Conjuring Nature’s Ghosts

Conjuring Nature’s Ghosts

In my last post, I pondered what survival strategies  the hickory horned devil caterpillar could be using in the middle stages of its larval life. On the one hand, they are not huge and threatening yet, and clearly not toxic to predators (no toxic foodplant, no stinging spines). They also are an inconvenient color (brown) among the green leaves they feed on during the day. This should be a recipe for disaster…

But they do have these freaky looking horns – harmless spines that are kind of like antlers. These can’t be easy to handle when you’re a caterpillar crawling around a tree… which means there must be some advantage to having them.

Looking at them a bit more, I remembered something from my own past experience. I actually had seen a hickory horned devil just once before raising them. Seven years ago, I was new to Charlotte, and out walking my dog in my neighborhood.  Watching the dog sniff things, I noticed some large caterpillar droppings on the pavement. I looked up, and there, just above my head, was a fifth instar hickory horned devil clinging to some sweet gum leaves. I jumped back, which, for me, was an odd reaction to spotting a spectacular caterpillar. As you’ve probably figured out from reading this blog, I think caterpillars are cool and I’m not at all frightened of them.

But I am, inexplicably, frightened of spiders (I always have been) and there was something spiderlike about this caterpillar. My spider fear is literally instinctual (I say it’s “hard-wired”) – I generally have no fear of other bugs, even ones that bite. This is actually a common human fear (psychologists call it “arachnophobia”) and I’ve read scientists who theorize that many of us come with it built into our brains because, in the distant past, our ancestors lived in places where spiders were frequently dangerous, so evolution has favored a built-in fear. I believe this, because it is definitely something primal in my brain, like a fear of barking dogs or of snakes.

So, look again at the photo of the fourth instar caterpillar above and look at these orb-weaving spider photos:

Notice the similarities between the spiders’ conspicuous legs and the caterpillar’s spines? There are eight large spines, and even in the fourth instar, they look distinctively like these native, long-legged spiders. Yuck.

And I can tell you (with an inner quiver) that we have a lot of these spiders in Charlotte, hanging their webs from tree branches. In August, about the time the hickory horned devil caterpillars reach their fourth and fifth instars, orb weavers also mature and come down out of the treetops where they have been growing in obscurity and hang their webs out in the open. At this time the hickory horned devils are becoming so large that they too become conspicuous.

Why would a caterpillar mimic a spider? Spiders bite, but these spiders are not dangerously venomous like, say, the black widow spider… one wouldn’t expect birds to be afraid of them because of that. Of course, they are dangerous to other insects, particularly to flying insects…

So here may be our answer – you may remember that I mentioned in earlier posts that parasitic wasps are one of the biggest dangers that giant silk moth caterpillars face, destroying large percentages of every brood. A big spider is a dangerous predator to a little wasp. The hickory horned devil’s spider mimicry may frighten away the parasites.

But, thinking about it, there still may be an effect here on bird predators as well. I’ve always found it puzzling that big orb weaving spiders (like those whose photos you see above) are so conspicuous – large, brightly colored, hanging in the middle of the web with no cover, where a flying bird could easily pluck them.  Yet birds don’t. I did a search of the literature on bird predation on orb weavers and found that biologists find that bird predation appears to be uncommon. In nature, bright coloration is sometimes a sign that an insect is toxic and not good to eat. Yet again, there is no mention of this in the literature – surely someone has looked into these big arachnids  being bad tasting… They are not poisonous in bite or taste, yet they advertise their presence… hmmm.

So let me propose an alternative –perhaps birds leave orb weavers alone because they are hard-wired to be instinctively frightened of large spiders just like I am, though there is no longer a real threat in these particular bugs.  In the tropics there are large orb weavers that are more venomous and sometimes prey on small birds. In other words, evolutionary history has given large spiders a fearsome reputation, and, though they may not still deserve the fear, they are still using it for protection. And so does the hickory horned devil… Deep instinctive fear is a force and it has echoes.

See -- totally harmless!

History has meaning to humans because we learn lessons from it.  In the battle for survival that we call nature, some lessons have value for other species as well, and they too find ways to preserve the history.

Anyway, it’s a hypothesis.

Midlife Crisis

4th instar hickory horned devil doing the limbo. Must have been the tequila.

Midlife can be… ugly.

