When I first started to become interested in insects two summers ago, I had a lot of inspiration thanks to the Catalpa trees in my backyard. I've always loved Catalpas: their flowers bloom starting in June and are spectacularly beautiful. They also smell really great.
One of the insects that can be found on Catalpa is the Catalpa Hornworm (Ceratomia catalpae). This caterpillar in the family Sphingidae can grow quite large (~3 inches) and goes through boom and bust cycles. During the summer of 2010, it was definitely a boom year for this caterpillar, which provided me with ample specimens.
There are also ample caterpillars for parasitoids to have a field day. Braconid wasps are especially prevalent, resulting in many of the caterpillars dying as they become vessels for broods of wasps.
Here you can see a Braconid wasp crawling on the cocoons.
This was a common sight, so I got used to seeing it after a few days. But once I found one that looked dramatically different (even from the cocooned ones), I took notice.
Yeah, I'd say that is dramatically different.
So....what happened? Good question! This guy lost the lottery in a spectacular way. He just exploded and that was that. Obviously, this is not like the parasitoidism of the Braconid wasps. If I were to venture a guess, I would blame a parasitoid fly maggot bursting out of the body: you can see a trail of liquid that comes out of the caterpillar's body and curves down by its head that looks like it could have been made by a maggot dragging itself along the leaf.
I haven't researched fly parasitoidism enough to put too much confidence behind that hypothesis, however. It also could have been a bird or other predator that decided to have a little snack, but wanted to leave a warning for the other caterpillars.
I would love to have the answer to the question, so I'll have to check for more caterpillars this summer. Maybe if I'm lucky, I'll find a maggot this time around, or see something else that explains it.
I came across a caterpillar today that was too cool to pass up sharing. You're in for a treat.
I was walking to the science center on campus today and looking around the fringes of the grass, as per usual, when I spotted a caterpillar on the sidewalk. It was serendipitous because I was just thinking about how as an entomologist, I've been training myself to focus on small things and watch for movement that might be overlooked by someone who doesn't constantly look for bugs. It was an interesting moment.
I noticed a small green thing that looked like a leaf, and at first thought it was. A breeze had just blown some leaves across the path, but this was a little different. It's a good thing I don't have a habit of crushing leaves, or this story would be sad.
Sidewalk is not its natural habitat.
A-ha! My first thought: "Definitely not a leaf." My second thought: "Sphingidae." This caterpillar has very pretty colors, like a grape popsicle shoved into a lime. The first thing to notice after the colors is the horn at the end of its abdomen--that's an identifying feature for the sphinx moths, family Sphingidae. Sphingidae is easily one of the coolest (if not THE coolest) families of moths, and they're easily identified by the posterior horn that the caterpillars have. (Unless you're dealing with Abbot's Sphinx - Sphecodina abbottii. In that case, the caterpillar will be staring right back at you with its raised knob that resembles an eye.)
Hemaris diffinis - Snowberry Clearwing
This Sphingid caterpillar is the Snowberry clearwing, Hemaris diffinis, which turns into a beautiful moth. When it reaches that stage, it will have clear wings (hence the common name) and look similar to a bumblebee. How cool is that? Notice the black dots around the spiracles on the side of the caterpillar, and how the horn changes colors from yellow to black towards the tip. Marvelous! While I was moving the caterpillar (first by stick, but when it wouldn't cooperate, just by picking it up), I turned around and noticed another person coming up the sidewalk behind me, looking at me like I was crazy. I tried to explain that I was moving a caterpillar so that it wouldn't be squished, but I'm pretty sure it didn't make me seem less crazy to him.
I've now been back in the United States for three days now, and each day I have made time to go out looking for insects. I've missed the familiar plants and temperate weather, and on top of that, I've had a lot of success, especially today. So far I've confined myself to my backyard, but that hasn't limited what I've found.
I pulled off some dead bark from some staghorn sumac, and found some ants which had set up residence inside the plant.
They quickly grabbed the ant larvae and ferried them away from view, and ran around feverishly trying to figure out what had happened. I was hoping for some beetle grubs, but this was fine too.
I also found a firefly crawling around in the grass. It's still a bit chilly for most of the fireflies to be out, but I've seen a few. Hopefully it will only be a few weeks until they're out in abundance and flashing. Speaking of fireflies, if you haven't yet, check out Firefly Watch, a project to look at firefly abundance and gather data on fireflies. It's a project you can help out with: just register your location, habitat, and watch fireflies for 10 minutes. It's great fun, and you'll learn a lot while you're collecting data.
Next on my list was finding a caterpillar. "Awesome!" I thought, being the first caterpillar I had found.
After taking a closer look, however, I realized that this wasn't a caterpillar at all: it has way too many prolegs. I'm not sure what exactly it is, but the only other group I know of that are similar to caterpillars in their larval stage are sawflies. Well, you also have beetles, but this is definitely not a beetle grub. So I'm leaning towards sawfly, and I'll see what I can figure out.
I next found something super cool: a molted spider skin. Until a few months ago, it never even occurred to me that spiders shed their skin. Of course they're arthropods, but it just never crossed my mind. Sort of like the clover thing, I suppose.
That's pretty awesome.
I came upon a tree which had its leaves full of small holes, and soon found the (alleged) culprits: weevils. There were many all over the tree, most of them mating. I'm not sure of what species of weevil they are, nor what species of tree they were on, so if anyone knows, leave me a comment. UPDATE: Thanks to Tricia, this mystery has been solved: they're a pair of mating Tulip Tree Weevils (Odontopus calceatus) mating on a tulip tree (Liriodendron tulipifera).
Once I was nearing the end of my romp around my yard, I came upon a particularly unfortunate beetle.
Its right wing and elytra didn't look to be in the best shape, but it was persevering. I'm pretty sure it's Atalantycha bilineata, the two-lined leather wing, but I need to look at it a bit closer first to be sure. I haven't ever seen this beetle before, it's great to know that I haven't found anywhere close to all the insects in my yard yet. We'll see what tomorrow brings!
All of these pictures were from yesterday, so I'll post my pictures from today's hunt tomorrow.
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.
View of the closed wings. The newly emerged moth cannot fly, but must first pump hemolymph into its wings.
Ventral view, with ruler for scale.
It has quite the wingspan.
Truly, it's a beautiful moth. It's a shame they're seen as pests.
This particular specimen is now located in the collections at Marietta College.
For more information about Manduca sexta, check out the following websites:
