A few weeks ago, I mentioned some of the butterflies I’ve
been seeing around; right now its all fritillaries and sulphurs, earlier this
summer is was Swallowtails, the Mourning Cloaks were earlier still this spring.
Later in the summer I hope to see some Monarchs. Have you ever wondered why
some butterflies are around at certain times but not others? Why are the
Mourning Cloaks the first ones here in the spring, and then they disappear? Why
are we seeing the fritillaries in such abundance now? Have you even noticed
that there is a pattern to the appearance of different butterflies? Believe it
or not, it’s not random happenstance. The answer is due to the fact that
although we’ve all watched the Discovery Channel and think we have the
butterfly life cycle down pat, it’s much more complicated and interesting than
egg, larva, pupa and adult.
The truth is, butterflies are short lived creatures that
come in seasons based on their life histories. Though it is hard to remember
now in the thick of the heat of summer, the main obstacle to all arthropods is
the cold of winter. Being primarily ectothermic or at the mercy of the external
environment for temperatures favorable for metabolism and activity means that
most insects have a difficult time being active in the winter here in Maine. The
question facing butterflies, and all arthropods, is how to deal with winter. There
are two main choices, the same ones we humans face, you can stick it out here
in some kind of overwintering dormancy, or you can get the heck out of Dodge
and migrate to warmer climes.
Migration is not the most common butterfly strategy for
dealing with winter, and it’s easy to speculate on why. First, only the adults
could possibly travel far enough, how much ground can a caterpillar really
cover? Secondly adults don’t tend to live very long, we’re talking about live
spans of days to weeks in most cases. Thirdly, and this is pure bias and
subjectivity on my part, butterflies seem so fragile, so wispy. I can barely
believe that tiny birds migrate successfully thousands of miles, but something
as insubstantial as a butterfly? The best known of the migrating butterflies is
of course the Monarch, and the details of just how they do it is still a huge
mystery. What is known is that it takes multiple generations for them to
complete the migration. The ones that leave here are not the same ones that
return the next year. The ones that return are the great or great great grandchildren
of last year’s butterflies.
So then, migration happens but is uncommon, and the timing
of the return of the migrators depends on many factors, weather and conditions
not only here but everywhere along their route, which can be thousands of
miles. For those butterflies that stay here year round, the majority of them,
they face a choice as well. We know that the basic butterfly life cycle
includes an egg, a larval stage, a pupa stage and an adult stage. Which stage
is the best one in which to wait out the winter? If you thought there was a
clear cut answer to that, you haven’t spent much time studying evolution. The
answer is of course, all of them. Different species have evolved different
tactics for dealing with winter, mainly guided by coevolution with their host
plant, the thing the larva or “eating stage” of the insect, lives on.
Mourning Cloaks are the first butterflies we see in the
spring, and the reason for this is that they over winter as adults. They hide
out under bark or in the leaf litter, emerge on the first warm days in early
spring, mate and die. We see them again in the fall when the new generation
emerges from a period of summer dormancy, fattens up on nectar and prepares to
over winter. The Swallowtails we see in June by contrast, after the Mourning
Cloaks are all gone, have over wintered as pupae, often attached to the trunks
of trees. Their larvae spend the summer feeding on the leaves of various
deciduous trees, and the last instar forms the chrystalis in time for winter’s
cold. The fritillaries flying about now started out last year as an egg laid in
late summer, near a patch of dead violets. The egg hatched and the tiny larva
that emerged, the first instar, is what over wintered, without eating. This
spring, when the violets emerged as a spring wild flower, the fritillary larvae
were already there, ready to start feeding. Each of these butterfly species
positions itself to be ready to make full use of its favorite larval food, and
these plant foods in turn, have their own seasons, thus so do the butterflies.
Watch nature long enough and it becomes a giant game of
connect the dots. The more you play, the more you learn, and the more you
realize there is to learn. Play long enough, and you realize all the dots are
connected.
References:
Canadian reference on Mourning Cloaks http://www.naturenorth.com/spring/bug/mcloak/Fmcloak.html