I changed the sleeves today -- I ignored my own wisdom and made the sleeves smaller than the ones I made as a teenager, which means I have to transfer caterpillars to new branches when the leaves all get eaten. With small caterpillars, this isn’t easy, so it’s not what I recommend other people do. Use an old double bed sheet, sewed in a tube.

Still, there are advantages to moving the caterpillars – you get to see how they are all doing.  Interestingly, I’ve lost about 2/3 of my hickory horned devils, primarily, I think to disease or natural mortality when they were in the 1^st instar (when they were really little). I have about 40 left, which is fine – 100 6-inch caterpillars would be pretty hard to maintain. This kind of drop-off between hatchlings and larger caterpillars is not unusual – I assume that it’s one of the reasons why the moths lay so many eggs. But the survivors seem very healthy, with “only the strong” surviving.  In contrast, the lunas have had very little drop-off in population – they are all smaller (still 2^nd instar) but I have at least 100.

A variety of instars hanging out together.

What I find most interesting though is a big variation in the rate of growth among the hickory horned devils – in the sleeves there are a lot of 3^rd instars (about an inch long) quite a few 4^th instars ( 2 -2.5 inches – they will grow to about 4 inches before their final molt), and a few that are still 2^nd instar (less than an inch). All these caterpillars hatched within five day period, so that’s quite a spread. Why some are growing much faster than others, I can’t guess. It could be genetic differences: as I said in an earlier post, caterpillar broods can have a lot of variation in appearance, for example. It could also be accident (some caterpillars found more nutritious leaves than others) or disease/injury – some got a little bit sick and it slowed their growth. Or it could be something totally different.

A shed skin. /This is something human kids don't have to do when they move to the next grade.

My guess is that this is genetic variation. The probable reason why there is so much variation in the appearance of brother/sister caterpillars within single broods (if you look at the previous post you will see photos of two Pandora Sphinx caterpillars – one green, one orange – I have seen four different color patterns within a single brood) is that trying out different looks is like giving nature a choice. Who knows which one may carry a hidden advantage in any given time and place? Similarly, caterpillars who grow rapidly may have a survival advantage over slower growing ones… or vice-versa.

…Which brings up the issue of the current appearances of the 3^rd and 4^th instar caterpillars. What’s up with that? As I blogged about a couple of posts ago, the mottled appearance of the 2^nd instar caterpillars mimics bird droppings, giving those caterpillars a good disguise from hungry birds. But the 3^rd and 4^th instars are solid chocolate brown, with relatively large (almost leglike) horns. While the brown color could match the bark color of a twig, these guys seem to spend all day chewing on the green leaves, where they stand out. The hors make them stand out more, but aren’t large enough to seem credibly threatening. I’m puzzled by how this works for the bugs at this stage in their lives – if you have any ideas, let me know. In any case, they’re beginning to look really cool, in an “Alien vs. Predator” kind of way, which is good enough for me.

Bugs & Moonshine

Regal moth caterpillars in sleeves in my front yard. In the close-up you can see them -- they are about an inch long now. I used nylon screen for my sheeting material because it's hot here in Charlotte and I didn't want them to cook, but normally any thin fabric will do.

When I was a teenager, I made a fair amount of money for a couple of summers farming some unusual produce. No, it wasn’t illegal, though it was ethically sketchy (more on that later).  Actually, drug dealing would have been more socially acceptable among my peer group at the time.

Who wants to go back to school and answer the question “what did you do all summer, man?” with “I raised caterpillars in my backyard. Thousands of caterpillars…” and then, as the line goes in “Alice’s Restaurant,” “they all moved away from me on the bench.”

I grew up in a fairly small, fairly idyllic college town in upstate New York, where a lot of college students decided to stay every summer, to fill up the local swimming holes and take all the temporary summer jobs. This meant that local kids were pretty much out of luck for summer employment, unless you had a parent who owned a small business that boomed in the summer. I didn’t, which meant I was generally without pocket money  and had to spend a significant number of hours painting our house, chopping fireplace wood, mowing lawns, etc., etc., FOR FREE, because may father wanted to be sure I “was occupied.” As we said then, it was a “major bummer.”

