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.

Food Loyalty in Bugs

In which I talk about picky eating in bugs, and decide to try an experiment…

Most bugs are picky eaters. Photo © Men’s Health

Another unusual thing about the Hickory Horned Devil (the regal moth caterpillar) is that it eats a whole lot of things besides hickory. According to The Wild Silk Moths of North America (WSMNA), it eats basically any nut tree (Juglandaceae – hickories, walnuts, butternut, pecan), sweet gum, persimmon, all sumacs, sourwood, ashes, sycamore, lilac and cotton. Other sources note that it will also eat oaks and cherries. This is an unusually broad range of food plants for a moth or butterfly caterpillar, and even a broad range for  a giant silk moth caterpillar (a group which often does eat a large number of deciduous tree species), and it notably includes leaves that are pretty different in taxonomy and texture and smell, which means that they are probably chemically different (I’m guessing).

Some of the regal moth's foodplants.

Why are many caterpillars only found on one or two foodplants? Well, science has worked out that it almost certainly has to do with past history. Imagine hundreds of thousands (or perhaps millions) of years ago, a species of moth found that a specific plant or group of plants suited its dietary needs – it had leaves with materials in them that the caterpillars were capable of chewing… could digest… had nutrients that the caterpillar needed to grow, and contained no poisons that could make the caterpillar sick. Perfect! However, if it was too perfect a food, this could be a problem for the plant because the caterpillar populations would thrive, eat a lot of the plants and quickly it would start threatening the plant’s survival. This growing catastrophe would then strongly favor the chance survival of individual varieties of this plant that happened to be a little different genetically so that they contained some different chemicals making those varieties more toxic – in other words, the leaves tasted worse -- to the caterpillars. The worse the plants tasted, the more plants would survive (perhaps this a defense mechanism the broccoli tribe has used against human children). In this way, over a lot generations, the plant would evolve a chemical defense against the caterpillar that was eating it until it wasn’t threatened any more. It’s an evolutionary process called natural selection – perhaps you’ve heard of it.

The back-and-forth “arms race” between plants and caterpillars.

However, natural selection also works for the caterpillar – since the plant was otherwise a good food, lucky caterpillars that happened to have genes making them less sensitive to the plant’s new toxins would tend to survive better, so after a while all the moth’s caterpillars might develop immunity to the new toxin (bad-tasting poison) in the plant. They might even evolve to use the toxin in their own chemistry to help them grow and survive. So then there would be a problem again for the plant, and varieties that had new toxins that the caterpillars didn’t like would survive better… and then the caterpillars… and then the plants… you get the idea. It’s an arms race between plant and caterpillar, and  the two begin evolving together, so the caterpillar’s chemistry is very finely tuned to eat a specific kind of plant that contains a lot of nasty chemicals. It may have even  evolved to use those chemicals to defend itself against caterpillar predators (like birds), because something that is toxic to caterpillars is often also toxic to animals that eat caterpillars. A familiar example is the monarch butterfly, which feeds exclusively on milkweeds, which contain some nasty poisons, which the monarch caterpillar (and butterfly) use to make it poisonous to birds.

 Toxic monarch caterpillar on toxic milkweed plant

Plant-eating insects like moths and butterflies have evolved this way with specific plants for a long time, which makes most of them picky eaters… which is why a non-picky insect like the royal moth is somewhat unusual. How would a moth come to be this way? My best guess (another “hypothesis” of mine) comes from something that I was talking about two posts ago – the fact that regal moths and their relatives seem to be relatively recent “invaders” from the south, taking advantage of ecosystems that are relatively “new,” thanks to past ice ages. In ecology, scientists note that the species that can best take advantage of a new place (where there are not a lot of established, well-balanced relationships between species) are creatures they call “generalists.” Generalists are species that can eat lots of different things,  aren’t fussy about living arrangements, etc. and thus are able to make do in new situations better than other organisms that depend on established relationships with each other. In older ecosystems where all the organisms have long worked out their relationships with each other, these “generalists” may not do as well, but in less-established situations they do better. Curiously, this points to the fact that the regal moth, apparently a relatively recent invader, is somewhat of a “generalist,” despite its unusual appearance (which looks very specialized) and freakishly large size. It’s well-suited to survive in a variety of conditions by eating a variety of foods.

 The thing is, evolution is still going on. WSMNA notes that some studies have found that for regal moth caterpillars “the effectiveness of host plant assimilation varies from population to population.” In other words, regal moths in different areas appear to be evolving preferences for different species of foodplant. The authors go on to say: “A study of localized preferences in this species, with its broad array of natural host plants, would be an interesting topic for further investigation.” Suggestion accepted: I think I will attempt a small (and poorly designed, since I’m winging it) experiment.

