The Risks of Agriculture

The Risks of Agriculture -- and of Caterpillars in Close Quarters

Factory farming caterpillars.

Should I try to become the Smithfield Foods of the hickory horned devil production industry? I think not.

Most of the remaining hickory horned devils have pupated by now and most of the remaining lunas have made cocoons (again, email me if you would like any – otherwise, I’ll return them to the wild) but, frankly, I had a lot of late losses. This brings up a problem that is near and dear to my heart  (and related to the title of my blog)– the ecological problems that happen with agriculture.

What does "agriculture" have to do with raising caterpillars? Well, think about it – what is agriculture? It’s picking some species (or a set of species) that originally came from nature and creating special conditions that allow those species to prosper, unencumbered by the normal ecological balances that control population size and rapid growth – predators, disease, resource limitations, competition, bad weather, etc. Though it seems counter-intuitive to people who don’t know a lot about biology, these natural “checks and balances” are actually good for most species in the long-term, as they keep populations within “sustainable” (a word you’ve probably heard before) limits and, through natural selection, force the population to maximize the “fitness” (ability to survive) of its members, as well as maintain a healthy, diverse gene pool to address future needs. (A number of my previous posts have addressed these issues, as you may have noticed.) As the song says, “you’ve got to be cruel to be kind.”

With agriculture  – in other words, with human tampering to short-circuit this environmental system in order to increase an organism’s population for our own selfish purposes – predictably comes problems because the long-evolved  environment and ecosystem doesn’t  in fact just disappear when we start fooling with their parts. These predictable problems are really familiar to human civilization: exhaustion of soil nutrients (probably the biggest thing limiting agriculture and human population growth until the invention of artificial nitrogen fixation in the early 20^th Century), land degradation, weather-caused crop damage (especially from drought, especially before modern irrigation projects, though this is still a big,  unsolvable problem) and pests, predators, disease. These last three are really the ecosystem striking back, as the big monocultures that agriculture creates invite the complex biology of the environment to, as they say in business, “institute corrective measures.”

Pests (a term usually applied to insects) are mainly a problem with plant crops.  Big monocultures – vast fields of corn, wheat, cotton, soybeans – invite huge population explosions of the bugs that happen to be evolved to eat those specific plants. We mainly solve this problem through using huge quantities of arthropod-specific poisons we call pesticides, which, of course, cause other problems and are really only short-term solutions, because insects evolve and adapt quickly to poisons (see my earlier post on insect evolution and food plants) and eventually will become immune to anything that is not prohibitively toxic.

A big cornfield -- a monoculture of corn. The corn borer moth
thinks it has died and gone to heaven. It's just Iowa.

Predators (this is really a false distinction, since insect pests are really just predators of plants) are generally those parts of the ecosystem that attack livestock or animal agriculture – wolves eating sheep and cattle, hawks eating the chickens, etc. Since the predators of the few animals we grow commercially are not all that common in nature, we generally have “solved” this problem by eliminating livestock predators from the environment. This may seems like a good solution, but it too causes big problems. The population explosion of white tailed deer (and mice squirrels and other rodents that aren’t quite as noticeable)  and Canada Geese (now plaguing suburbs throughout the east), is a direct result of wolf elimination. And there are further downstream effects from that – you have perhaps heard of  Lyme Disease? It’s all only going to get worse.

And, finally, agricultural diseases – the problem topic I’m most interested in.  We don’t think much about the microbial ecology out there (the complex interactions is the biosphere of millions of species of bacteria, viruses and fungi) because we can’t see it . Our knowledge of these organisms is very new--we, in fact, really didn’t fully understand what the causes of disease were until the development of biology in the 20^th Century.  But  whether we know that microbes are playing a big part in the ecosystem or not, they still profoundly affect things. In plant agriculture, you simply can’t grow certain crops in parts of the country where certain diseases range. Wine grapes, for example, are basically impossible to grow in much of the American south where a certain leafhopper ranges, carrying with it a bacterial disease lethal to European grapes; the introduction of chestnut blight, a fungal pathogen, to North America basically eliminated the American chestnut, a once dominant tree in the eastern forests. If you are a wheat grower, just a single incidence of wheat rust is a major catastrophe, because it eliminates wheat as a crop, if it ever gets established.

