My life goal is quite clear: inspire more people to appreciate and conserve nature. Even though I still don’t know how to achieve it, I decided to be an ecologist/entomologist first.
Now that I have decided to be an entomologist, I am going to start writing about my research and life. I recognize that people don’t really read blogs anymore, but lengthy articles are still one of the best ways to communicate science.
This is my last semester at the University of Illinois as an undergraduate student. It so happens that the insect physiology class I am taking now requires an article documenting an insect rearing project. The class is taught by insect physiologist Dr. Marianne Alleyne, who always try to promote scientific communication to the public.
Manduca sexta, is a sexy moth species (order Lepidoptera: family Sphingidae) with the common name “tomato/tobacco hornworm”. Our task is to rear these worms from eggs to late-stage caterpillar, then perform experiments on them. This insect species has been heavily studied because it is easy to keep and breed.
It starts on Jan when they were a bunch of eggs (on artificial diet substrate):
On Jan 27th, a baby caterpillar came out, you can see the egg shell on the left:
Then he molted in two days:
So how does he grow as he molts? Unlike us, insects don’t have internal skeleton structure. Instead, they are armored by “exoskeleton“, which is basically segments of hard plates connected by softer tissues. Imagine body armor in the middle age, and then imagine that is your skin. The sclerotized cuticle in insects provides structure support of the body and it can’t grow with the body. So insects have to shed the skin, at the same time grow a bigger set of exoskeleton.
Here is a short video on [ Giant Texas Katydid molting ] .<< click
The same goes for caterpillars. They have relatively soft and flexible cuticle but they need to molt to grow bigger as well.
Below is the 3rd instar, meaning that he molted twice already. Now you can see he is much bigger than the dead egg at the center:
You may notice the black “tail” on the end of its body. We call it “tentacle“. It is technically not a “hair”, but more like an extended tube from the body. In early instars, the tentacle is relatively large and you may imagine how the tentacle diverges predators to attack it instead of the head. Actually, the tentacle is an exocrine gland equipped with chemical defense (Vegliante & Hasenfuss 2012).
In the case of Manduca sexta, the caterpillars feed on leaves of tobacco plants, which contain a toxic compoundnicotine. Nicotine is produced by plants to deter herbivores, but M. sexta uses the toxin for its own protection against predators because they are able to excrete ingested nicotine. Here is an article about a recent study (Kumar et al. 2014) on the mechanism behind how the caterpillars utilize nicotine.
As days past, my Manduca is getting much bigger! Currently, he is at his fifth instar, (I am going to assume he is a dude, because, well I just feel like he is a dude). This is his last larval stage, meaning that he is going to become a pupa when he molts again. For the most part, I would say rearing Manduca sexta is not too difficult, which is one of the reasons Manduca can be a good study organism. They don’t really need much of attention. You just have to clean the container and replace food items every two days. Well if you have an entire tobacco plant that would be even easier: you just put the worm on the plant.
Look at how huge he is!
But that statement is only based on my experience with this one caterpillar. If you look at the overall survival rate, the proportion of eggs that hatched and grow up into final instar, you would find the rate is quite low. Only one of the four eggs hatched in my cup. I notice many of the eggs in my classmates’ cups did not hatch as well. As an insect species, the tobacco hornworm follows the type III survivorship curve (fig. 1). They produce a large number of offspring in the form of eggs, hoping that some of them would make it through the year. Even though most individuals die young, the remaining insects are numerous enough to continue the generation. In contrast, parents of big mammals, like us human, give birth to a few babies but provide extensive feeding and care so each baby has a high chance of surviving.
Figure 1. An illustration of the three idealized types of curve (from Wikipedia)
And of course, we are rearing Manduca in the lab. In nature, baby hornworms would face more challenges. They would have to deal with a more variable temperature and humidity condition. Most importantly, natural enemies, such as predator and parasites, will take out a large proportion of the population.
Here is a picture taken in my backyard.
The many white cocoons attaching to the caterpillar were made by larvae of a parasitic wasp species. The larvae feed and develop inside Manduca‘s body until Manduca reaches final instar. They would then emerge out of the caterpillar and made cocoons, in which they pupate and become adult wasps.
Like I said, Manduca has been used as a model organism to study insect physiology. The caterpillars have a relatively big size thus easy to manipulate and dissect. In a classic experiment done with Manduca, Dr. Carroll Williams and Dr. Frederik Nijhout, who are famous entomologists, investigated the control mechanism of insect molting and introduced the concept of “critical weight” (Nijhout & Williams 1974a). They showed that a cut-off point for body weight needs to be reached for the caterpillar to release hormones signaling molting and metamorphosis. We knew that insect hormones such as
We knew that insect hormones such as PTTH (prothoracicotropic hormone) and JH (Juvenile hormone) signals critical morphological and psysiological change, but we didn’t know how the release of hormones are controlled. During the experiment, Nijhout and Williams noticed that PTTH releases only happen during a particular time period each day, and only when the caterpillar reaches a certain weight. Nijhout and Williams continued to study the relationship between JH and PTTH. In the following paper, they revealed that the decrease of JH to a minimal level is critical for the release of PTTH (Nijhout & Williams 1974b).
In a simpler word, just like a computer program having many components and regulators, the hormonal control system relies on the variation and balance of the difference type hormones in the insect body.
If you a biologist who has some spare time and you wanted to know a bit more about insect hormones, I would recommend a paper by Nijout and Wheeler (1982) that talks about juvenile hormone.
Coming back to my Manduca, I took this guy home and place him in a bigger container. Although the homework part of the rearing project is pretty much finished, I would want to see the transformation of this guy into an adult moth. I gave him some green bell pepper as food since the pepper belongs to the same plant family (Solanaceae) as tobacco. And… he ate it! Please wait and check out my next post.
Update of Feb. 23, 2016
My Manduca has stopped eating and pretty much stopped moving as well. I believe he has enter pre-pupa stage getting ready to molt. With my experience in butterflies, this stage usually only last one or two days. However, if something went wrong, this would pretty much be the end of his life…
Update of Mar. 3, 2016
I thought he was gonna die, but when I checked again few days later, I saw this beautiful pupa! He will emerge as a big fat moth in another week.
Crazy how he transformed.
A closer look at the long nose, which is actually its proboscis. You can see the pupa has given each body part the shape. Guess: which is the eyes, the antennae, and the wings?
He emerged… but not in a good shape. So I put him at rest…
Kumar, P., Pandit, S. S., Steppuhn, A., & Baldwin, I. T. (2014). Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. Proceedings of the National Academy of Sciences, 111(4), 1245-1252.
Vegliante, F., & Hasenfuss, I. (2012). Morphology and diversity of exocrine glands in lepidopteran larvae. Annual review of entomology, 57, 187-204.
Nijhout, H. F., & Wheeler, D. E. (1982). Juvenile hormone and the physiological basis of insect polymorphisms. Quarterly Review of Biology, 109-133.
Nijhout, H. F., & WILLIAMS, C. M. (1974a). Control of moulting and metamorphosis in the tobacco hornworm, Manduca sexta (L.): growth of the last-instar larva and the decision to pupate. Journal of Experimental Biology,61(2), 481-491.
Nijhout, H. F., & Williams, C. M. (1974b). Control of molting and metamorphosis in the tobacco hornworm, Manduca sexta (L.): cessation of juvenile hormone secretion as a trigger for pupation. Journal of Experimental Biology, 61(2), 493-501.