I didn’t have a real job but, as I have said in a previous post, I did have a significant hobby – insect collecting. As a huge bug-geek, I got some insect collector newsletters, and one of them had classifieds, where people sold collecting equipment and … specimens. Reading these ads carefully, I noticed that some of these advertisers sold live bugs – notably giant silk moth cocoons. Collectors liked hatching moths to get “perfect condition” moths for their collections.  This gave me an idea.

I had grown caterpillars from eggs before and I knew the basic mechanics – catch female moths,  force them to lay eggs in paper bags, then find a source for the food plants. In the past, I’d grown caterpillars in dry aquariums. It was a lot of work, because you had to go out every day or so and cut fresh branches and clean the terrariums regularly or you got mold, which was yucky and killed the caterpillars. I knew there was a better way (there are instructions in some of the field guides) to do this outdoors with live plants, called “sleeving.”

Sleeving is a method by which you can farm caterpillars on a large scale. It’s a basic technology that, I suspect, was invented to grow silk moth caterpillars for the silk trade. “Technology,” in this case means a fabric tube that you can slide over a branch and tie tight at both ends, enclosing the caterpillars and the leaves. The idea is to keep the caterpillars from escaping and to protect them from predators – especially birds and parasitic wasps, which kill them in droves in the wild. If you make the sleeve big enough, you may never have to handle the caterpillars at all until they pupate. A bed sheet, sewed together at the long edges, is ideal, because the tube is big enough to enclose a large branch which can feed maybe a hundred caterpillars through their larval lives. Bed sheets are thin enough to allow the tube to “breathe,” so moisture doesn’t collect inside the tube and the caterpillars stay dry and healthy.

June was the prime collecting season for most of the large, showy, “collectable” moths in my region, and I had some great places to hunt them, a couple of miles away in the countryside, where mercury lights on some industrial and commercial buildings happened to be near some big open spaces and natural flyways. I and a couple of friends would ride out every night it wasn’t raining and collect. The year I started “farming,” we caught multiple gravid (fertilized) of all the locally common species of giant silk moth – luna, polyphemus, cecropia, io and even one promethia moth (not a common insect in upstate NY). We also caught egg-laying females of about 10 species of sphinx moths – otherwise known as hawkmoths – large bodied, often beautifully colored moths with long, powerful narrow wings. I had the raw materials to stock about a dozen large sleeves, though I knew that I might need even more when the caterpillars got bigger. It’s a little hard to guess how much forage a hundred  (or so) large caterpillars will eat.

I paid about $10 to put an ad in the newsletter, listing the pupas/cocoons I would have available later in the summer and their prices (I guessed at what I could ask, charging more for rarer species, and factored in the cost of mailing and supplies). I really had no idea if anyone would buy any, but I assumed somebody would buy something. I assumed my customers were kids like me, and a couple of years earlier, I would have bought some.

My mother, fortunately, had a lot of old, worn bedsheets which she didn’t mind getting rid of (I had to buy a few cheap sheets at Woolworth’s)and one of my friends’ mothers had a sewing machine and was willing to sew the tubes (our parents were largely academics, and they were willing to encourage strange hobbies). I had many of the food plants in my backyard, and another friend had a small patch of woods out behind his house which had the rest. I simply tied  the sleeves on the trees, cut the eggs out of the paper bags, put them in, tied the sleeves closed and waited.

Meanwhile, the mail started coming. I got nearly a hundred orders. Some of my customers ordered a lot of pupas. Some were business addresses in NYC, which, I  realized later, were dealers selling specimens to collectors around the world. My sphinx moth pupas were particularly popular because few of the semi-professional “growers” (there were not many back then, but there were a couple) bothered trying to growing them. My father, seeing the money roll in, largely let me escape from painting the house.

There was a folk song that I liked to sing back then:

Get you a copper kettle, get you a copper coil
    Fill it with new made corn mash and never more you'll toil
   You'll just lay there by the juniper while the moon is bright
Watch them jugs a-filling in the pale moonlight.

It was like that: vaguely weird, vaguely criminal (was shipping insects around the country through the mail legal? my dad wanted to know), and somehow beautifully lazy. It was fun riding around in the midsummer night  on my bike like a bandits, collecting moths, building my stock. It was easy work tending the caterpillars – I just made the rounds every few days, checking the sleeves, letting the droppings fall out, making sure there were plenty of leaves still left. It took me no more than 4 hours a week, at least when the caterpillars were small. It left a lot of time for lying in the hammock with a book and going swimming with my buddies in the gorges.

