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.”
“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).”
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.
“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.”
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.