Monarch Butterflies Use Magnetic Field to Navigate

Long known for using a type of solar compass in the brain, the insects can also migrate when skies are overcast, suggesting a magnetic compass.

Paris (AFP) - The North American monarch butterfly uses the Sun as well as Earth's magnetic field as navigational tools for its famous long-distance migration, scientists said Tuesday.

The insects with their characteristic orange-and-black wings flutter thousands of kilometres each year from the United States and southern Canada to the Michoacan mountains in central Mexico, where they overwinter.

The butterflies, whose Latin name is Danaus plexippus, have long been known to use a type of solar compass in the brain.

Yet, curiously, they are also able to migrate when skies are heavily overcast, which suggested co-reliance on a magnetic compass.

Now, biologists from Massachusetts say they have found evidence for this, making the butterfly the first long-distance migratory insect thought to use magnetic navigation.

They placed monarchs in a flight simulator, which they surrounded with different artificial magnetic fields to test the insects' directional sense.

Most headed equatorward in initial testing but turned north when the inclination angle of the magnetic field was reversed. The compass worked only in the presence of light at the upper edge of the visible light spectrum.

The butterflies' antennae appeared to contain light-sensitive magnetosensors to make this all work, the team found.

The research, published in the journal Nature Communications, sees the monarch join a lengthening list of birds, reptiles, amphibians, turtles and insects, including honeybees and termites, believed to use the magnetic field for navigation.

"Our study reveals another fascinating aspect of monarch butterfly migratory behaviour," the authors wrote.

"Greater knowledge of the mechanisms underlying the fall migration may well aid in its preservation, currently threatened by climate change and by the continuing loss of milkweed (plant) and overwintering habitats.

"Another vulnerability to now consider is the potential disruption of the magnetic compass in monarchs by human-induced electromagnetic noise, which can apparently disrupt geomagnetic orientation in a migratory bird."

Old World Swallowtail Egg

The transformation of a caterpillar into a butterfly is nothing short of magical, and now a new study shows what happens from the inside out. Here we follow the life course of an Old World swallowtail butterfly, which begins life as a colorful marble-looking egg. Adult butterflies lay the eggs, which hatch in about 3-7 days.

Old World Swallowtail Caterpillar

The hatchling is born to eat. It devours plant material over a period of 2-3 weeks, increasing body mass by a few thousand times.

While its colorful body is a standout here, when viewed from a distance, the shades can blend into plant material, helping to camouflage the caterpillar. When viewed up close, the colors can also warn predators that the caterpillar’s body contains unappetizing chemicals that can be toxic. Some species can even deliver poisonous stings.

X-ray: Day 1

After the 2-3 week period of feasting, the caterpillar spins a hard shell around itself, which becomes the chrysalis. This is where the real transformational magic begins, leading step-by-step to a butterfly.

The study, published in the Journal of the Royal Society Interface, used high-resolution computed tomography to capture 3D images of what goes on both inside and outside of the chrysalis. This X-ray shot provides just a glimpse of all the changes taking place inside.

Day 1 From the Inside

This image, resulting from many different X-ray slices showing the chrysalis interior, reveals the developing digestive system and wings. Co-author Russell Garwood, a research fellow at the University of Manchester’s School of Materials, told Discovery News, “What this (also) shows is that at the beginning of pupation the tracheal system -- which the insect uses to breathe -- is already fairly well formed. Either this forms very quickly, or it is carried over from the breathing system of the caterpillar.”

Old World Swallowtail From the Outside

This is the more typical view of a chrysalis, showing its protective outer shell. A mucous-like substance allows it to attach to a plant surface. Note that the color resembles a dead or dying leaf, providing life-saving camouflage during this vulnerable time.

Days 1 Through 13, Step by Step

Most of the major changes occur during the first week of pupation. This image shows the changes that take place from day one through 13.

“You can see that as the butterfly develops, the tracheal system grows in complexity," Garwood said, "so by the end, the adult's has a far larger volume of air tubes, but the same basic layout as the first day -- it just has lots more small branches.”

Day 13

“The red structure in the middle is the midgut -- part of the digestive system, which is also the dark structure you see in the X-rays of the chrysalis,” Garwood shared. “Early in the development this is quite large, like the one we see in the caterpillar, but as it develops it shrinks and moves backwards, and then changes shape over the next few days to the structure we find in the adult form.”

“On day 13,” he said, “the yellow things you can see are structures called malpighian tubules, which help clear waste out of the adult's body (a bit like kidneys).”

Day 14

An X-ray image again shows what the changes look like both from the outside and the inside. By this day of development, the chrysalis is already well on its way to becoming a butterfly.

Day 16

“The light green structure you can see growing in size is an air gap between the chrysalis and the edge of the body of the developing adult,” Garwood said. “You can see that as the adult takes shape within the chrysalis, this air gap expands -- this is because there is a narrowing of the butterfly's body here, which forms throughout development. This is actually the margin between the thorax, which has the wings and legs, and the abdomen, which is the back part of the body.”

Butterfly Complete

After just over two weeks, the chrysalis cracks open, unveiling an Old World swallowtail butterfly. After about one hour of adjustment, its wings are full sized, dry and ready for flying.