How do insects smell, taste and hear? Do they feel pain? These questions and more!

Insects for Fun!

This week instead of focusing on a specific insect I wanted to talk about some Physiology, that is how the body works. Not all of it of course, but just some things that I personally find incredibly interesting.

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Transcript:

Welcome to Episode 55 of insects for fun! The entomology based podcast hosted by me, your not so local entomologist. Today I will go into the topic of Insect Physiology. Basically how the bodies of insects work, and the different ways insects can hear, smell, taste and feel.

Let’s get to the basics, an Insect has three body segments, A head, thorax, and abdomen. I believe I mentioned some of this already as well in my episode about bugs versus insects, but we’ll be going into a lot more detail today! Insects also have six legs which is where the class name Insecta comes from. Now what makes insects and all arthropods really unique is that they have an exoskeleton which simply means they wear their bones on the outside which is the complete opposite of other animals.

Now because they wear their bones on the outside they also happen to have strange methods of hearing, tasting, and smelling. They also don’t have the same internal structure that mammals or other animals do!

Let’s start with the inside of an insect. They have three guts, the foregut, midgut, and hindgut. Each section serves a specific function for digesting and absorbing different parts of their food. The foregut also known as the crop, is where the food gets broken down into smaller pieces. Of course some insects are fluid feeders like mosquitos for example and in those situations the foregut prepares the liquid into something easier to digest and absorb. The foregut in mosquitoes contain blood thinners which prevent their meals from getting sticky and hard.

The midgut is the area where most of the nutrients get absorbed into the body. Enzymes are released to digest the transported food particles into absorbable nutrients for cells. The food is actually wrapped up in a special membrane which is believed to protect an insect's innards from pathogens that may be present within the food.

Not everything gets absorbed through the midgut though. The hindgut is actually where salts, and water are absorbed from the leftover waste and is incredibly important for maintaining a balance of internal fluids. This is also the landing place for nitrogenous waste which gets evacuated from the insects “blood” through a series of small tubes.

Insects don’t have a closed circulatory system the same way we do, which is to say they don’t have a heart that pumps blood through veins and arteries to their organs. The system of an insect is known as an open system in which the insect’s blood, referred to as hemolymph flows freely through the body, bathing all the organs. They do have something similar to a heart which is called an ostia and this is a long tube that runs from the head down to the abdomen but it actually pumps hemolymph from the back to the front. The reason is because as the hemolymph moves its way down the body it can relay messages from the head region.

Insects do have a brain, but they also have multiple bundles of neurons throughout their bodies that can send messages in response to stimuli like touch, smell and taste. The bundles of neurons are called ganglions. That being said the head does contain three very important ganglions that together form what we would think of as a brain which is again why the ostia pumps hemolymph up there to flow back down.

So now we know that insects have a brain, bundles of neurons called ganglia, an esophagus that leads to three different guts, and an open circulatory system, but what about lungs?

These animals do not have lungs but they have holes on the sides of their bodies which are called spiracles. These holes are controlled by muscles, and when open, allows air to freely move through them and carbon dioxide to leave. Once the air is taken into the body it moves through a series of branches trachea directly into all the cells of the body. In most cases these tracheal tubes are connected to a reinforced spiral of tissue that keeps them from collapsing. However! Some insects, like aquatic ones don’t have this and can use the tracheal tubes as air sacs to store extra reserves.

As for other organs, well, I think we all know they have eyes and sex organs by now but what about tasting and hearing organs?

Turns out that insects have small sensory organs all over their bodies in the form of tiny hairs, spines, nubs, etc . These organs are used for smelling, tasting, hearing, feeling and sensing pressure changes in the environment.

Most insects have most of these on their antennae, but these can be found all over an insects body! For example butterflies can taste with their feet which is how they can determine which host plants to lay their eggs on. Crickets and Grasshoppers can taste with their ovipositors which is how they decide which soil is best to lay their eggs in. On the other hand some insects taste with their antennae like many parasitoid wasps. This is not to say they can’t taste with anything else, because in most cases they can also taste with special structures next to their mouths called maxillary palps.

