|The Question: I just saw a caterpillar flying on a silk thread. It was moving along with the air current about a meter (3 feet) off the ground. It was suspended on a long thin thread with no apparent ‘parachute’ on the end. Eventually it descended to the grass level and walked off. I was not aware caterpillars could do this.
Submitted by: Mark, Berkshire County, United Kingdom
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The Short Answer: Yes, caterpillars can “fly,” although insect scientists actually use the more whimsical term “ballooning” to label the behavior. And if you see a caterpillar flying or ballooning again, I encourage you to take some pictures and video, because although it is a fairly common phenomenon, I was unable to find any good pictures or videos I could share. If you search for “flying caterpillar,” most of what you’ll find is larger caterpillars dangling in mid air. This is not true flying or ballooning, however. Instead, this is a behavior that usually occurs when a caterpillar falls off a branch or drops off a branch deliberately to escape a predator. The caterpillar releases a silk line as it falls, and then climbs back up to return to its branch. I wrote about this previously in an article about dangling caterpillars.
What you’re talking about, however, sounds more like true ballooning. Some species of moth, including the notorious gypsy moth (Lymantria dispar), use ballooning as a way to disperse. Right after the eggs hatch, the tiny caterpillars drop down on a line of silk, sometimes only a few centimeters long, but in some cases a meter long, and then wait for the wind to break the thread. At that point, the caterpillar is carried along till it lands in another plant or on the ground.
Dr. James Bell, a senior research scientist who studies insect behavior at the Rothamsted Research agricultural center in England, says conditions have to be just right for this to work. “For the neonate caterpillars to be successful, the right meteorological conditions have to be present. The weather has to be warm, with rising air, and wind speed needs to be less than three meters per second.”
You might think ballooning is not a very effective way to disperse, and in some ways you’d be right. The tiny ballooning caterpillars don’t seem to have any control over where they land. They just let go and hope. And the fact is that quite a few end up on the ground. When that happens, they may quickly dry out and starve.
By chance, some do land on the right trees. Those caterpillars, feeding on their preferred food, tend to grow faster and produce more eggs than those that land on less suitable food plants. In some species, the caterpillars that land on the wrong trees have the ability to launch a second time. So the caterpillars may start out randomly distributed on nearby plants, but since the ones that land on the right food source stay put and the ones that land on the wrong food source sometimes get lucky on the second ballooning, the end result is higher densities on the correct plant, despite having no direct control over where they land.
The other thing that helps is that the caterpillar species that use ballooning to disperse are nearly all generalists about food plants. They usually have a preferred food plant, but can survive and grow to become reproductive adults on many different food plants. Other moth species are often specialized to feed on a single host plant species, or only on the species within one plant genus or family.
Why Doesn’t Mom Pick the Spot: But all of this makes you wonder, why doesn’t mom moth do the dispersing and find the right food source the way most moth moms do? In most moths, adult males and females have wings. They find each other, they mate, and the female flies to the preferred caterpillar food source plant, lays her eggs, and the caterpillars hatch right onto their first meal.
In species that balloon, however, females are wingless. They feed on one plant until they reach adulthood. Flying males locate the females and mate with them. The females then lay eggs on the same plant. When the eggs hatch, some will stay put and others will balloon away to find a new food plant. There are conditions under which it seems that this is a very successful strategy. For example, if you are a moth that lives in a forest where most of the trees are your preferred food, then your odds of landing in a good place when you balloon are pretty good. If that particular habitat is also stable for long periods of time, it means dispersal never has to be very far – or very accurate.
The overwhelming majority of moth species disperse by flying, however, so ballooning is definitely a fringe method for dispersing, but under the right circumstances, it can work. Since females whose offspring disperse by ballooning don’t need to fly, they can use their energy to grow fatter and carry more eggs. Over the long run, females without wings may outcompete females that fly.
Extreme Ballooning – Ballooning has evolved multiple times in several families of moths. Some spider mite species and many species of spiders also independently evolved ballooning as a means of dispersal. Given that ballooning evolved multiple times, it must be a successful trick. The real stars of the ballooning world are spiders. While caterpillars and spider mites typically balloon only a short distance before landing on a plant or the ground, spiders can be lifted high into the air. Caterpillars and spider mites have never been collected in the air out at sea, but spiders can be found ballooning hundreds of kilometers from shore. When he was traveling the oceans on his journeys, Charles Darwin noted that spiders would often rain down onto the famous ship, The Beagle. And when new volcanic islands form in the ocean, often the first animals that show up are spiders that wafted there on a line of silk.
Interesting video on ballooning spiders – http://www.youtube.com/watch?v=kYPABcMzbEg
Cool video that shows a spiderling molting, and then shows ballooning spiderlings – http://www.youtube.com/watch?v=aICZqRY3_d4
Bell, J R, Bohan, D A, Shaw, E M, et al. (2005). Ballooning dispersal using silk: World fauna, phylogenies, genetics and models. Bulletin of entomological research, 95(2), 69-114.
Barbosa, P, Krischik, V, & Lance, D. (1989). Life-history traits of forest-inhabiting flightless lepidoptera. The American midland naturalist, 122(2), 262-274.