Normal Biology: manduca sexta

I had some extra time after work today, which I used to visit Marietta College's Special Collections. They keep scores of old documents, many dating back to when the Ohio Company of Associates first established Marietta in 1788 as the first settlement in the Northwest Territory. What I was after today, however, had an Entomological bent to it.

A young Hildreth, via Wikipedia.

Samuel Prescott Hildreth, a doctor who lived in Marietta during the early to mid-1800s did some of the first work in Ohio studying insects. He was a naturalist and published the first observations of the periodical cicada's 17 year life cycle, which is what I was after. I didn't find any of his cicada papers, but the librarian did bring me a book he wrote and illustrated, entitled "Portfolio of Insects."

This book includes paintings by Hildreth of various insects from Marietta and elsewhere in Ohio. When I first opened the book, I was greeted with beautiful illustrations of the tobacco hornworm (Manduca sexta) during all of its life stages. I had an immediate visceral reaction to that page--the tobacco hornworm is one of the first insects I raised when I became interested in Entomology, and it was emotional for me to be confronted by this right after opening the book. It was a direct connection to a naturalist who had written this Portfolio 150 years ago.

That caught me by surprise. I was excited, but sheesh, that almost brought tears to my eyes.

I soldiered on, invigorated by each page and by the connection I felt with Hildreth. His book was a veritable menagerie of the most charismatic insects you can find around Marietta, and which are still around today--one of the perks of having a lot of woods and natural areas in the county.

The book is only about 40 pages long, and each page showcases a colorful new creature: a brilliant Luna moth, a royal walnut moth, a smattering of other large moths and butterflies, a giant water bug...even a red eft (Notophthalmus viridescens) and a sketch of what Marietta College looked like during his time! As for his handwriting, it was difficult to read. I could make out some descriptions and names of plants and insects, but that was about it. Fortunately, his labels for each illustration were neater. I recognized the scientific names for each species and even noted some that had changed during the previous 150 years. Imagine that, eh?

Hildreth was truly a Renaissance man: in addition to being a naturalist, he was a doctor, geologist, and historian. He was active in the community as well, and today has a street in Marietta named after him.

Sadly, I only had an hour to look at the book before I had to leave, but I'll definitely return soon. I have some more research to do vis–à–vis Hildreth's cicada research, which I hope will also feature some of his illustrations.

Hildreth's Portfolio of Insects is an achingly beautiful direct connection to the past. It means a lot to me, being a native to Marietta, and for me, it inspires a timeless feeling of camaraderie with Dr. Hildreth. I recognize these insects drawn so long ago and I can still go out and find them. Had we lived at the same time, I think he and I would be good friends.

I wish I had at least one picture from the book, but I'll have to speak with the librarian about any effects taking pictures would have on it. Who knows, maybe we can get it digitized. In lieu of any pictures, I will leave you with a quotation Hildreth used to begin a paper on cicadas he published in 1830:

"No part of natural history more abounds in wonderful and extraordinary productions, than that portion of it embraced in the study of Entomology."

In Ohio, there's a delightfully gregarious caterpillar known as Manduca sexta, or the tobacco hornworm. It's placed within the Sphingidae family, which is composed of the hawk moths. Take a look at the Wikipedia page for the hawk moths: they're all spectacular. Some of the adult moths resemble birds or bees, they're just amazing. To get back on topic, the tobacco hornworm feeds on tobacco (obviously), as well as other plants within the family Solanaceae, which includes tomatoes. This makes them pests to gardeners, which is pretty unfair. Tobacco hornworms develop into some beautiful moths and are quite large, so they're truly a sight to see once they reach maturity. Luckily for me, they also feed on Datura wrightii, a plant within the same family as tobacco and tomato, which grows near my house.

I had the fortune of finding a few of the caterpillars on the Datura plant, so I plucked them along with some leaves and placed them in a container in my room. They were still in their early instars (An instar is simply a stage in the life of a caterpillar. As it grows, it will molt and enter a different instar, gradually getting bigger as it enters each successive instar.) and so were relatively small, as you can see from the pictures. Of course, they did not stay small for long. I ended up plucking five or more leaves each day for each caterpillar in order to give them enough food, and I still wasn't sure if that amount was enough. They would eat through all the leaves in short order, and sometimes I found myself getting even more leaves for them throughout the day.

