Episode 58: Where Do Insects Go When It’s Cold?
Image: “One of the world’s rarest and least understood insects, a grylloblattid ice crawler (Grylloblatta sp.). Photographed live at night on an ice field, Northern California, USA.” (Source: Wikimedia Commons, by way of Alex Wild – a very nice guy I met at a BugShot workshop one year)
Our new episode is available from our Podcast host here: Episode 58
We’re also listed on:
- Freezing to death as a human
- Hypothermia (Mayo Clinic)
- “Which body organs are most at risk during a heat wave?” (The Conversation, 2019)
- “In cold blood: how therapeutic hypothermia can save lives” (BBC Science Focus, 2019)
- “Frostbite” (NCBI Bookshelf)
- “A New Species of Orthoptera Forming a New Genus and Family” (The Canadian Entomologist, 1914)
- Grylloblattodea (NCSU)
- “Grylloblattids: A Critically Endangered Insect Order” (Entomological Society of America, YouTube)
- “Phylogeny and biogeography of ice crawlers (Insecta: Grylloblattodea) based on six molecular loci: Designating conservation status for Grylloblattodea species” (Molecular Phylogenetics and Evolution, 2006)
- “Conserved and narrow temperature limits in alpine insects: Thermal tolerance and supercooling points of the ice-crawlers, Grylloblatta (Insecta: Grylloblattodea: Grylloblattidae)” (Journal of Insect Physiology, 2015)
- Cryptocercus punctulatus (supercooling wood roach)
- “Cold-Acclimation of the Wood Cockroach Cryptocercus punctulatus (Scudder) (Dictyoptera: Cryptocercidae) by Robert Lewis Hamilton” (thesis defended 1985)
- “Phylogenetic Status, Distribution, and Biogeography of Cryptocercus (Dictyoptera: Cryptocercidae)” (Annals of the Entomological Society of America, 1999)
- The Insects: Structure and Function (5th Edition)
- Other specific insects
- “Seasonal Variation of Trehalose and Glycerol Concentrations in Winter Snow-Active Insects” (CryoLetters, 2008)
- “Mechanisms underlying insect freeze tolerance” (Biological Reviews, 2018)
- “Snow Scorpionfly (Boreus westwoodi) Mating” (YouTube)
- Monarch Butterfly Migration and Overwintering (USFS)
- Mantises (Mantodea)
- “Freeze tolerance in an arctic Alaska stonefly” (Journal of Experimental Biology, 2009)
- “European Corn Borer Ostrinia nubilialis Huber” (Purdue University Field Crops IPM)
- Antarctic midge (Eretmoptera murphyi)
- “An insect invasion of Antarctica: the past, present and future distribution of Eretmoptera murphyi (Diptera, Chironomidae) on Signy Island” (Insect Conservation and Diversity, 2020)
- “What Do Insects Do in Winter?” (Ask Smithsonian, 2017)
- “Overwintering Strategies of Insects and Implications for IPM” (University of Illinois Extension)
Hello, and welcome to Assigned Scientist at Bachelor’s. I’m Charles and I’m an entomologist.
And I’m Tessa and I’m an astrobiologist.
And today it’s just the two of us to discuss the topic of insects, because I noticed that we haven’t had an entomology episode since we talked to Aaron, which was fully 20 episodes ago. Unacceptable.
Unacceptable. So today, I wanted to bring you here, Tessa, to discuss the question of where to insects go when it’s cold outside. I’m interested to know how much you know about this topic going in.
I know, like in broad strokes, that, you know, different insect species use different strategies. Some of them are basically able to one way or another preserve themselves over cold temperatures. I know some are essentially just annuals that, you know, they’ll lay eggs, and then they’ll just kind of die when it gets too cold, and the next generation will hatch in the spring. I know, some – like monarchs – are migratory. But like I don’t know any specifics beyond that.
Well, as always, Tessa, you’re informed, and you’re tall. And those are the two things that people love the most about you.
Well, thank you.
It basically, as you said, it’s only fitting for insects, for there to be a diversity of strategies for dealing with the cold. And one of the best strategies for dealing with cold is living in a place where it doesn’t get cold to begin with, right, which is really just – the highest concentrations of diversity and insects is often in tropical and subtropical environments, because insects don’t have to deal with that external limitation of low temperatures.
