An eyespot that blinks?

Eumorpha phorbas caterpillar, final instar. Photo by Kennedy Warne (http://laststands.kennedywarne.com/ay-caramba)

In the summer of 2011 I travelled to north-western Costa Rica to visit a place called the Area Conservacion de Guanacaste (see an earlier post about the ACG here). The plan was to meet up with researchers Dan Janzen and Winnie Hallwachs who have been running a long-term inventory project involving continuous collection, identification, and rearing of butterfly and moth caterpillars. Dan and Winnie had co-authored a paper on eyespots in caterpillars the previous year (Janzen et al 2010) and had previously collaborated with  Jayne Yack, another researcher who is on my PhD committee. Jayne suggested strongly that I visit the research station in Costa Rica, and I'm very glad I did. I organized the trip and my supervisor Tom Sherratt came along for the first week of my four-week trip.

My supervisor Tom Sherratt during our trip to Costa Rica.
He is holding fresh foliage for a Sphingidae moth caterpillar (can you spot it?) 

I had hoped to observe and study first-hand some of the spectacular tropical examples of caterpillars with eyespots and during my visit I was told about a caterpillar that had been collected by one of the parataxonomists at Estacion San Gerardo located in rain forest habitat. This particular species is only rarely encountered as a caterpillar and Dan said it was a 'must-see' given my interest in caterpillar eyespots - it did not disappoint.

The caterpillar I was about to see was Eumorpha phorbas, a large-bodied Sphingidae caterpillar. Caterpillars from this family typically have a "horn" protruding from the end of their body, but sometimes this horn is reduced in the final instars. This was the case with E. phorbas. We were viewing the final instar of this caterpillar when the anal horn had been reduced to a hardened, black "button". I would learn later that another closely related caterpillar, Eumorpha labruscae, has a similarly reduced anal horn during the final instar.

Close-up of hardened "button" at the rear end of a Eumorpha phorbas caterpillar, final instar.
Photo from Janzen and Hallwachs' database.

Close-up of hardened "button" at the rear end of a Eumorpha labruscae caterpillar, final instar.
Photo from Janzen and Hallwachs' database.

What's unique about these caterpillars is that when they feel threatened they are able to move the skin around this hardened button. Specifically, a quick depression of the around the eyespot conceals/reveals the black “button”. 

Posterior end of a final instar Eumorpha phorbas caterpillar.
This caterpillar can actively move the area around the black "button" such that it looks like a blinking eye.

Watch the video clip below of the Eumorpha phorbas caterpillar's response to see what I mean:

Here is a video clip of a Eumorpha labruscae that can do essentially the same thing during its final instar as a caterpillar. This video was recorded in Costa Rica by The National Biodiversity Institute (INBio).

It appears that this phenomenon is restricted to these two Eumorpha species. In many ways this hardened button in combination with the localized twitching behaviour creates what resembles a blinking eye. We took some time to examine the Eumorpha phorbas caterpillar and to see how it responded to a perceived threat (anterior or posterior pokes).

Both caterpillars can "blink" their posterior eyespot upon perceiving a threat. That is, they can move the skin around the eyespot such it either conceals/reveals the eyespot or flashes (i.e. reflects light) conspicuously towards an onlooker. Interestingly, both Eumorpha caterpillars also inflate their thoracic body segments, while pulling their head into their body, to form a diamond shape which appears similar to the head shape of dangerous co-occurring snakes (at least to human observers).

Final instar of Eumorpha phorbas caterpillar in its defensive posture.
Photo courtesy of Kennedy Warne (http://laststands.kennedywarne.com/ay-caramba)

Dorsal view of a final instar Eumorpha labruscae caterpillar in its defensive posture. 
Note the snake-like head. Photo from Janzen and Hallwachs' database.

We found these caterpillars so interesting that we synthesized our notes and wrote a short natural history paper about them. We also included our thoughts about how this "blinking" eyespot might function. You can find the full paper here:

Hossie, T.J., Sherratt, T.N., Janzen, D.H., Hallwachs, W. (2013) An eyespot that “blinks”: an open and shut case of eye mimicry in Eumorpha caterpillars (Lepidoptera: Sphingidae). Journal of Natural History 
DOI:10.1080/00222933.2013.791935

If you do not have institutional access to the article the publisher has granted 50 free downloads for people who follow the link below. Please only open the link is you can't get access otherwise.

Free download if you do not have access 

Over the next few days I will post additional photos and more detailed information about these two spectacular caterpillars. Once available I will link those posts below:

• Eumorpha phorbas: a caterpillar with an eyespot that blinks
• Eumorpha labruscae: another caterpillar with a blinking eyespot

Acknowledgements:

I want to thank Kennedy Warne for allowing me to use his photos here and for providing me with addition information about the caterpillar at the time of his encounter. To find Kennedy Warne's original posts (and photos) about his encounter with Eumorpha phorbas check out these links:

• Ay caramba! A caterpillar that thinks it’s a snake
• More on the snake caterpillar

Also, Dr. Dan Janzen and Hallwachs were great hosts when I travelled to Costa Rica. They were the ones who took us to see Eumorpha phorbas. The opportunity I had to observe and work with so many live caterpillars in the Neotropics is largely a result of their encouragement and support.