When it comes to hearing and smelling insects also have some crazy places where this takes place. For example, crickets, grasshoppers and katydids all have hearing organs located around their knees with katydid’s actually having theirs directly on the knees. Some moths have their ears on their abdomen which can detect frequencies way beyond a normal range. For example there are families of moths which have evolved to hear ultrasonic sounds in order to escape bats like moths in the families Geometridae, Pyralidae, and Thyatirinae. As for where they smell, that usually happens with their antennae, but it’s not always the case. Some insects can smell with their mouths. What’s even more amazing is that insects can hold onto memories past metamorphosis. For example caterpillars that were trained to avoid a certain smell will remember that smell even as a butterfly or moth.For example scientists trained caterpillars to avoid a smell by giving them a small shock in the presence of it. Once the caterpillars had become butterflies, they still avoided the smell they were trained with.

To make things more crazy, insects don’t just see with their eyes. A study in Japan found that yellow swallowtail butterflies have light receptors on their genitals which they genuinely need in order to successfully mate and even to lay eggs on a plant's surface. A study found that if these receptors were blinded then success rates for mating dropped from 66% down to 28%, and the success rate of laying eggs went from 80% down to 15%. That's pretty big.

I now want people to think about two questions. Why exactly do insects need to taste things, and what exactly are they tasting?

Insects like us rely on taste to tell us whether something is good or bad. Oftentimes when we find something unpleasant there is a biological reason behind it, at least when it comes to those of us who don’t mind our vegetables. For example in countries with malaria it was found that people have a higher tolerance to bitter foods and cyanide. It’s actually thought that perhaps cyanide at low levels can combat malaria. In Japan many people like the taste of yuzu which is an extremely bitter citrus, but I can’t stand the stuff. Like seriously if I get a mouthful it makes me wanna vomit and maybe that’s because my ancestors came from areas where bitter plants were poisonous.

Now let’s talk about what exactly insects are tasting. Could a grasshopper prefer buffalo wings over BBQ? Insects can differentiate salty, sweet, and bitter flavors just like us, but they prefer sweet over everything else. Perhaps some of our listeners can relate (insert clip from willy wonka or something). They’re actually pretty keen on the different concentrations of salts and sugars as well with honey bees being able to differentiate different types of sugars and their concentrations, and they aren’t a fan of splenda by the way.

As for what insects can feel, given that they have receptors all over their bodies it's no question that insects can feel physical stimuli, but what about pain? For a very long time insects were thought to just reflex on autopilot based on different stimuli, but more and more studies now are showing convincing evidence that insects actually can feel pain, the same way other arthropods can like lobsters for example which have been adopted into the animal welfare act. In fact studies found that bees among other insects will actively avoid stimuli known to be painful, and of course you could argue that it was reflex based like when a doc tests your reflexes with that rubber hammer they always use. But it goes beyond that. When bees were faced with painful food sources that contained more or better quality food than the nonpainful sources, the bees actually chose to endure the experience for the reward. These same bees had previously avoided the same exact food source before when the prizes were the same between the painful and nonpainful food source. These kinds of interactions aren’t just found in bees either. Cockroaches and fruit flies along with wasps, and other insects had similar results. Actually, so far no insect that has been tested like this has shown complete indifference to a painful experience.

I for one fully believe that insects feel pain because I’ve raised caterpillars for years on end handling them in various ways. They get used to touching like anything else, but I’ve pinched a couple before when trying to pull old molts off and man, they do not like that at all. They also lash out at their brothers and sisters if one of them gets bit by accident as opposed to just getting crawled over.

This wraps up today's episode on insect physiology! I hope you enjoyed it and as always if you enjoy the show it’s super helpful to rate it so the podcast can continue to gain reputation and build more listeners. It also helps if you decide to share it with others! Now speaking of sharing, I’ve got some designs ready to go for merchandise so you’ll soon be able to rep the podcast! If you want more insects for fun you can also support the show on Patreon.com/insects for fun where I have exclusive bonus episodes and vlogs for Tier 2 and above. If you would like to reach out to me you can send me a dm on IG or an email message, and links to everything will be in the show notes.