This is all because monoculture itself is basically a complete denial of the realities of microbial ecology. In the environment, pathogens (microbes that kill or severely damage multicellular life) are relatively rare because (1) if a microbe evolves that destroys its host, it thus eliminates its food source (at least in the populations where it is present) and cause its own elimintatio; (2) pathogenic microbe populations are controlled by competition/predation by other more benign (and more successful) microbes in the microbial environment and (3) they are balanced by anti-microbial adaptations (like your immune system) that multi-cellular life forms have evolved over time. Generally, successful microbial species are those that have evolved to come to an ecological balance (think of it as a truce, or a trade agreement) with the hosts they live on/in and with the other microbes that also share that environment.

Monocultures mean growing organisms in unnatural crowds.
Pigs and chickens in factory farms.

Monoculture, which makes radical changes in the ecology, changes all that, eliminating many of the checks and balances and cooperative agreements that have been negotiated by the evolutionary process. This change thus allows microbes that otherwise not be well adapted to survive to actually flourish, and it gives a huge, easily accessible food supply for them to spread through with terrifying speed (hence the apt metaphor you sometimes hear of diseases spreading “like wildfire”).  “Easily accessible” is actually one of the largest ecological weaknesses in human agriculture: We grow vast fields of corn, house thousands of pigs in giant barns; in nature, populations are spread out across landscapes, mixed in with other competing species – if a new, deadly pathogen emerges, it may kill off a local population but then die off and subside before it’s presence in the environment becomes massive and it is a threat to the existence of the entire species. In other words, there are natural barriers to the uncontrolled spread of most pathogens.  By re-arranging things to suit our needs and wishes, we have unwittingly eliminated most of those barriers.

 The struggle with disease and microbial ecology is really the biggest problem in animal agriculture, and, frankly, it’s not a winnable battle.  We are currently using some very serious (and seriously dangerous) tools to combat this problem. Read up on the widespread use of antibiotics in livestock farming and Google “antibiotic resistance” if you want to begin understanding the catastrophe this is leading to. We simply can’t eliminate the unnatural spread of pathogens in monocultures of animals without causing still larger problems.

Make no mistake about it, growing caterpillars in sleeves is exactly the kind of unsustainable monoculture practice I have been describing, and, like any unwitting farmer, I’ve been plagued by totally predictable problems. As I’ve described in earlier posts, my unnaturally large populations and limited natural supply of nutrients (in my case, the sweet gum tree leaves that I can reach without a 12-foot step ladder) has led to problems – stunted growth and the lunas chewing through their sleeves, allowing parasites to attack. Then, further, the weakened populations of caterpillars became more susceptible to bacterial diseases, which, because of unnatural crowding, spread rapidly through my stock. The end result is that I only have about a dozen live pupae of each moth, after starting with over 200 eggs. You may remember me talking here in an early post about the natural adaptation of giant silk moths to spread their egg laying over a large geographic area and many trees. The catastrophic spread of disease under crowded conditions is yet another reason why that is necessary for the species.

Of course, my agriculture here has been small scale, and I have still had a modest success – I’ve successfully taken more caterpillars through the larval stage than  would have likely survived in nature.  Think of me being like a small, subsistence farmer, who has many farming disasters but still manages to grow more food crops than he could ordinarily forage from nature.  It’s still a disaster, but the practice is less unsustainable than what happens when you try to ramp it up to a larger scale. If there really was a big market for hickory horned devils, it might lead someone like me to think about developing a factory farm for moths… At which point, a wise person might consider pursuing another profession.

Mimicry, Shmimicry. It's the Fear, Stupid

Mimicry, Schmimicry. It's the Fear, Stupid.

What’s the reason for  the copy, or, perhaps as Yeats said, how do we know the dancer from the dance?

Two posts ago, I discussed mimicry in caterpillars and adult Lepidoptera again, mainly focusing on adaptations that allow these yummy bugs to hide in plain sight from all the creatures that want to eat them.  But this is really only half of the story. The other half – using mimicry as a kind of offensive weapon – is really more to the point when considering the case of the 5^th instar hickory horned devils and the spider mimicry that I have proposed their horns are helping them accomplish.

If camouflage is one way caterpillars and moths keep safe from birds and other predators, another is to develop a visual association with things these animals actively avoid – things that they find aggressive, unpleasant, or are outright life-threatening. If done effectively,  this is an even better strategy, because when the predator succeeds in discovering the bug, it still works. Birds and other predators actively avoid things that they are scared of. I say “if done effectively,” because mimicking a threat animal may, in fact, be harder to evolve working adaptations for than developing camouflage. Nonetheless, there are a lot of examples of effective fear-based  mimicry in nature, so it can’t be that hard.