 What I am doing is picking two different foodplants -- pignut hickory and sweet gum -- and testing whether one seems in any way to be a preferred choice for the caterpillars I am raising. Do they accept one readily but not the other? Do a larger percentage survive when fed one than when fed the other? Do caterpillars grow at different rates on the different plants? (etc.) I picked these two plants because both are really common here, and both are on the list of “most common host plants.” They are not closely related plants, so they are likely to have significantly different toxins in their leaves.

Hickory horned devil caterpillars from Charlotte, NC seem to prefer glass to hickory leaves. This being the South, maybe I should have offered them Chick-fil-A,

I will try to gather some data relating to some of the questions above, but don’t expect too much from my informal experimenting here… I may produce some evidence for a local food preference, but it sure won’t be conclusive. Think of this as a test run for a much-better-designed experiment that you might think up and run.

Bibliography:

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

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

The Caterpillars Have Hatched and They Look...

Awww... cute? Really, somewhat badass. Like they want to be punk rockers when they grow up.

Honestly, I can say that regal moth caterpillars have more "hair" (actually, they are spines) per body mass than any other creature I've ever seen. They are about 1/3 an inch long(say a little over 1 cm, if we go metric -- the largest just-hatched caterpillars I've ever seen too) and about a third of them is spine.

Why they would evolve to have such a bizarre body at hatching is anyone's guess. At this size, "protection" seems pointless, and this is an huge encumbrance, not to mention a waste of resources. I guess the "designer" wasn't so smart on this one.

Anyway, I have them started on a foodplant -- sweet gum. This is going to be my primary foodplant for both the regals and the lunas, since I have a lot of it in my yard, with plenty of branches that are within reach. I'm also going to do a little experiment, feeding a small number of the regals pignut hickory instead, but more about that in my next post. Both luna regal moths eat pretty much the same trees (a lot of different nut trees and others with aromatic leaves...) though regal has the bigger palate.

I'm going to "sleeve" these caterpillars outside soon, but when they are this small I don't want to lose any, so I'm starting them on potted saplings inside. I've made a tube of nylon screening that I can put over the plant and tie at the top for easy access. This allows the cage to "breathe" and hopefully will keep the babies from drowning in condensation drops or getting mold. A lot can kill you when you're little. I'll lose some anyway, but with any luck, not too many.

Giant silk moth caterpillars (and most large moth caterpillars) grow in five stages ("instars") and sometimes radically change in appearance each time they shed and enter the next stage. In some species (not in this one, I think) there can be a lot of variation in coloration (for example, some can be green, some can be red, some can have stripes, some just a few spots) even within the same group of eggs from the same mother -- variation that is much rarer in the adults. Why? Again, who knows. It's a mystery I may speculate on later. It's a topic I've never heard entomologist talk about much.

By the way, another strangeness with these bugs: A group of them hatched VERY rapidly. I looked at the eggs and none had hatched at about 9:10 this morning. I looked again just a little before 10 and there were a couple of dozen crawling around. The eggs were not laid very simultaneously, so hatching must be triggered by some kind of environmental factor -- light? Anyway, interesting. I notice that there seem to be big groups at different (but similar within the group) stages of development (judging by color).

Imagining the Moth

The female regal moth I caught on July 26 was dead by last night, having laid 135 eggs (which entomologists call “ova” – scientists like to have their own jargon) . I’ve cut them out of the cardboard container I kept her in and put them in the clean glass container you see above for careful monitoring. The standard egg laying container is a paper grocery bag, but I didn’t have one, so I made due with  a stationary box, which did the job. They are the largest lepidoptera (butterfly/moth) eggs I’ve ever seen and they are a beautiful translucent green, except for a few, which are now partially brown. Those are that way because the eggs are transparent and you can actually see the embryonic caterpillars developing inside – the brown is their heads—I’m guessing some will hatch tomorrow. I have saplings of their foodplants ready in pots – when they are really little I want to grow them inside the house where I can control the environment.

Before you feel too sad for the moth who died for this , I need to say don’t fret – she accomplished her primary mission in life: laying eggs. Giant silk moths only survive a few days as adults in the wild, living entirely off fats that they accumulated as caterpillars. They have no functioning mouthparts and they don’t eat.  They have only two missions – to mate, and to lay as many eggs as possible.

I”ll talk about their mating behavior another time (because it’s an important topic), but their egg laying is also interesting. Regal moths, like many of their close kin, often fly great distances, depositing an egg or two on one tree, then flying a quarter mile to lay another egg. There are a lot of possible explanations for how this behavior evolved.(I like to call these “explanations” “hypotheses” because no one actually knows how the pressures of nature encouraged the behavior’s development, and, frankly, it’s hard to imagine an experiment that would prove the “real” cause. Honestly, what follows is all unproven speculation – intelligent guesses. In science, the next job would be to find some clever way to test these ideas – to design an experiment and then collect solid information – “data..” However, nature – the world outside is a messy, complicated place and coming up with intelligent designs for such experiments is very, very difficult. For now, I’ll stick to the fun, imaginative part – the hypotheses.

