Unlike humans, butterflies are not disposed to seeking altitude
records. Indeed, they will not fly higher than is strictly necessary in
their everyday lives, whether looking for a mate, food, somewhere to
lay eggs, avoiding predators or migrating.
Worldwide
there are many thousands of species of butterfly, each adapted to its
own particular habitat and needs. Some spend their whole lives on a
patch of coastal grassland, the larvae feeding on low plants or living
in ants' nests, and the adults never flying more than a few feet above
the ground. Others spend all their time in the tree canopy many metres
above ground level.
Still others are only found on
high mountains. So even though they don't actually fly very high above
the ground locally, butterflies that live on the mountains of Peru
spend their whole lives at altitudes of around 6000 metres.
Butterflies that migrate tend to fly the highest in general. The most famous migratory butterfly is probably the monarch, Danaus plexippus.
These leave Mexico each year and fly north to Canada, albeit taking
several generations to get there. Monarchs have been sighted by glider
pilots flying as high as 1200 metres. Interestingly, they seem to fly
in the same way as a glider, using updrafts to gain sufficient altitude
so that they can glide for quite a distance before needing to use
energy to climb again.
Europe also has plenty of migratory species. The painted lady, Vanessa cardui,
makes its way to southern France from north Africa. It has to leave
Europe in winter as no development stage of this insect can survive a
frost.
To get to France many will cross through the
mountain passes of the Pyrenees, which in general lie at about 2500
metres. During late summer and autumn one can observe butterflies
drifting southwards. If they encounter a high building, they just fly
straight upwards and over it. If they encounter a high mountain range,
they will do the same. So you need only to stand for a while on any
mountain pass during the migration period to see them coming over
either singly or in swarms, flying close to the ground as they travel.
The
mountain passes of the Caucasus are higher, while those of the
Himalayas are higher still at 7500 metres. I wouldn't be surprised if
migratory butterflies could fly straight over mount Everest if they
encountered it in good weather.
However, insects of
any kind cannot fly if they are too cold. Butterflies can keep warm to
a certain extent by beating their wings, though if they fly too high in
the wrong conditions, they may become too chilled to maintain a
wingbeat.
On average, the air temperature reaches
freezing at an altitude of just below 8000 metres, suggesting that this
would be their physical altitude limit. They might on occasion be
carried higher on updrafts, but this surely doesn't count as autonomous
flight.
Terence Hollingworth, Blagnac, France