A common kind of offensive mimicry in caterpillars is to evolve color spots that look like eyes, so as (obviously, due to their long, cylindrical shape) to look a bit like snakes.  Birds really do not like snakes, for good reason – snakes are one of their few predators that can get up in the trees. Here are a few examples of caterpillar-snake mimicry:

Eye spots are apparently relatively simple to evolve through changes in the genes that affect surface coloration, which may contribute to this being a popular (commonly arrived at through the forces of natural selection, that is) survival trick.  Eye spots also occur a lot in adult moths and butterflies, especially in larger ones – for instance among the giant silk moths. Here are two different North American examples:

Io Moth

Polyphemus Moth

Obviously moth eye spots are not there to mimic snakes, so what is the function? Well, birds have other predators as well in the trees – climbing mammals such as raccoons and opossums (and squirrels, which, believe it or not, sometimes eat bird), and especially hawks and owls. Behavioral biologists generally assume that the wing spots on moths are there because when a disturbed moth flashes them it gives an attacking bird the shock of thinking that it has stumbled on an owl.  If you are a small bird, always on the lookout for ambush attacks from these winged predators, this would be a very nasty surprise and an effective scare.

“Avoidance,” which we humans think of as a rational, learned thing, is actually more a behavioral adaptation in nature, and an unseen property in ecosystems. Creatures living in the natural environment have a day-to-day struggle to stay alive: to not be eaten, to not starve, or to not fall into a state (through lack of food, injury, sickness, etc.) where they cannot maintain the strenuous activities they absolutely must perform daily to keep living. Generally, they have no backup, no support system, the way humans do with our human society. Consequently, when there’s something that is pretty constantly bad/dangerous/risky for them in their natural environment, natural selection tends to evolve an automatic behavior in the animal’s population that causes them to avoid the risk – natural avoidance . Let’s call it an instinctive fear.  Humans can’t see these instinctive fears in nature directly (you’d have to be inside the bird’s head), but it can be a real thing, with really obvious signs – a pattern in the ecosystem, if you like – that you can detect indirectly.

Take the strange pattern of the commonness of blue-black butterflies in the American South.  Here are a few examples – notice the common pattern:

How do we explain the commonness of this pattern of colors? Okay, you might say, these are all swallowtail butterflies – they are all related to each other, so maybe they’ve all just inherited the pattern from a common ancestor. But then consider these two butterflies, which actually come in two different color forms,  a blue-black form, and one that is not:

Red spotted purple (Limenitis arthemis)

Banded purple (Limenitis arthemis)

Eastern Tiger Swallowtail (dark form)

Eastern Tiger Swallowtail

Notice first, that the red spotted purple is not related to the swallowtails (it belongs to the brush-footed butterflies, a very different group) and that the different forms of the two species are really different in appearance. One form of each is blue-black, while the other has vertical stripes. (Vertical stripes are an interesting defensive adaptation that helps flying butterflies survive bird attacks by making it difficult for the bird to distinguish between the butterfly’s body and wing when it is in motion – if the bird pecks the wing, the butterfly gets away.)

A key piece of information to note is that the blue-black forms of these two butterflies are not common everywhere in each butterfly’s range: they are only common in the south. When I collected butterflies as a kid in Ithaca, NY,  I never saw a red spotted purple or a dark form tiger swallowtail though banded purples and regular tiger swallowtails were very common.  But when my family traveled about 100 miles south (say, to Long Island or Pennsylvania ) I did finally see these elusive insects and collected them excitedly. I was really confused why they were so common a relatively short drive away.

There is a reason – the pipevine swallowtail (the first photo in the series) feeds pretty exclusively on the toxic southern plant pipevine and, with that plant, has a range whose northern limits are about at Long Island and central Pennsylvania. The poison the butterfly gets from its food plant is, apparently, very toxic to birds – so toxic, in fact,  that birds are really afraid of it (avoidance again), and other butterflies (none of which are poisonous themselves) have adapted (though natural selection) to take advantage of this fear through mimicry of the pipevine swallowtail.  This is a kind of mimicry that biologists call “Batesian Mimicry.”

I’ve read descriptions of how this works that go basically this way: “any bird that tastes a pipevine swallowtail has such an unpleasant experience that it doesn’t ever forget it and any butterfly that looks like the noxious one gets left alone as well.”  Is this what is happening?