 My personal best guess  for why regal moths have evolved to fly long distances and lay single eggs is that, since these eggs grow into very big caterpillars, it would be a bad idea for too many of them to be on one tree because they are likely to attract the attention of predators (such as birds) and once a predator has found one caterpillar, it is going to be primed to see another one nearby. So, this hypothesis says I saved the female moth the effort of flying all over the forest to lay her eggs, so she probably was able to get more laid before her fat-stored energy ran out (fat is really just a chemical battery, if you think about it). She had to lay them all together, which might be disastrous in nature where there are birds, but I’ve got her back – I’ll protect her caterpillars from predators. This makes me the good guy. Really.

However there is another “hypothesis” for why this moth has evolved single egg rather than mass laying behavior and this explanation perhaps means that I’m not being so clever in “farming” these caterpillars. Some of the regal moth’s near relatives, the oakworm moths (species in the genus Anisota), do tend to lay their eggs in big masses, and the caterpillars feed together in big troops, especially when they are young. (Herding together must serve some kind of protective function for these caterpillars, like it does for bison and schooling fish and ducks and starlings and the like, but I have no idea how that works – make your own hypothesis.) Oak moth caterpillars are considerably smaller than regal moth caterpillars at maturity and they grow up faster. Importantly, this means they have less time chance to catch a disease that could spread from caterpillar to caterpillar quickly, wiping out all a moth’s brood in one rapid epidemic.

       Young oakworm moth (A. osalari) caterpillars. National Park Service photo by Sally King.

This is a familiar problem to anyone who has ever tried to raise a lot of giant silk moth caterpillars  – you have to grow them in crowded conditions (you’re really “farming” them) and various diseases can appear and destroy your project. So perhaps another explanation (hypothesis) for why the regal moth lays single eggs is that this protects long-maturing caterpillars from spreading the diseases a few are likely to catch during a long period of infancy. This could be the explanation, or both explanations could be right or… something else entirely might be the cause. Nature is complicated and it’s hard to imagine all the problems that might come up in the life of a moth.

But it’s still fun to make guesses about the evolution of unusual characteristics in a species. One other feature of regal moth egg laying behavior that I also think is a bit strange is the fact that these moths produce only one generation a year, even here in the south where summers are long. This is actually not typical of most giant silk moths. Lunas, for example, have at least two generations a summer here in North Carolina, and researchers have noticed that they have three in Texas. The first lunas hatch and lay eggs in April here, but the regal moth waits until the end of July or August to hatch, mate and start laying eggs. Why?

We can reject the explanation that the moth has only enough time for one generation a year because it is so big that it takes a long time to grow. It does take a relatively long time to grow, but not really that much longer than a luna, which goes through the whole cycle twice. So let’s add another piece of evidence: its closest relatives (other species in the sub-family Ceratocampinae) also have only one generation (one “brood” as scientists say, or “univoltine” behavior) though many are much smaller moths. The authors of The Wild Silk Moths of North America  have suggested what I think is a convincing hypothesis for this: “In the United States and Canada the greatest number of species occur in the Southeast. Midsummer flight in warm, humid weather maybe an adaptation related to the tropical affinities of the group.” In other words, the authors note that relatives of these insects are a lot more common in the American tropics, so species like the regal moth may represent relatively recent invaders from tropical climates and they may still have hold-over physical adaptations from their not-too distant past, such as favoring the time of the year when North Carolina feels like Mexico or Honduras. In the tropics, the moths lay eggs in the rainy season, since that’s when the food plants are freshest. Perhaps these tropical moths have carried the home country behaviors here.

 This is another perhaps un-provable hypothesis regarding evolution, but I like it because it again makes you expand your imagination to consider yet another dimension that can affect things: history. Consider the fact that only a little over ten thousand years ago (a blink of an eye in biological history, really) most of North America was covered by the expansion of a thick ice sheet. (Ice might have been a mile thick where I live now.) When that happened, where did all trees, bugs and other animals go? They either died out or they moved south to where there was no ice (Florida would have been ice free, but much cooler than now.) Then, when the ice receded back to the north, old species returned and some new species invaded from the south. The regal moth and its relatives may have been just such invaders – they still carry the signs of their history in the way they live.

So, in order to come to grips with how a moth works you need to imagine all kinds of things – what it’s really like out there – and what it must have been like out there a long time ago. Who knows what the most important “causes” were? There were certainly many, all working together and the result of the biological evolution they caused is the moth we have now. To really know we await clever minds, brilliant experiments and … data.