And so, they each progressed through their instars, growing larger and larger. I was surprised at just how big they were getting: these aren't small, inch-long creatures. If you're creeped out by large worms or snakes, then you won't like these guys (but you should anyway). Taking a look at the pictures now, you can start to notice their stripes and small eye-like spots on their sides. The stripes are one way to distinguish them from a similar species, the tomato hornworm, which also feeds on tomatoes. It has eight V-shaped markings, while the tobacco hornworm has the seven diagonal stripes*. The stripes serve another purpose as well: camouflage. The stripes break up the hornworm's pattern while it's feeding on leaves, helping it to blend in with the overall shape and color of the plant. If you encounter a plant with skeletonized leaves, it's a good indication that a caterpillar has been feeding on the plant, but even if the caterpillar is still on the plant while you're looking at it, it's no guarantee that you'll find it. They're remarkably hard to find, even when you know they're there (which is remarkably frustrating).

Now, the orange eye-like spots on the hornworm's side are its spiracles. Spiracles are how insects breathe, simply put. It probably doesn't hurt the insect's chances of survival that its spiracles look like eyes, either.

In the picture below, one of the caterpillars has undergone ecdysis (molting), and has entered its next instar. Tobacco hornworms usually have five instar stages, but can have more depending on growth conditions. Near the caterpillar's posterior end, you can see the remains of its previous molt. The caterpillar proceeded to eat these remains. When a kid is growing and needs nutrients, they're forced to eat vegetables. When you're a caterpillar, you eat what used to be your skin. Except for the head case. That part is too hard, so it just pops off and is left alone.

Pictured: a delicious gourmet meal

You can tell that the caterpillars have grown much larger than they used to be. The most mind-blowing fact about it is that these pictures were all taken over the course of about a week. Caterpillars can grow pretty quickly.

Caterpillar excrement is known as "frass." It fertilizes plants and all that good stuff. Also, you can hear it falling down the leaves of trees and other plants like rain sometimes.

The small reddish brown object is the pupa of another caterpillar, Ceratomia catalpae.

Still eating.

At a certain point during the hornworm's fifth instar, it starts undergoing physiological changes signaling its body that it's time to pupate. Different hormones start being produced, and the hornworm's behavior changes. Effectively, it's puberty, but with less pimples. One of the behavioral changes is quite pronounced, and results in the caterpillar entering a so-called "wandering stage." The caterpillar descends from its plant and starts walking around on the ground, looking for a good place to burrow into the dirt. Or, if the caterpillar is in an insect cage, it will walk around the enclosure and stomp on everything. (Various sources recommended that I remove excess frass to avoid a messy situation. It was a very helpful recommendation, and saved me a lot of cleanup time later on. If you decide to raise a hornworm, you would do well to heed the same advice.) After a little while of wandering and being much more active than it was during its feeding stages, the hornworm will start to dig into the soil. It's a fascinating process: the hornworm uses its body as a shovel. It doesn't so much as dig a burrow as pulverizes its way down into the dirt and decides "Yep, this'll do." I found I had new respect for my hornworm after watching it go underground.

Lateral view, the hornworm at work.

The problem with not having quite enough dirt in the insect cage.

A fascinating visual cue that the hornworm has entered its wandering stage is the presence of its aorta (its heart). On the "back" of the hornworm (its dorsal side, for those of you anatomically inclined), the hornworm's aorta pumps its blood (technically hemolymph, I do believe), which is quite conspicuous.

The hornworm's aorta, hard at work.

After the wandering and digging, the hornworm settles in for some huge changes to its anatomy and physiology.

Also notice how the posterior end of the hornworm looks a bit like a face. Some mimicry, perhaps.

While that hornworm is busy pupating, let us catch up with the other hornworm.

.....or not.

