Orientation of Attaching to Pupate

Why do caterpillars tend to attach with their legs toward the light? Or do they?

We noticed that most of the Monarch chrysalises in our lab were oriented with their legs to the light. We did a test to see if this was right. Was it our imagination? We selected four containers with Monarch caterpillars and placed them in specific spots in the room, on opposite walls, to determine whether this was so. Each container was marked and placed in the exact same spot, level, and position each day.

Containers of Monarch butterfly caterpillars in the lab, fed daily, removed when they became chrysalises, and were then shipped to exhibits throughout the US.

In each container, Monarch caterpillars were housed and fed fresh milkweed daily.

Monarch caterpillars and chrysalises in the lab.

Out of 30 chrysalises in each container, only two or three attached with their backs to the light. Most were angled with their legs more to the light than to the back.

Nearly all the Monarch chrysalises had pupated with their legs toward the light.

More studies need to be done by others to see if this proves true in all areas. Our one study does not make a definitive scientific conclusion other than, in our lab, they pupated with that orientation.

If this position is an advantage, why? We believe that pupating with their legs toward a solid object, as they pupate on in nature at times, can cause life-threatening damage to the finished chrysalis. When attaching to a fence, side of a house, or other solid object, pupation direction may be a life-death instinct. If they attach on or to a flat object, that side will be darker. If the back of the abdomen is flattened by a stem or wall, the adult butterfly will look and function normal. If the leg/antennae/proboscis area is flattened, it often leads to failure to pupate properly which can cause death or disfigurement that leads to death.

If it is an advantage, those that had an instinct to pupate with their backs to the light would be less apt to survive. The number that die would be minimal, perhaps not enough to make a difference. Was our experience a coincidence or is this the normal ratio of angle to the light that happens all over in the Monarch world?

Fatal damage to the leg side
of a Monarch chrysalis

Please do your own studies. Do they pupate the same way in your house? Do you see more damage with those that pupate with their backs to the light? When we all observe what happens in our own caterpillar rearing environments, we all learn.

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