Papilio machaon - osmetrium

[Esther Cornelius]

Martin: This is my theory on saturnidae larva tubercules, it might also
be extended to osmetrium. The larvae use it as a wepon against other
caterpillars when food gets scarce. this theory comes from only one
observation of Hyalophora euryalus, involving four final instar
individuals that I saw getting 'bopped' as they all congregated at the
last few leaves once when I let their food supply get low. Hyalophora
has four (I think) tubercules on the segments near the head. The
tubercules have tiny black bumps like truncheon spikes. The individuals
that got bopped appeared stung, stopped eating and assumed an "S"
position reared back like a cobra. They stayed like that for several
hours (over night), even after more leaves were supplied. The largest
caterpillar did the most effective bopping and it was left with the food
when the others retreated. I didnt record which cocoons were associated
with the bopped larva, however there were noticibly more that did not
hatch associated with that batch, and I dont believe it was caused
because one feeding was 12 hrs late. 
    
     Now I'm gonna go further out on a limb (Annie wll love this) to
suggest that this mechanism produces large species. Something like this: 

Plan 'A': Those species that DON'T limit their competetors access to
food all share the available food supply so whatever energy is available
goes into 3 survival strategies 
1):smaller size (the best). 
2): more eggs at the expense of longevity or flight energy (bad news if
the food supply is limited). 
3): more flight energy / longevity at the expense of ova production
(fewer offspring). 

Plan 'B': Those species that DO limit competetors access to food. The
biggest get all the remaining energy so a few individuals benefit from
haveing a lot of eggs and the size and energy for long distance flight,
so long as they can find mates, which Saturnia are well adapted to do.

Osmetrium probably have some kind of properties that repell other
Papillio larvae. But this guess is only because both species are large.
I can see how Hyalophora boppers could at least cause infection, but I
don't see how chemical boppers could keep from having the same effect on
the user. Possibly the smell warns of something physical like biteing or
an acid or feeding supressant to which the area surrounding the
osmetrium is immune. The easest way to find out would be to put a few
together in a low food situation.

OK, come & get me! I'll chop Y'all to chum!

Bill