As a human, it’s natural to imagine it this way, since we learn most of the avoidance we practice – like learning that the flame on the stove is hot, etc.  For birds to learn the poisonousness of butterflies this way however, seems somewhat unlikely – it would likely take a little while for the association of sickness with the specific insect to be learned, and birds only live a year or two. It seems likely that only a small part of the bird population would be experienced enough to practice avoidance. There’s also the issue of how clear a lesson birds are being taught, given all these harmless, tasty mimics that are perfectly good to eat also flitting around the birds. In my observations, both here and in other parts of the south, pipevine swallowtails are not rare, but they are by no means the most common of all these insects. If you were a bird regularly eating blue-black butterflies, and only occasionally ate one that made you sick, how long would it take for you to get the message? A while, I think. If each bird has to learn for itself not to eat a blue-black butterfly, the result seems hardly likely to be effective enough to make mimicry an effective adaptation – which it must be.

So let me propose a slightly different hypothesis that I think makes a little more sense, given all the evidence of mimicry: I propose that the toxin that is in the pipevine swallowtail can be seriously dangerous to birds’ health, having a significant impact on their ability to survive.  If the toxin is this harmful, natural selection would favor the development of an instinctive fear (natural avoidance again) of butterflies that look like that. This instinctive fear is really a simple, built-in behavioral rule that keeps the birds away from that particular form of harm (like a built-in fear of snakes and owls, or like my fear of spiders).  This behavioral rule is powerful and widespread (it evolves in many species for the same reason), so natural selection (in turn) favors many other butterfly species adapting to take advantage of its invisible presence. It doesn’t matter that most of them are perfectly good to eat – natural selection has hardwired in a dislike for black butterflies as a general principle, so if they evolve black coloration the avoid being eaten.

Pipevine -- a pretty plant, but the source of birds fearing
black butterflies.

My basic point here is that there are features in the landscape you can see directly (watch an owl eat a songbird), but there are also important forces out there that exist invisibly (in behavioral adaptations, like fear of black butterflies for example) that you can’t see directly. You can, however, still come to understand what these forces are by looking for patterns (common forms of mimicry, in this case) that provide clues.  In some ways,  I think it can be more exciting in nature study to see the invisible than it is the visible.  Who would have known that the innocent little pipevine plant could be responsible for  a significant piece of predator-prey behavior in eastern North America or for making many butterflies in the south black? Perhaps something similar is responsible for the bright coloration of the adult regal moth, but I’ll leave that for a later post.

Pudgy on the Outside, Tough on the Inside

Pudgy on the Outside, Tough on the Inside

The hickory horned devil caterpillars have become large and
scary looking, but really they're sweethearts.

Some updates on my rapidly maturing caterpillars. First, the hickory horned devils are beginning to pupate. Thank God! They eat so much that I have run out of reachable limbs  (even by step ladder) on two different trees. I have four out of about 30 survivors currently pupating and more, I'm sure, to follow soon. The mortality in this brood was higher than I expected, but I'll still get a good crop of live pupas. If you would like me to send you a couple, email me at jbhathaw@uncc.edu. Again, I would prefer to send these to kids and/or parents with kids, but if you want some, let me know.

Other than the die-off (mainly when they were little) the hickory horned devils have been relatively easy to raise. When they are ready to pupate, they very helpfully turn from emerald green to blue-green,  which eliminates all worry that you are putting a caterpillar that still needs to eat more into a pupation cage with nothing but dirt.

Luna caterpillars in about 3rd instar. They're tougher than they
look.

Wish I could say the same for the lunas, which were as hardy as all get-out, yet became ornery as they became older. You wouldn't guess this from the cute little florescent green fatties, but they are one tough bug.

I've been astounded with them regarding their appetite. Lunas don't seem to grow any where near as fast as hickory horned devils -- they're currently only in 3rd or 4th instar and about one quarter the size of full-grown horned devils -- but they seem to eat a lot more per caterpillar. They will frequently eat an entire sleeve full of leaves in less than a day. This is a pain, because changing sleeves is a significant amount of work. If you try growing these, definitely use a full bed sheet sized sleeve, which might allow you to not have to move 50 or so caterpillars more than once a week.

But that's not the only reason for using large, linen sleeves. As I've mentioned before, I decided to use nylon screening for my sleeve material during this project because it's hot here in Charlotte, NC in August, and I thought the better air circulation would lead to healthier caterpillars. But last week I had a surprise -- I went to check the luna sleeve and found it riddled with holes. At first I thought a squirrel or a mouse had been at it, but on closer inspection I realized that the caterpillars (about an inch long at that point) had been chewing through it themselves. I caught several hard at work. About 20 or so had staged a jail break and were busy eating on nearby branches outside the sleeve.