But there were some anxieties, as there is with any entrepreneurial venture. After I first put the eggs in the sleeves, I didn’t check them for two weeks, knowing they would take a while to hatch. We had a lot of rain and I worried that they might be drowned by it. When I opened the first bag, I saw nothing, absolutely nothing at first, and I immediately thought “Oh, no – I’m going to have to send all that money back!” Then I noticed a fine powder of minute droppings at the bottom of the bag… I started gently flipping through the leaves – there were tiny black cecropia caterpillars everywhere.

All-in-all, most of the caterpillars I was raising were very easy to grow, and sometimes they grew surprisingly quickly.   I would open a bag where nothing had been visible beforeand there would be a whole herd of large, odd-looking caterpillars grouped together on the twigs, like an alien invasion. Some of them thrashed or clicked their mandibles (they don’t really bite) when I alarmed them. I discovered that many of the sphinx caterpillars actually had multiple color forms within the same brood. Some of them were startlingly lovely, at least to my bug-loving eyes. Here are photos showing a few of the species I raised:

And, of course, there were farming “issues.” Raising these large caterpillars in a high density environment is not natural, and sometimes a sleeve would catch a disease, which would wipe out most of the caterpillars in it.  (All farming is unnatural in this way, and human farmers have had to deal with diseases and pests since the beginning.) One sleeve got raided by a raccoon, which tore open the tube and ate all the caterpillars.  (This is the caterpillar farming equivalent of the “cow in the corn.”) Fortunately, most of the losses were among the giant silk moths, where I had lower demand and duplicate broods.

And there was no agriculture textbook or cooperative extension officer for me to go for answers to some of the rearing questions I had. I had to figure a lot of things out by guesswork. Cecropias make their cocoons in the branches where they feed (easy) but lunas (harder) usually make their cocoons on the ground in the dead leaves. I put the luna caterpillars, once they stopped feeding, in a big cardboard box with a bunch of freshly picked loose leaves. An even bigger problem were the sphinx caterpillars (hardest) – they normally go to the ground, dig a burrow and pupate inside it. How do I duplicate an environment for that? I tried putting loose dirt in a box, but they didn’t like it and wouldn’t dig (I guess the dirt was too loose). I finally solved the problem by putting a six-inch layer of damp sphagnum moss, mixed with a little bit of regular dirt, in the boxes – this made a soft soil they could tunnel through. (The moss made good packing material for shipping later.) I learned some things about insect biology in raising these insects. How do you efficiently mail pupas? Art mailing tubes worked, and could be cut to custom lengths to match the order.

A sphinx caterpillar that is about to pupate. It's easy to tell: fifth instar caterpillars suddenly change color slightly (note the brown back), stop eating and start wandering. 

One big thing I learned was that I needed to pay attention to the insects if I wanted to understand them.  As they got big, I got nervous about how I would know to remove caterpillars when it was time for them to cocoon/pupate – if  I pulled them out too soon, would they starve?  As it turned out, it was a no-brainer. Caterpillars about to pupate change color slightly and begin to wildly wander. Every day, I would find a few at the bottom of the bag, wandering around: they were ready to go into a box.

In the end, I was able to make almost all my orders (I still had a lot of cocoons from some species left, which I let go when they hatched the next spring – I more than replentished my wild source), and, after paying for all my supplies and mailing, I made around $500, which was real money back in the late 60s. It was more than anyone else I knew made that summer – at least legally. I didn’t brag to other kids though --most people just wouldn’t understand.

I farmed caterpillars the following summer as well, and was almost as successful. I had repeat customers, which meant that the pupas I shipped had survived mailing. When I was in college years later, I was still getting inquiry letters asking if I had “stock” to sell.

Thinking back on it, caterpillar farming was both the best and oddest “job “ I ever had.  I didn’t know it, but I was an entrepreneur, and a biotech entrepreneur to boot. It was a job that required imagination, vision, a little bit of craziness, and just plain luck. It was fun, a-watchin them sleeves a-fillin, in the pale moonlight.

Who Cares How You Look?

The birds do, Mr. Doo-doo.

After my last post, I have to say that the hickory horned devils are now second instar and, ahem,  big enough to be outside now, and everyone is happy about this, especially the hickory horned devils.

