Tuesday, 2 July 2013

Eumorpha phorbas: a caterpillar with a blinking eyespot

Eumorpha phorbas (Sphingidae)

Dorsal view of Eumorpha phorbas caterpillar, final instar. Photographed during my 2011 trip to Costa Rica.

 Above is a photo of the caterpillar as I observed it - a final instar larva about 60 mm long and 12 mm in diameter in the middle of the body. This specimen had been collected by parataxonomists in an earlier instar while feeding on Sarcopera sessiliflora (Marcgraviaceae). Eumorpha phorbas caterpillars are only encountered on very rare occasions - I just happened to be fortunate enough to be visiting the research station in Costa Rica as the caterpillar reached this final instar. This species is restricted to rain forest habitat and its host plant (Sarcopera sessiliflora) is a canopy-level large woody vine. A preference for this canopy-level host plant partially explains why the caterpillar is observed so infrequently. This one had been discovered by parataxonomists upon searching a 25 m tall tree had been uprooted.




Early Instars:

During the earlier instars these caterpillars rely largely on colour-matching the green leaves of their host plant Sarcopera sessiliflora. You will notice however that the caterpillar has a bright marking on the thoracic body segments, what we would consider a weakly-developed eyespot. It is unclear whether these markings provide much protection to these early-instar caterpillars. We aren't even sure if the marking indicate the beginning of eyespots evolving in this species or the reduction of conspicuous marking as they are being selected against. We do know however that a closely related species, Eumorpha labruscae, has well developed eyespots even in these earlier instars.
Eumorpha phorbas caterpillar, pre-penultimate instar. Costa Rica.
Photo from Janzen and Hallwachs database.
Eumorpha phorbas, pre-penultimate (3rd) instar. Costa Rica.
Photo from Janzen and Hallwachs' database.
Dorsal view of Eumorpha phorbas, penultimate instar. Costa Rica
Photo from Janzen and Hallwachs' database.
In the photos above you can see a somewhat damaged red "anal horn". It is relatively delicate and will become limp and deflated if is gets damaged. E. labruscase has a similar anal horn during these early instars that it waves at predators when it feels threatened. We suspect the red anal horn of E. phorbas, would typically look similar and can be similarly "waved" when not damaged. This red anal horn will be lost by the final instar in both species, and replaced by a hardened black "button".


Final Instar:

When the caterpillar is threatened it adopts a posture where the anterior body segments are pulled in, while it inflates the thoracic body segments. The caterpillar appears to have the head of a snake, which probably helps protect it from attacks by its predators. What do you think, would this guy scare you?


Eumorpha phorbas caterpillar, final instar. Snake-like defensive posture - dorsal view.
Photographed by Kennedy Warne in Panama, see his original post here.
Eumorpha phorbas caterpillar, final instar. Snake-like defensive posture - lateral view.
Photographed by Kennedy Warne in Panama, see his original post here.
Close up of the anterior end of a final instar Eumorpha phorbas caterpillar in its posture - dorsal view.
Photo from Janzen and Hallwachs' database. Caterpillar from Costa Rica.

Remember that what may look like a "head" in the above photos are really just inflated body segments (specifically the thoracic body segments). The caterpillar's true head is actually relatively small in comparison, but can be seen if you look closely at the very tip of the "head" above. Inflating the thoracic body segments into a snake-like "head" isn't the only trick that this caterpillar has for scaring off predators. Where the caterpillar had a bright red anal horn in the early instars it now has a hardened black "button" that the caterpillar can manipulate such that it looks like a blinking eye - but it's not a real eye. The only other species known to have a "blinking" eyespot like this is its close relative Eumorpha labruscae. 


Posterior end of a final instar Eumorpha phorbas caterpillar showing the hardened button capable of "blinking".
Photo from Janzen and Hallwachs' database. Caterpillar from Costa Rica.
Eumorpha phorbas caterpillar, final instar, showing the hardened button capable of "blinking"
Photographed by Kennedy Warne in Panama, see his original post here.

I have written about this "blinking" eyespot in a previous post here. A more complete description of the "blinking" eyespot in the two species and its possible function in deterring predators can be found in our recent manuscript published in the Journal of Natural History:


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


Still frames of a final instar Eumorpha phrobas caterpillar from Costa Rica revealing the hardened button, thereby creating what appears to be a blinking eye, at least to human observers

It may have struck you that the colour of the caterpillar in this final instar does not match the leaves of its host plant. From what we can gather it seems that the final instar larvae spend much of their time resting on the trunks of trees where it blends in among the lichen and moss, rather than sitting on leaves as it does in earlier instars. My correspondence with photographer and naturalist Kennedy Warne who observed a final instar E. phorbas caterpillar in Panama provides some anecdotal evidence to confirm this:
"Our specific interest was Pelliciera mangrove forest, but the encounter with the caterpillar was in in lowland forest abutting the mangrove area. The caterpillar was head-down, stationary and completely exposed to view, vertically oriented on a tree trunk covered with a fuzz of moss and liverwort."
Many of the species  from this genus burrow underground pupate below the leaf litter so this colour pattern may also help it blend in while it searches for a safe place amongst the forest floor. In our e-mail exchange Kennedy Warne also provided the following observations on the caterpillar's behaviour:
"We were struck by the fact that whenever we came close (perhaps six inches) from the critter, it slowly raised its head to assume a more snakelike pose. This was a slow movement, unlikely to have in itself caused a startle reaction in a predator (my conjecture) -- however the effect was to make the caterpillar more imposing, and certainly to reveal the full snakelike mien of the head. The blinking of the "rump" eye appeared to be continuous, perhaps every few seconds, though I didn't time it and can't remember the precise frequency. We searched nearby trees but this was the only specimen we encountered..."
These observations are certainly in line with what we had noted during our encounter with E. phorbas in Costa Rica. In a recent interview on a New Zealand radio show Kennedy Warne discusses his encounter. You can find the interview here; discussion about the caterpillar begins at 08:30.

Adult:

In the adult life stage this moth is relatively drab except for the small flashes of orange and black on the hindwings. The brown colouration probably helps the moth remain concealed on tree trunks during the daytime. A website run by Bill Oehlke reports that adults range in size from 112-121 mm wingspan and nectar at various flowers. They apparently breed continuously with adults reportedly caught every month of the year in Central and South America. In other related species the male moths track females at night by following pheromone plumes, and this is likely the case for this species as well.


Eumorpha phorbas adult (male), wingspan 112 mm. Arrows point to features for species diagnosis.
Photo from Janzen and Hallwachs database.
Eumorpha phorbas adult (female), wingspan 116 mm.
Photo from Janzen and Hallwachs database.

 According to a note in the specimen record from the Janzen and Hallwachs database the colouration of the forewings can range from a more intense green to almost blue-black. Below is a different adult specimen with a slightly different colour as an example.
Eumorpha phorbas adult, wingspan 113 mm. Costa Rica.
Photo from Janzen and Hallwachs database.

 More information about the adult lifestage of Eumorpha phorbas can be found at these links:


Find out more about "blinking" eyespots, check out these posts!



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