the mystery of Vanessa unidirectional migration

Ron Gatrelle gatrelle at
Tue May 1 14:35:55 EDT 2001

My comment follows at the end. Ron
----- Original Message -----
From: "Royce J. Bitzer" <mariposa at>
To: <leps-l at>
Sent: Tuesday, May 01, 2001 1:26 PM
Subject: Re: the mystery of Vanessa unidirectional migration

> To Liz Day and others,
> I think that Red Admiral migration in North America is actually
 My own observations do suggest that the offspring of the spring arrivals
themselves migrate south. But is that a genetic "dead end?" Most likely
The reason why it's probably not is because their offspring, which are our
"summer brood" and appear to be stationary here, have enough time to breed
another generation during the summer. Butterflies of this third generation
start emerging in mid- to late August, and these in turn probably are the
that fly south from here during September and early October.

> This suggested phenology somewhat parallels that of Monarchs in the
 midwest, in which one generation arrives here from more southerly regions
in the spring and
gives rise to a summer, stationary brood, which in turn breeds yet another
generation that is the one that actually migrates south.

Big snip
 Monarch migration north and south is stimulated by photoperiod - the
relation of Earth and Sun. Any generation of Monarch could be induced to
"migrate" by subjecting it to a specific sustained photoperiod. If I
remember correctly, the migration mode kicks in from northern latitudes
southward like "clockwork" as the same reduced photoperiod progressively
moves south with the Earth's axis. In other words there is no gene that
kicks in in alternate broods or conscious will of a brain (I know Royce was
not saying this. This is just info and commentary.) This is strictly a
mechanical instinct based on a stimulus and response.

That which triggers the dispersals of these Vanessa taxa are likely due to
completely different set of stimuli than with Monarchs - though photoperiod
is a common factor in the behaviors and activities of many organisms and
could be expected as a factor here - but not primary as it is with NA


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