(I’ll talk about the arcane practice of “sleeving” caterpillars in a subsequent post.)

Each time a caterpillar sheds its skin to grow larger, it is said to have entered the next “instar.” The hickory horned devils are now in the second instar out of five. Some caterpillars look pretty similar in every instar (lunas, for example), but some change their appearances fairly significantly. Hickory horned devils are in the latter group. They are about an inch long and look like this:

What do you notice about the appearance of second instar hickory horned devils? Well, to birds they look like bird poo… or so entomologists think (who really knows what birds think?).

A fairly typical bird dropping. Yes this is gross – but the demands, of science, etc. If you look carefully at tree leaves, you see this all the time because a lot of birds perch in trees.

 Why do they see that in this harmless little caterpillar? Well, the splotchy pattern and the squiggly curve of the body does look a bit like the dropping photo, but this is also something that they have seen before in the bug world. Here are some other examples from both moth and butterfly caterpillars:

Viceroy butterfly caterpillar

 "Orange Dog" -- caterpillar of the giant swallowtail butterfly

Moonseed moth caterpillar

Ruddy daggerwing butterfly caterpillar

Sure, these caterpillars look pretty different from each other (and from the hickory horned devil), but you see the overall pattern – bumpy shape, white splotches mixed almost randomly with other colors, especially dark browns. Behaviorally, each of these caterpillars tends to rest curled in a questionmark shape. You can see the resemblance with the bird poo and with the hickory horned devils (though my photos aren't great).

Why do so many caterpillars use this kind of camouflage? (This is actually a form of what we call “mimicry” – but much more about that in later posts.) Well, there are some obvious answers. First, it’s a common pattern on tree leaves, so it really is a good pattern to copy if you want to blend in. Second, to birds it looks like… poop. Most creatures have a built-in dislike for putting that stuff in their mouths (or even getting near it) because being in contact with it is a great way to catch diseases from your fellow creatures. Think about your own disgust – it’s hardwired in us by evolution.  Though not all species share this built-in disgust  or “aversion” (dogs come to mind as an counter-example) to feces from their own kind, it’s common enough to assume it’s likely to be present in many birds.  

So that’s why this appearance pattern works as a survival “strategy,” but how did so many different kinds of caterpillars come up with it? (I’ve only shown a few examples – there are hundreds, if not thousands  of others out there.) You need to remember here that bugs don’t actually consciously “come up with” these natural tricks that help them survive, any more than you “came up with” your natural hair or eye color to make you look cool and attractive to other people.  The caterpillars’ appearance was, originally, the result of a random set of mutations -- or a random new combination of existing genes -- that accidntally happened to create this appearance. But the pattern happened to work for the bug that first was gifted with it by chance, and it survived ... and made a lot of similar bugs that also survived, and so the genes – the genetic instructions for how to look like bird poo – got passed on and became common. Because the trick works, the genes then stay common, even as the species evolved and passed on its genes to many other species, its ancestors.  This is basically how natural selection works on genes over time – genes that have a lot of usefulness stay in species’ genomes because they keep coming in handy in staying alive.

From the examples I showed above, you might have caught the fact that there are both butterflies and moths that use this pattern.  Think about what this means – biologists know pretty much for certain that all butterflies and moths are relatives – they have evolved from common ancestors, with butterflies first evolving from moths about  100 million years ago (this is a guess, of course – the oldest fossils of butterflies are about 48 million years old, but these fossils are very modern butterfly-like, indicating that the split from moths had to be earlier).  This means that genes that are shared between moth and butterfly species would have to be pretty ancient – at least 100 million years old.  I would guess that the basic genetic package for the bird poop-camouflage trick has to go way back  and still be hidden out there in the genes of most butterflies and moths for it to currently be so common in this group of animals.

Of course there is another possible explanation – it’s called “convergent evolution” – where different species independently “come up with” the same common set of features because, well, it works. This is possible here. The thing is though, for so many different varieties of moth and butterfly to come up with this same pattern there would still have to be some common underlying feature (a broken white/color pattern for example) in order for it to come up again and again and again. A better bet is that the bird poop imitation was so successful for the moth/butterfly ancestors that it stayed around long enough to eventually drift together on the genome as a kind of package -- a big and complex group of genes that is a fundamental tool for survival through the bird poop trick.