But even in cold or temperate places, particularly in temperate areas that go through warm and cold cycles, we see more insects in part of the year and many fewer insects in the other other part of the year. You could ask, why are cold temperatures a problem for insects really to begin with. You might ask why does freezing kill insects, but freezing also kills you a human person.
So the real question is, why do insects die at much higher temperatures than many other animals – for example, humans – and why is cold temperature in general a much higher deterrent to insect activity than e.g. human activity?
I’m gonna guess it’s partially due to the size, you know, square cube law – the bigger you are, the slower you lose heat. But also mammals and birds are endothermic. Reptiles also have some control, although not as much insects, as I understand that don’t really have much at all. So they have less margin when it comes to dealing with cold temperatures.
This is all fantastic. So we actually, earlier this year, we talked a little bit about freezing damage and how cold temperatures hurt organic bodies – when we talked about cryonics and cryopreservation. And a lot of the problem as one quoted scientist in an article put it, you can think of insects as basically small bags of water. And you can also think of larger bodies as essentially bigger bags of water. Where, a lot of the problem is that we are chock a block full of water, and water loves to freeze. And when it freezes, it expands. That freezing, in the formation of ice crystals, can cause all kinds of damage internally because of multiple reasons. But among them that difference in volume between liquid water and frozen water. So that’s part of it.
But then I really wanted to get into I spent a couple hours honestly looking for answers, very precise answers to what is happening. When you are freezing to death. You as a human person, I’m not speaking to any sentient insects, right, right, we listening when you are freezing to death, your core body temperature drops, which starts preventing organs from functioning normally, and the temperature at which your body is captured because in human bodies hypothermia is actually defined as your internal temperature dropping below still a pretty high, it’s like still in the 90s.
Because it’s not a sort of a sense of it being cold in the sense of when we touch something and it feels cold, but rather, is it sufficiently lower than your normal temperature that it will cause problems with like the biochemical reactions that are keeping your body going? And so if it drops too much, then the biochemical reactions can’t proceed normally. And so that starts shutting things down, eventually the heart stops beating, and then your body becomes depleted of oxygen and it starts producing toxic chemicals, which then eventually can get up into your brain. And then it’s it’s game over, right?
As, as it’s happening, you’re experiencing vasoconstriction, where there is a decrease in blood flow and sort of it gets concentrated in your core to protect where all the vital organs are. And so the important bits, it’s basically like a kind of the physiological reasoning there is it’s better to lose some toes than a kidney, for instance. So that’s humans, because as you said, humans are and mammals and birds in general are endothermic, where we have internal metabolic mechanisms to maintain our internal temperature at sort of a pretty constant, high temperature, also known as being quote unquote, warm blooded. And I did find some sources describing quote, unquote, endothermic insects, but there does seem to still be a little bit of a distinction between endothermic as it’s used to refer to mammals and endothermic, as it is selectively used to refer to certain kinds of insects, the idea being, you know, broadly speaking, endotherms, maintain sort of a constant internal temperature, versus ectotherms, are really dependent on environmental conditions to determine what their temperature is, right?
Some sources talking about quote, unquote endothermic insects, the use of endothermic there seems to refer to insects, these insects having more deliberate internal mechanisms that they can use to maintain a higher than ambient internal temperature tend to be larger, they tend to be flying in most insects are what we would consider ectothermic, where they are very responsive to external temperatures to determine their own internal temperature. And so that’s part of the problem in why it is difficult for them not only to continue living, but just to function in general in the cold. And if you are very familiar with insects, then you will know that if you just cool an insect down, they tend to go into you know, a state of inactivity, I remember when I was in high school, and I was in my biology class, and we’d like did a section on Drosophila, quote unquote, fruit flies, you know, it was like…
Mutant genetics using Drosophila, right.
Did you ever do one of these, Tessa?
Yep, yep, in high school breeding horribly mutated Drosophila for science.
Exactly, we would do crosses, and then we would count out the phenotypes of the offspring, and what we would do is, instead of killing them, because we wanted to preserve them so that we could do more crosses, we would just have a dissecting microscope and we would have icepacks underneath our petri dishes and keeping the insects cool would keep them relatively inactive, so that we could keep them alive, but still deliberately look and is this male or female doesn’t have this or that phenotype, etc, etc. Also, the the Drosophila inevitably got out. And so that section of the building for a period of a couple weeks, every year, would just have a bunch of just like random fruit flies everywhere.
