Caterpillar of the day: Arsenura drucei

Almost every day that I am here in Costa Rica I am busy photographing caterpillars and recording their behavioural responses to simulated attacks (a light pinch with blunt forceps). I am going to try and share some of this amazing diversity here on my blog. I want to share the most interesting ones, but some of those specimens may not be identified down to species right when they go up. I will give the preliminary ID or a descriptive name and update my older posts once that information is confirmed. Once I confirm an ID I will mark it in the blog post like this: [CONFIRMED]. Until a post is confirmed please do not rely soley on my posts as an accurate ID for a given species.

Caterpillar of the Day: Arsenura drucei (Saturniidae)

This specimen was collected in the San Gerardo sector of the ACG. To me it looks aposematic (i.e., advertising its unpalatability), particularly because of the contrast in the caterpillar's colour compared to the host plant. This is the penultimate instar (PU), and to me it is the most beautiful part of this species whole life. 

Here is what the species looks like in other stages of its life:

Note: The caterpillar photos below are from the Area de Conservación Guanacaste (ACG) caterpillar database.

Instar II

Pre-penultimate Instar:

Ultimate Instar:

Adult (Photo from http://www.boldsystems.org/views/taxbrowser.php?taxid=10094):

The second instar also looks noxious, but the ultimate instar is actually quite cryptic in comparison. If the caterpillar is still noxious in the final instar and as an adult why doesn't it advertise it? Does the caterpillar lose its noxiousness? Probably not as many caterpillars sequester toxins directly from their host plant. Instead my guess is that the caterpillar has just become big enough to be a worthwhile meal despite its toxicity (these caterpillars get to be ~120 mm long in their final instar!). Being cryptic in this final instar probably secures sufficient protection from predators without having the bear a cost of becoming more noxious (toxic or unpalatable chemicals are thought to be very costly to produce). Adult Saturniidae moths don't feed as adults and die shortly after mating so crypsis is probably sufficient here too.

Update: It is also possible that this strange colour and morphology is a kind of fungus mimic. The caterpillar may not be toxic at all and simply mimicking an unpalatable fungus growing in the forest. This was suggested by Dr. Dan Janzen and may provide a more parsimonious explanation than the one I provided above. A type of Ophiocordyceps fungi might be the model.