Monarch caterpillars make ideal subjects for experiments because they are slow-moving, docile, easily handled, and harmless.
If a caterpillar gets knocked off a milkweed by a predator or is blown away in a storm, it will starve unless if finds a new food plant. So, locating milkweed plants is an important skill for survival.
An unexpected observation poses a question
While I was cleaning out a bin containing large caterpillars, I placed the jar with caterpillars on a large rock slab in my garden. One escaped and quickly disappeared into nearby vegetation.
After 15 minutes found him. He had gone two thirds of the distance towards the only milkweed plant in my back yard, located about 7 feet from his starting point. Too bad I didn't leave him alone, to see if he was really able to reach the milkweed. I wondered, had he selected that direction just by chance? How good are they at finding milkweed? I decided to find out by doing a simple experiment.
"Find Milkweed" Experiment #1
Caterpillars have very poor vision (we think). They probably find milkweed by their sense of smell with sensors on their antennae. By waving their antennae and heads about, they can detect the direction of an odor, especially when there is little-to-no wind. So...
Since I made the first observation outdoors, I thought I would try to repeat it outdoors, but in a larger "test area," my driveway. where
I would place a caterpillar on the driveway at the center of a square, with a milkweed stem at each corner, to the NW, SW, SE, and NE of the starting point in the center.
I would observe whether the caterpillars could find one of the four milkweed stems, and how long it took (dividing distance by time to get speed of travel). What percentage of caterpillars would make it to the milkweed? Is it harder for them to find milkweeds located further away? I would make a sketch map of their path of travel, recording whether they took a direct or roundabout route.
Does wind direction or speed make a difference? I thought that if the caterpillar always went to the milkweed stem located directly upwind, this would be evidence that they were finding it by odor.
Since I hadn't worked out how to put numbers on caterpillars, I would test each of ten caterpillars only once, moving them from an "untested" container to a "tested" container.
Results
The first and second caterpillars I placed at the center of the test square froze and didn't move at all. I realized this is typical of caterpillars exposed to an unfamiliar situation. For example, when at night you enter a room where you keep your caterpillars and you suddenly switch on the light, they freeze. The sudden light and vibrations from footsteps startle them. These caterpillars had never been outdoors. No wonder they were "scared."
"Find milkweed" #2
I would have to do the experiment under conditions more familiar to the caterpillars. I set up a card table in the room where they were raised. I put a milkweed stem at each corner of the table, then released the caterpillar at the center of the table, where I marked a tiny X.
I was testing two things. 1) Can caterpillars find milkweed? 2) By giving them a choice of directions, I might find out something about how they find milkweed.
With only slight hesitation, each caterpillar set out on his quest. They waved their heads about a lot and sometimes changed direction. Nine out of ten found a milkweed stem, most within about 10 minutes. Eight out of ten went to the same NW corner (upper right).
Now, ants will follow a scent trail laid down by other ants coming back from finding a good food source. Did the first caterpillar lay down a scent trail, which the others then followed? To rule this out, I rotated the table 90 degrees counterclockwise after each experiment, so the "scent trail," if one existed, would lead them to a different corner of the table. But they still mostly went to the NW. I also exchanged milkweed plants between different corners, to rule out the possibility that one plant was particularly attractive.
Did they go to the NW because of light? Monarch adults are strongly attracted to light. NW was the direction of the setting sun, when the experiment was done. But it's shady outside the test room, and the NW window wasn't necessarily the brightest. There were also windows to the W and SW. So, I couldn't conclude that attraction to light was pulling them NW.
Did they go to the NW because a draft in the room was blowing milkweed odor from the NW towards the center of the table, where I released them? I lit a stick of incense, to see which way the smoke blew. But the slight wind direction in the room was this way and that... inconsistent. Again, no conclusion as to why they went NW.
But we did learn some things
- Caterpillars are easy to experiment with, provided you don't "scare" them.
- Caterpillars nearly always find nearby milkweed quickly, a key survival skill.
- Caterpillars prefer a certain direction (during our setup), but we don't know why.
- We still don't know how they find milkweed.
Good experiments are hard to design. Many things conspire to hide or block any meaningful results. On the first try, the caterpillars were "scared" and wouldn't perform.
An experimental setup can have a built-in "bias." It's like a tilted playing field--the physical setup leads to some result that's not what you're looking for. On the tabletop, they all went NW but we don't know why. Maybe bright light in this direction was the "bias." Next time, use a room with no windows--with uniform light in all directions. Scientists work very hard to eliminate any bias that might ruin their experiment.
Inexplicable results: We do experiments to "explain" something. Our results--most caterpillars going NW--were consistent. The NW preference could point to something interesting about caterpillars--provided we can rule out a built-in bias. But so far, the meaning of the NW direction is unknown--there is no "explanation."