Migration?

[Anton Marais]

On Fri, 5 Jun 1998, Andrew Daw wrote:

> >      I was wondering what drives butterflies and moths to 
> >      migrate (in this case northwards into southern UK), 
> 
<SNIP>
>
> In Europe for example, the Painted Lady (Cynthia cardui) is resident
> in North Africa.  By early spring the local population has grown to a
> point that the local habitat cannot support them, and they head off for
> pastures new.
> 
<SNIP>
>
> Being an African species a cold snap (overnight frost) can kill
> them off quite easily compared to our more hardy resident species.
> 
This is a very exciting topic - especially the migration of Vanessa cardui!
Here in South Africa we also get the Lady, but only during our Southern
Winters. Probably because the foodplants only grow during this time as
the summers are too hot! I have seen some Ladies in the mountains also
and they seem to be migrating Southward!
What I am therefore very curious about is if there has been any study
into the migrating pattern of the Ladies.  Where did they originate
from in the first place!  Is it really an African butterfly or could it
be an American butterfly?  (Because there is some others in the same
genus like Vanessa atalanta)
Also: How did they manage to cross the Atlantic Ocean and when? (Probably
before the break-up of the continents?)

Regards
Anton