Category Archives: SEM
Word of the day: Tubercle
(Who am I kidding, these words aren’t random. I pick words that relate to Lepidoptera – from the Torre-Bueno Glossary of Entomology)
Tubercle, a small knoblike or rounded protuberance… in caterpillars, body structures, sometimes bearing setae, e.g., pinaculum, verruca.
Bonus word! Tuberculiform, shaped like a pimple or tubercle.
Caterpillars usually aren’t just hairy all over – the primary and secondary hairs, or setae, are arranged in an organized fashion that is mostly conserved throughout Lepidoptera. Chaetotaxy, or the arrangement of setae, can be important for distinguishing between families or even species. The length of hairs and their relative positions to each other may seem subtle, but can have important phylogenetic implications.
The hairs are all named based on their position on the body; we currently use a system devised by Hinton (1946). For example there are usually two hairs toward the dorsal (upper) side of the segment, so they are called D1 and D2. These may arise from a small or unnoticeable protrusion, to a large conspicuous wart. This is where terminology gets tricky. Many scientists have their own preferred names for these structures, and their own cut-off points for when to use which word. They are usually describing the same structure, it is just a matter of how large it is. In our lab we tend to use pinaculum for a small one, tubercle for medium, and wart for large. When in doubt, you can call it a tubercle.
Here is an example of a small one, in an SEM image I took of a setae (Sympistis species).
Frivolous forked fungus beetles
Just for fun! Perhaps not frivolous, but I wanted some alliteration… well, frivolous in the fact that I’m a sucker for side projects that do not relate to my dissertation research.
I am collaborating with a very enthusiastic alumni of our department, who is still quite keen on collecting, observing, and learning about insects. His name is Stan and he has his own website where he uploads photos and commentary about wildlife found along the Airline Trail here in CT. I like mornings, but it takes a special sort of morning person to be out there photographing bugs and birds every day throughout the year. You can check out his website here! He also joins our weekly lab meetings and always has fun things for show-and-tell.
During his travels Stan has come across these awesome beetles, called forked fungus beetles (Bolitotherus cornutus). They are unusual, slow moving beetles which live and feed inside large mushrooms. They have some really interesting life history traits and behaviors, such as the fuzzy horns of the males, and their defensive secretions. You can learn much more about forked fungus beetles and see lots of photos on this page of Stan’s website.
Most interesting (isn’t sex always the most interesting?) are their courtship behaviors. The males appear to create a stridulatory sound by rubbing the tip of their abdomen against the top of the female’s prothorax. You can see this behavior in the following video (taken by researchers in Virginia).
So far, it appears that the actual mechanisms behind how this sound is generated, or even reasons why the males do this as part of their hours-long courtship rituals, are unknown. Since Stan had captured several fungus beetles, and I had some extra allotted SEM time for this semester, I thought it would be fun to get some beetles under the scope to see if we could get good images of the structures on the male’s abdomen and female’s prothorax – the contact points for the scraping sound.
And that is what we did! After cleaning (the beetles were covered in mites and dirt and fungus) drying, mounting, and sputtercoating we were ready to examine them. I don’t want to give anything away, but we came across some small structures that had gone unnoticed under a dissecting scope, which could potentially be involved in this strange behavior. Stan got to watch as I used the scope. As you can tell, this machine is a few years old. But it works!
And here are the fuzzy horns of the male, just because they are kind of nuts.
Our next goal is to set something up to capture our own video and sound recordings of fungus beetle courtship. No matter what it will be fun to continue working with these funny looking beetles.
Here are some caterpillar images I took during my last session with the SEM (scanning electron microscope). I have cropped and edited them, these versions are just for fun – I’m saving most of my shots for potential publications.
I would like to give people a sense of what is hidden in the world around them – these are landscapes that exist on such a small scale. Yet they do indeed exist, and can be found with enough patience and determination. Awaiting you could be great beauty, or potentially nightmarish scenes. Regardless of how you feel about insects on an emotional level, I encourage you to consider the complexity these creatures hold and the wonder they can provide.
Acronicta falcula. Crochet hooks (little claws on the abdominal prolegs), 500x magnification.
Acronicta falcula. Skin texture, 1000x magnification.
Acronicta falcula. Skin texture, 2000x magnification.
More images to come soon!
Here is my first attempt at colorizing an SEM image (using Photoshop Elements 8).
This is the micropyle of a moth egg in the genus Acronicta. As you can probably guess, it is not these colors in real life.
And this is the skin texture of Acronicta afflicta. Again… they are not green and orange. This was tougher and I got a little lazy with my selection (and using just the touch pad on my laptop makes it tricky) but the effect is pretty cool.
Caterpillar feet are scary
Today I had my first session learning to use the SEM (scanning electron microscope). It was a blast! I spent the past week preparing some egg and caterpillar samples. I took an Acronicta afflicta specimen, cut it up (because the caterpillar was too large to fit into the scope whole), and came up with a protocol for fixing, drying, and mounting the pieces. I really enjoy the step-by-step precision and organization in the electron microscopy lab. There is something about wearing gloves and pouring things from one vial to another in a fume hood that feels so… science-y.
This round was just for practice, I will be gathering more critical specimens to image throughout the rest of the semester. Here is one of my favorite views, a caterpillar’s proleg, or “fake foot”. These are the fleshy nubs that look like legs on the abdomen of a caterpillar. They have hooks on the bottom to grab onto the substrate. I think this view is rather creepy.
This is a close-up of the texture of the caterpillar’s skin.
And one of my favorite structures to view under the scope, a spiracle. This is how caterpillars perform gas exchange, or breathe. While on most insects the spiracles are less obvious, they are typically big and bold on caterpillars. All those little fuzzy looking structures really increase the surface area for gas exchange to occur.
I am so grateful for my instructors and the advice of fellow grad students so far, and I’m excited to continue learning. I think the trickiest part with imaging caterpillars is going to be figuring out my collection and fixation protocol so that I don’t distort the body shape too much. We did an ethanol series (since they were already in 70%) up to 100%, then a few changes of HMDS. If you have ever dealt with preparing caterpillars or other soft bodied invertebrates for SEM and have any tips to share, I’m all ears! The eggs I imaged were either collapsed and infertile or just the shells after the caterpillars had hatched (air dried for several months), so the images weren’t that great. That will be another trick, figuring out how to preserve fresh eggs so they can be imaged in their usual shape.