What’s fascinating to me about this kind of thing in caterpillars is that you can literally see the animal taking advantage of different parts of its genetic heritage as it grows up and its situation changes. When the caterpillar is really little (and too small to be mistaken for bird poop), it doesn’t look like this at all, as we have seen.  When it gets bigger, it’s too big to be mistaken for bird poop, and so its appearance changes again. But when the caterpillar is in its second instar, it is really just about the right size, so the time is right for natural selection to again pull these genes out of an ancient bag of tricks. Pretty cool, I think, the way you can see all kinds of pieces of ancient history coming out and showing itself to you in a living bug.

Bibliography

Wagner, David L. 2005. Caterpillars of Eastern North America. Princeton University Press, Princeton.

Scott, James A. 1986. The Butterflies of North America. Stanford University Press, Stanford.

Citizen Science – Not.

There are hazards to having 250 caterpillars crawling around your house…

...#1 would be withstanding the silent disapproval of your family.

Epizootic's family were less than thrilled with the caterpillars in the house.

Well, maybe not so silent. One evening, we were sitting as a family, watching the Olympics. I asked one of my daughters to get a book from my study. She came back, looking… concerned.

“Dad, where did all those big potted plants on your floor come from? What are you doing?? asked Epizootic’s youngest child, a little louder than was necessary.

“You know – I’m using them to feed the caterpillars…”

“Caterpillars?! In the HOUSE???”  Epizootic’s daughter has a mysterious fear of all things arthropodic.

“Sure, you know, the hickory horned devil and luna caterpillars I’m rais…”

“Hickory horned devils IN THE HOUSE???”  Ms. Epizootic interrupted, getting into the swing of things. Girl things. Screechy things.

“Yeah, you knew I was doing this – I have a couple hundred little caterpillars. it’s only until they get big…”

“A COUPLE OF HUNDRED BIG CATERPILLARS!!!” (Then she used my proper name in a way she only does when she’s a leeetle bit miffed.)

Some of the plants no one noticed me carrying into the house.

Then they started to get upset. Frankly, I was a little bit confused that they were surprised about all this, since I had mentioned that I was planning on hatching and raising hickory horned devils… true they had all been watching TV at the time., and I didn’t go into real details. I also told them I was blogging about it… and I thought everyone  was reading this blog…

Frankly though, I think it was a bit of a dead giveaway when I carried four large pots with saplings in them right past where they were sitting a few nights before. My family is not very observant. They apparently also never look in the open door of my study.

I like to think that I would have noticed (though I have been known to take a few days to register new haircuts).  That’s kind of the point of what I’m doing here – noticing things that are around us all the time, unnoticed.  And thinking about them a bit.

Though I’m interested in science and I like talking about it, what I’m doing here is not science. Nor is it “citizen science,” whatever that is. I actually think the term “citizen science,” is demeaning, as if you have to have a badge or a uniform to do official “science.”  Science is a method, not something you have to have a license to operate.

What I’m doing here is really closest to something us old-fashioned people used to call “nature study.” It’s not that the information that I’m discussing here is not scientific, or that some of this couldn’t lead to science… but what is really happening in my posts is just observation and musing. It’s the first step o being ready to science though, to understand that the world is full of interesting things we don’t know much about. This is where all sceintists start. With nature.

You just have to be curious. And observant. And not mind having a few little bugs in the house.

Note to family -- the caterpillars are big enough now to go outside. Can I come back in to get them?Pleeeeze?

A Southern Strategy

The votes are in…

… and surprise, surprise, the southerners have simple tastes.

As I noted a couple of posts ago, I decided to try a simple experiment with the hickory horned devils (aka regal moth caterpillars) and how they react to different foodplants. I did this because they are known to eat a wide variety of trees, but some of my source books  say (this is what scientists call “reviewing the literature”) that ” local populations may have different preferences.” I decided to try my caterpillars on sweet gum and hickory, both of which are described in the books as “primary” choices for the moth.  I assumed that they would be willing to eat either, but I was going to test which produced bigger, healthier caterpillars.