And now it is time to learn about one of the most interesting natural phenomena in the animal kingdom: Parasitoidism. You may know about parasites: nematodes, flatworms, and according to some, politicians (we won't get into that argument here). Parasitoids are similar to parasites in that they also live inside of a host and feed from it. However, parasitoids take parasitism one step further. Parasites live and feed off of a host, yes, but they do it in a way that doesn't kill the host. If the host dies, the parasite also dies since it loses its food supply. Parasitoids, on the other hand, have little concern for the well-being of the host. In a parasitoid-host relationship, the parasitoid feeds off the host in such a way that the host dies. The parasitoid has special adaptations that allow it to survive after the death of the host, so to the parasitoid, the final result for the host doesn't matter.

So what does that mean in the context of Manduca sexta?

Manduca sexta has the misfortune of being prey for some species of wasps in the family Braconidae, which contains a variety of parasitoid wasps. These wasps seek out various caterpillars, and when they find them, they lay their eggs inside of the caterpillar. This is bad news for the caterpillar, as its defensive options are limited. The plants it feeds on contain toxic chemicals, and there is evidence that some caterpillars can ingest more leaves containing these chemicals to self-medicate against parasitoid infection**, though such a study has not been researched specifically for Manduca sexta. However, it is not too much of a leap to hypothesize that the hornworms could make use of toxic chemicals in an attempt to kill the parasitoids.

As a side note, such parasitoidism is obviously beneficial for the host plant of the caterpillar, as it can suffer severe skeletonization of its leaves due to the caterpillar's voracious appetite. The plant is not entirely defenseless, however, and is able to release chemicals to attract such parasitoids to rid itself of the caterpillars. To learn more, check out this wonderful article from Scientific American.

So, the hornworm has had the misfortune of being parasitized. What can it look forward to? Aside from the hope that the eggs won't hatch, not much. Its future is pretty bleak. First, the eggs will hatch. The wasp larvae will start eating the insides of the caterpillar. Since they are parasitoids, not parasites, they won't have much respect for the normal, healthy functioning of the various organs and systems of the caterpillar. The little respect for the caterpillar's health that the wasp larvae will have will simply be not eating the vital organs first, allowing the caterpillar to live so that the larvae will have a safe place to develop. Once they have grown sufficiently, the larvae move on to phase two.

The larvae eat their way through the caterpillar's skin. If you go back to the picture of the larvae coming out of the hornworm, you will notice a few dark circles on the caterpillar. Those are wasp larvae eating their way out, almost breaking through the skin. Once the larvae have made it out of the caterpillar, they will immediately start spinning cocoons. They will stay in the cocoons for about a week or two, and then emerge as adults. During all this time, the caterpillar is still alive. Depending on nature's mercy, the caterpillar may die soon after the wasp larvae have burst from its body, or it may stay alive until the adult wasps emerge from their cocoons, and even for a few days after. I imagine the caterpillar curses its existence if it is in that predicament.

Adult braconid wasps emerging from their cocoons.

The cocoons look like small white egg sacks. There can be as many as 50 or more on one caterpillar, depending on its size and unluckiness.

These adult wasps will each seek out other caterpillar targets.

The resulting caterpillar.

For an even more gruesome lesson on parasitoidism, National Geographic has you covered.

Now, time to move on to a healthy hornworm.

Pupal stage.

The caterpillar has undergone quite a change while underground, resulting in its pupal stage (also known as Metapod, to Pokémon fans). The most outstanding feature is the curved tube at its anterior end, which its tongue grows in. The pupa is not entirely defenseless. When touched, it will jerk its posterior end violently, which is quite startling if you do not expect it (I speak here from personal experience). The pupal stage also takes place while buried a few inches underground, which isolates it from many predators.

As the moth gets closer to eclosion (emergence from the pupal stage), the pupa will get darker and more translucent.

It will turn almost purple in some areas.

Finally, the moth will emerge from the pupa. I assume the orange-brown liquid to be hemolymph, though I haven't found a source identifying it for sure.

The pupal stage lasted a little over two weeks, and the final result was...

It's quite a beautiful moth, very "furry." It's also quite large, to the degree of a small bird. Knowing the size of its caterpillar and pupa, it's no surprise.

Check out those eyes! Also notice the tongue curled up into a ball. The adult feeds on nectar.

A full view of the body and wings. Notice the series of orange spots on its abdomen, absolutely stunning. There are six on each side, twelve in all. I assume these are what the sexta (Latin for six) in the species name refers to.