Caterpillar Mysteries: Systematics, Evolutionary rates, and Mimicry in Acronictinae (Noctuidae)
That was the title of my talk at the EEB grad symposium on Saturday. It sounds pretty impressive, now that I take a step back and think about it. I got a lot of great feedback before my talk to polish it up, and a lot of positive reviews once it was over. I felt like I forgot half the things I wanted to say, which meant I had perfect timing. If you know me, you know that once I get going on a subject I can talk your ear off, so sticking to a 15 minute time limit and leaving a minute for questions was quite an accomplishment.
Anyway, I thought I’d give some teasers as to what my talk entailed. I might delve into a few more as longer posts as I develop more of my ideas and start getting desperate for advice. My biggest problem is having too much material to work with.
First of all, meet some of my caterpillars. These are all in the genus Acronicta, which is in the subfamily Acronictinae in the family Noctuidae. I bet at first glance (and even at a second or third) you would have no reason to believe they are closely related at all! My advisor has called them the “warblers of the caterpillar world”. There are 50+ species in North America, with the highest diversity in the North East. There are about 150 species worldwide. Some closely related genera in the subfamily which may enter into my research are Agriopodes, Cerma, Comachara, Harrisimemna, Polygrammate, and Simyra.
Evolutionary rates – The caterpillars show a huge phenotypic diversity. They extend in all directions in morphological space – different colors, hair types, hair patterns, coloration patterns, camouflage, aposematism, mimicry of other caterpillar species, and even wide variation in behavior. Some rest in a hunch back shape, some sit in a “J” shape, some are conspicuous and some try to hide. However the adults are all dull, gray, unexciting moths. They are growing on me, but drawer after drawer they get a little monotonous. Many species are difficult to distinguish (even to experts), though their caterpillars are distinct. Why is there this apparent de-coupling of evolutionary rates, where the adults don’t deviate much and the caterpillars race into different phenotypes?
I want to address this with a molecular tree (we already created one tree with 25 species to start) and character matrices for the eggs, larvae, and adults. Before we try more molecular techniques I need to collect more specimens, I want to capture as much of the worldwide variation as possible. This will involve some loans and assistance from colleagues around the world, and hopefully some trips.
Taxonomic questions – I don’t want to give anything away by showing our first tree, but so far it has helped address a few small questions we’ve had. The taxonomic history of Acronictinae and Acronicta has involved much shuffling around of species, and I would like to see if we can sort out any more troublesome issues.
Larval description – One project of mine is to fully describe the larva of one Acronicta species to serve as my “atlas” for creating my larval character matrix. It might also result in a publication since it is a species whose larvae have never been described beyond a couple of sentences. I am working with A. falcula, an uncommon species which feeds on Corylus. The description will include a written morphological description, photography, habitus illustration, chaetotaxy (drawing of the hairs), SEM images, and life history information. It’s a charismatic little caterpillar. The drawings are currently underway, and I’ve got a specimen ready for the SEM later this week.
Behavior and mimicry – This part is the most fun! Many Acronicta species are aposematic and/or mimic other caterpillar species Mimicry in caterpillars is supposedly rare, but maybe it is just rarely reported? And why is mimicry so prevalent in this genus? Is it Batesian or Müllerian mimicry? Are the caterpillars chemically protected? Does their resting posture on their host plant offer any insights? Do they flaunt themselves or generally stay cryptic? To address these issues I want to raise several species and observe their behavior on their host plants in the wild or in a greenhouse. I want to try some bird exclosure experiments and use clay caterpillar models to test the effects of bird predation. I might also try offering them to ants to see if they have defenses against invertebrate predators. These experiments offer the possibility of some great collaborations with scientists from other universities.
And that’s enough for now!