Well, as I said, surprise, surprise – the experiment was over almost before it began. When I gave some of the caterpillars pignut hickory (the most common hickory in our area), they clearly refused to eat it. Newly hatched caterpillars sat together on the glass walls of their hatching cage, like kids huddling in a crowd outside a classroom they don’t want to enter, and wouldn’t go near the leaves, though they were fresh and clean. After a day and a half, I began to worry that they were going to die if they didn’t eat, so I also put in some sweet gum leaves, which their earlier hatchlings had already accepted readily. Within an hour, they were on those and eating – you can see this in the picture below.

 1st instar caterpillars on sweet gum leaves. Untouched hickory leaves are in upper left corner.

So, these Charlotte-area hickory horned devils  have a definite preference for sweet gum – I would actually call it a requirement, since I’m pretty sure they would have died before eating the other plant. If I had more caterpillars to and if I was willing to let them die, I would have tried a lot of other plants and I would have pushed the experiment to the bitter end. This might have given me what scientists call “solid experimental confirmation” of the conclusion that my caterpillars exclusively feed on one tree. Actually, a fuller experiment would have been to run the experiment with caterpillars from multiple local moths (to make sure this moth’s caterpillars were not just freakishly picky) and at the same time also running the experiment with caterpillars from a moths caught in other places, say somewhere in the deep south and somewhere in the upper midwest. That would take some work, so forget it for now.

Nonetheless, I have a solid “hunch” that this piece of information is “meaningful.” Here’s why: the food preferences of my caterpillars is clear, it agrees with other “field sightings” I’ve had (the only fully grown hickory horned devil I’ve ever seen here was feeding on a sweet gum tree).  I know from long experience that clear insect behaviors are rarely “anomalies” (something unique).  Insect populations are large in any given area, population interbreeding happens every year (many times a year in some species) and natural selection weeds out “bad” behaviors with a fine-toothed comb. In other words, in the normal word of insect evolution, the major details regarding bugs in a local environment tend to average out to what works best. If my bugs like sweet gum over all other foods, chance are the whole extended family (the whole Charlotte-area hhdevil tribe) feels exactly the same way, just like I can bet you most of my red-state neighbors won’t be voting for any atheist-socialist-gun-control candidates this year (I could be wrong). The regional cuisine of Citheronia regalis  appears to be limited to sweet gum (ideally, deep-fried).

Though the best source books I have don’t say anything about this, a quick web-search of some insect/nature sites, does have some people commenting that southern populations of the moth prefer sweet  gum.  Most of these reference are vague/uncertain, clearly because no one has really studied the matter. The truth is, research in insects in general (except crop pests), not to mention research in something specific like the larval behavior of a single moth,  is not done much or well-funded, so no one really knows what’s going on here. In this age where we know a lot about a lot of biological fine points like individual genes and proteins, it sounds strange to say, but common features of nature around us have not been well-studied – by professionals. So the science of this really depends a lot on the “field reports” of amateurs like me. Think of it as mobilizing a lot of unpaid interns to do data collection.

So, I say, let’s not let our lack of scientific validity stop us from going forward with speculation here:  back to the “tentative data” I have regarding the food choices of a mid-southern population of the regal moth…

Consider the history…

 If southern regal moth populations are much more specifically focused on eating sweet gum, how does this make sense, when other populations (northern populations) are known to eat a lot of other things – and these other foods are all around in the south?

Let’s look at it the other way – do northern populations eat other things because of what is available in the north? Let’s look at range maps for both the regal moth and for the sweet gum tree:

 Though the ranges look pretty much alike (sweet gums can’t take the long, cold winter of the north and regal moths, which pupate in the ground, without insulating cocoons, probably also have an issue with cold), I notice that the range of the regal moth is somewhat larger, going a little further north than the sweet gum tree. So … northern populations of regal moth in, say, New York or Massachusetts,  must eat something besides sweet gum. This may be why the books describe the other plants as known food for the moth. If the moth’s range started in the south, eating sweet gum there, and expanded north, then it had to evolve to eat other foods in order to move further north…

We actually know that the moth started in the south and expanded northwards (as I explained in an earlier post) because we have a geological record of relatively recent (only 13,000 years ago) ice ages when glaciers covered North America about as far south as I live now. When the ice receded, plants and animals (including the regal moth and the sweet gum) moved back in to recolonize the land, scrubbed clean by ice. apparently, the sweet gum has not been as rapid in its northern expansion as the regal moth. Trees can’t fly, so perhaps the range difference is simply the result of difference of the two species spreading at different speeds across the landscape, but the maps suggest different ideas. Notice that the sweet gum isn’t present in higher elevations in the Appalachian Mountains  but the Regal Moth is – this tells us that the sweet gum is probably more sensitive to cold than the moth.

