In my last trip to the Amazon basin of Ecuador I had the fortune of meeting Paul Bertner, an acclaimed photographer and adventurer. I have been following his image posts and trip reports since I cannot remember when (and you should too!), and was excited to spend some time in the field with him, in hopes of learning some new tricks. It was great fun to discover hidden gems in the rainforest together, while discussing arthropod biology, conservation and photography.
During one of our night hikes we came across a tiny green caterpillar that was resting on a silky retreat on top of a leaf. At first glance it did not look very special, but then I noticed that its head featured two enormous antler-like horns. The horns were almost half the caterpillar’s body length! They were not simple straight horns, but rather complex structures that included many branches and hairs. I recognized the caterpillar as a member of tribe Biblidinae in the butterfly family Nymphalidae, but only later learned that it belongs to genus Diaethria.
Despite their small size, Diaethria butterflies are quite well-known thanks to the characteristic pattern on the underside of their hindwings. Circular bands in black and white surrounding black dots, giving the impression of the letters BB, Bd or the the numbers 88, 89, 69 etc’. The common names “88 butterfly” and “89 butterfly” are typically used for species in this genus. They are often seen puddling – an interesting behavior in which butterflies take up minerals from mud, sweat and feces.
Upon seeing the caterpillar I knew exactly how I want to photograph it. I wanted a frontal, head-on photo of the caterpillar’s head with the antlers stretched up in their full glory. What photo is more suitable for the holiday season than a festive caterpillar? Unfortunately, I did not have my high magnification MP-E lens with me (as mentioned, the caterpillar was tiny), so I gently collected it to photograph later. Let me tell you, photographing it was not an easy task. It seems that the caterpillar’s default behavior is to rest face down on the leaf, preventing any view of its antlers other than a dorsal one. It literally took hours to get it to change position, and I had to come up with a creative solution to get something remotely similar to the photo I had in mind.
One question that comes to mind is how do these caterpillars molt with such long head protrusions? Do the horns come out already stretched from the old head capsule or are they compressed as horn “buds” that inflate later? And what are those horns good for? They are most likely an adaptation against predators, but it is hard to say exactly how they are used. They can be used as defense to push away ants from attacking the caterpillar, or maybe the caterpillar drives away parasitoid wasps by waving the antlers from side to side. Hopefully someone will be able to document their function in the future and shed light on these remarkable structures.