much energy to fly dozens of square miles
around, searching for the right food plant to place their eggs. She can
only get along with this task if she was one of the fattest caterpillars
in town ;o)

But: Compared to saturnidae, you can find only one or two eggs on one
plant (meaning umbelliferae, don't know about - bigger - citrus), so
there isn't too much competition among the caterpillars. Of course, many
saturnidae place hundreds of eggs at one plant, sometimes even at one
single leaf. In this case, all rules of Charles Darwin show their cruel
truth: Only the biggest and strongest will survive in order to improve
the average "quality" of the whole species.

I've never become aware of a food fight among P. m. larvae until now, so
I'll have to keep on breeding to find out if there are any, observing if
or how the osmetrium plays a role is these fights.

There is another topic to be discussed in this context: The larvae seem
to "sleep" for many hours a day. They don't move, they don't eat and -
that's the point: they don't or hardly react with the osmetrium if they
are touched in these phases of the day (as far as I watched this within
my breedings). So, the stories saying this defense mechanism is against
birds or wasps don't make sense to me. Birds or wasps don't ask the
caterpillar if it's paralyzed or not to give it a fair chance to
survive, do they? Your theory of food competition is quite better
because: The phases of activity and inactivity are exactly synchronized
among the individuals, as far as I experienced it. Even if the
caterpillars are located on separated plants, they seem to "be switched
on and off" together. (Maybe Rupert Sheldrake's theory of morphogenetic
fields can explain this behavior...)

So, if the osmetrium only works in phases of common activity, it must be
supposed to affect the competitors, not enemies...

Martin