Vanessa at night - migrations

Paul Cherubini monarch at saber.net
Tue Feb 5 04:07:06 EST 2002


> The significant question here involves butterflies (such as Vanessa) in
> strong dispersal mode that find themselves over open water at nightfall.
> Do they keep going?  Some believe (well, okay, one person believes) that
> they must fold up and drop into the drink within an hour after dark.
> That's not impossible, given that some episodes of butterfly dispersal
> are suicide flights for all practical purposes.

Yes, Ken some of us think Vanessa butterflies are likely functionally incapable
of oriented flight within an hour after sunset because they lack dark 
adapted eyes - in plain english they are probably functionally blind at night

Page 228  R.F. Chapman - The Insects, Structure and Function:
(College textbook used for Courses in Insect Morphology & Physiology):

"The flight activity of many insects is limited by light intensity and
Lewis and Taylor (1965) conclude that this is the major factor 
controlling times of flight.  For instance many day-flying insects, such
as butterflies and Hymenoptera, are not active in the dark and
aphids will not take off when the intensity falls below 20 ft.-candles."

Then the book goes on to describe and illustrate the structural
differences between light adapted and dark adapted eyes
http://www.mindspring.com/~cherubini/eyes.JPG

A few years ago on this forum the tables were turned and I was
the one making a fantastic claim about insect flight capabilities.
I reported that about a half dozen or so tagged monarchs released
near Bishop, Calif (north of Mt. Whitney) during the first week 
of November 1991 were recaptured along the south-central California
coast just northwest of Los Angeles about 10-20 days later. I tried to
claim this was good evidence the butterflies likely flew over the frigid 
13,000 foot snow capped Sierra Nevada Mountains north of
Mt. Whitney in early November.

Everyone else thought it was highly unlikely the monarchs flew
such a fantastic direct route back to the California coast. They thought
the butterflies proably flew a less direct route down the Owens Valley
and avoided crossing the Sierra Nevada Range.  I wasn't too
thrilled, but accepted their challenge as constructive
criticism.  In later years, I did another release further north near 
Reno, Nevada during the first week of November and had
one recaptured on the central California coast at Morro Bay
within a week or two later.  This experiment more definitively
suggested that some of the monarchs did indeed likely cross
the central Sierra Nevada Mountain Range in early November. 

Paul Cherubini
Placerville, Calif.

 
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