Holes the little buggers chewed. Smart, real smart.

I want to stress that nylon screening is not a delicate material -- it's made to replace wire window screens. These little suckers can chew! They're not so bright though -- by making holes in the sleeves and by spending some time outside its protection, a number of them may have doomed themselves. As I've mentioned, parasitic wasps are major predators of giant silk moth caterpillars, and I have no doubt that a few took advantage and laid eggs in some of the lunas. I've already found one that was dying from parasite infection. So it goes -- in farming you have loses because nature is merciless. I've never had this problem before with linen sleeves, so I'd suggest that anyone reading this who wants to try raising caterpillars use that material -- I don't think the luna mandibles can get an handle on the more densely woven material.

Interestingly, I have had no such problem with the hickory horned devils, though they are much, much larger and stronger. Why not? Well, as usual, I don't know, but I do have a guess.  I think the horned devils are adapted to feed on a relatively few number of species that have relatively tender leaves, while the lunas, with their more northern range and much broader range of food plants, are a much more generalist species, and have adaptions that allow them to handle broader food choices, including leaves from some trees with dense, cold-resistant leaves. In other words, the foods they eat require that these caterpillars, soft and chubby as they look, to have strong, capable jaws. Jaws that can clip handily through nylon screening designed to block tougher  bugs.

As I have alluded to before, the hickory horned devils are all bark and no bite -- just soft southern bugs with big hairdos designed to keep them out of fights. Lunas are northern brawlers -- put them in a cage?   Fuggedaboutit.

Why Are Insects Deceptive?

Why Are Insects Deceptive?

In my last post, I talked about mimicry in hickory horned devil caterpillars and I’ve talked about it before.  It’s an interesting issue in looking at insect ecology, behavior and evolution. When you start looking for it, you see it everywhere.

Why are luna caterpillars bright green and ribbed? They are mimicking the green, ribbed surface of the leaves they live among, since the leaves are everywhere and don’t attract attention. Birds and other predators don’t eat leaves.

It’s essentially camouflage, and it can be remarkably specific. Remember an earlier discussion I posted in which I talked about how regal moths and their relatives were tropical invaders who breed only in the late summer rainy season? Well, if you think about it, this explains some of the unusual coloration of some of these moths – coloration which,  when they come to the light on your front porch, certainly seems to stand out.

Take for example, one of the regal moth’s cousins, the large, bright yellow, imperial moth:

This moth, seen by itself, certainly seems to be showing off rather than hiding… but that’s only because we’re seeing it out of context from the woods in which it lives, during August, the season when it is flying. At that time in the late summer in the south, a number of trees prematurely begin to shed leaves. A common forest tree that does this is the tulip tree. Here is a picture of a tulip tree, taken in the season when these moths are flying:

… and here is a picture of a fallen tulip tree leaf. Notice the resemblance to the color pattern of the imperial moth?

Here are some other members of the subfamily Ceratocampinae, all also late summer flying, all also dead leaf mimics:

 Why do they mimic dead leaves instead of the more common green leaves? Remember that dead leaves fall and flit around on the breeze and moths fly. It’s a better match.

Remember my post about the second instar caterpillars (and many others) looking like bird poop? That too is taking advantage of a common piece of the specific environment the caterpillar lives in and using camouflage to blend in with it. Similarly, a lot of caterpillars are also twig mimics:

Why are bugs so deceptive? I’ll talk about this more in a later post, but the short answer is first that, among animals,  arthropods are small (their size is limited by the way they absorb oxygen into their bodies) and thus tend to be prey to larger animals (birds, lizards, amphibians, etc.), which means that predator avoidance is a big survival issue.  Secondly (and more importantly, really) because they are biologically equipped to take great advantage of the adaptive capabilities of evolution – they produce large numbers of offspring which get heavily “selected” by the struggle for survival – 500 hundred eggs may result in two surviving, breeding adults (if they are lucky); they also breed frequently, producing from one to several generations every year. In short, their genes have a lot of opportunities to try experimental combinations, and, when they hit on some new design that shows some promise, they get to make a lot of drafts to get it right.  When you live like this, you can develop some amazing adaptations to fit your environment. 


Bibliography

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

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.