So what happened when the ice receded? It didn’t all happen at once, but when it did, large areas of land were first colonized by more cold tolerant species like hickories and walnuts, sycamores and ashes --  the sweet gums could only arrive when the south became pretty warm. The regal moth, if it had an ancestor living in the deep south (or if it was an invader from Central America, as I speculated before), had to be/become a “generalist” with very flexible food habits if it wanted to take advantage of the new landscape, so it learned to tolerate a wide variety of foods…

But the moth wasn’t always a generalist. Back before the ice, when there was no new, unsettled landscape suddenly open, the moth’s ancestors surely had done what most other moths and butterflies do – evolved a special interdependence with a specific plant or family of plants (see earlier post for explanation of how this works), because that strategy seems to work well. What was the foodplant that the ancestor ate? My “hunch” (based on extremely sketchy evidence) is that it was the sweet gum or its ancestor. One thing we tend to forget about evolution is that, though new gene combinations result in new features, the genes that made the organism what it was before are generally not lost, at least not for a while.  this means that most of  the genes that helped the regal moth ancestor live successfully with a specific host plant are still back there in the genome, and, once the conditions  return that made them useful  in the past, natural selection will encourage those genes and the traits they cause to return. The sweet gum is back… and the regal moth is reverting to its ancient ways, its ancient strategies of survival, its ancient tastes.  I certainly can’t prove it, but that’s my best guess.  Again, it’s fun to start with a little hard information and… speculate.

“Oh, I wish I was in the land of cotton, old times there are not forgotten…”  Now there’s an idea the bugs share.

Bibliography:

Tuskes, Paul M., Tuttle, James P. and Collins, Michael M. 1996. The Wild Silk Moths of North America. Cornell University Press, Ithaca, NY.

Petrides, George A., 1988. A Field Guide to Eastern Trees.Houghton Mifflin Company. Boston.

Hatching Magic

Adult male luna moth from "The Butterflies and Moths of North America.". It may be hard to believe, but these moths are much more beautiful in life than in photos, though this one is good.

Well, the luna moth eggs finally hatched today – I was beginning to wonder if the eggs were fertile. Like the regal moth caterpillars, they began hatching in a large first wave, about 30 or so hatching in the first hour. Since the eggs were not layed so rapidly, it leads me to wonder if they emit a pheromone or something that coordinates hatching.

Hatched luna caterpillars -- click to enlarge.

The caterpillars are light green and tiny – they are about one half the size of the regal moth hatchlings (as were the eggs). I have raised lunas before, so I know what they are going to grow up to look like – beautiful fat, green corrugated caterpillars. I'm feeding them sweet gum, like the hickory horned devis, and they appear to like it just fine.

If you have never seen a live luna moth, I have to say that I think it is one of the most beautiful creatures in the insect world. There’s a reason why the Lunesta sleeping pill people use the moth (or a bad cartoon of it) to symbolize their product. I have read numerous stories about people finding one by their front porch light and calling the newspaper or the local natural history museum to report it, assuming that since it is so spectacular looking, it must be spectacularly rare. They’re not rare, in fact, they’re just hard to see.

As I said, I’ve raised this moth before – in fact it’s the last giant silk moth I raised before the current batch. When my children were small (about 20 years ago), I brought them to my mother’s house in upstate New York. It was summer and my oldest daughter was about 6. One night, on a whim, I took her on a drive to visit the places I used to go to collect moths when I was a teenager, and we caught a luna – it was high on a wall, underneath a light and its extravagant, long tails hung down like the wings of a magical fairy or an exotic bird. It was a female too, and I got it to lay eggs and brought them back with us to Arizona, where I raised the caterpillars. The whole thing made  such an impression on my daughter that years later she got a tattoo of a luna on her back.

I can’t say that I was pleased when I first saw the tattoo (no parent that I know of is pleased by their children’s tattoos, though science writer Carl Zimmer may try tell you otherwise) but it did soften the blow a bit when she told me that she got it in memory of that special evening with me, so long ago. I have to admit that the tattoo artist really did a good job, accurately capturing the luna, and the moth is printed on me too, in my memory.