I seem to recall we had an issue with that too, except for a lot of the ones we were working with one of the mutations made their wings not functional, so they just never really got very far.
That’s one of the classic ones, there’s like no wings, there’s two sets of wings, which is honestly, it’s, it’s very cool. We can’t get into the genetics of Drosophila… what was I even talking about? Oh, if you just get an insect at a cooler temperature, then they will likely slow down and so why is this right like why to cool temperatures decrease insect activity so much. So insects are typically exothermic. They’re reliant on environmental conditions to be active. So they are much more sensitive to relatively cooler temperatures. And many of the activities that insects do for example, flying are very energetically expensive, flying takes really high metabolic activity to do. So, even when you look at insects, which are relatively active in cooler temperatures, they tend to be flightless insects, because it is so metabolically costly.
And because insects are small does, as you said, they are much more liable to lose heat very rapidly. It is basically that insects rely on external temperature to be an adequately high temperature to be able to do a lot of their activity. And so if it is cool around them, they do not have the same mechanisms available to them typically, as for example, humans to keep being equivalently active, right? And because they’re very small, they lose heat pretty rapidly, because as you said, the sort of surface area to volume relationship is is let’s put it let’s put it this way. It’s not in their favor when it comes to heat loss and heat retention, by which I mean they lose the heat very quickly. And speaking of you know, just into terms of dealing with temperatures in general there are multiple kinds of adaptations that different kinds of insects have for temperature regulation, not even specific to overwintering.
So for instance, morphologically some insects for example, some bees some mas have particular insulation around their abdomen through dense scales are hares or some of them that live in relatively cool environments have darker scales so that the energy that they absorb from solar radiation is much higher they have behavioral adaptations where many insects will in effect shiver you know, they will they will use their flight muscles to, you know…
Oh, wait, I did know that, yeah – bees do that, don’t they?
Yeah! One example being when there’s like an invader and honeybee hives, I think they could like surrounded and and basically shiver and heat it to death just pretty metal, pretty metal, they will also they can orient themselves towards or away from the Sun to decrease or increase the amount of heat that they are getting from solar radiation as well as moving in and out of sunlight altogether. So, if it is a relatively cooler day, you might see insects more in the sun, if it is very, very hot day, then they might take refuge in the shade.
And then there are also physiological means particularly in like in flying insects, which regulate body temperature by regulating heat loss, because the outline the body plan of an insect is you got your head, which has the brain such as it is eyes, sensory organs, most of them, and mouthparts, which are pretty straightforward. And then you have the thorax which is like a big thick bundle of muscles because the thorax is the point of connection for both sets of wings and for all three legs. And then the abdomen is where all the gooey stuff is right more or less where like the various particularly the digestive and the reproductive systems are in the abdomen. So if you take an insect for example, if you take a mantis This is very, very clear, I think in a way that it isn’t in like a beetle or fly. A mantis typically has a very clear difference between the head, the thorax, the abdomen, and particularly in mature female fantasies that you might see in the fall, they will have really large abdomens, and they don’t have highly sclera stylized cuticles, meaning that their exoskeleton is relatively thin compared to a really crunchy guy like a very hard beetle, right? Like if you step on a scarab beetle, it’s going to be a it’s gonna be a sharper crunch than if you step on a mantis.
Great way to quantify that.
And I remember reading in a paper once on mantises that described females as having quote, “voluminous ovaries,” which I thought was very…
Certainly gives you an image.
Yes, yeah. And it’s basically if you see like a mature female mantis in the fall that’s been developing eggs, it has a large it has a visibly distended abdomen, which is the point that I’m trying to get at where that is where all of the like soft stuff is, this is relevant because there will be like heat generated in the thorax that then gets sort of not totally dissimilar to the system of warming floors that a lot of countries have, where there will be like tubes running underneath the floor that carry heated and unheated water around so that it’s sort of ambiently warm, not totally dissimilar to that there will be through sort of the movement of hemolymph heat will go from the thorax into the abdomen and then can get diffused out of the abdomen to the surrounding air and thereby cooling down the insect overall.
Finally, before we get into some specifics, I wanted to clarify cold tolerance as a general idea versus freeze tolerance as a specific idea where overwintering basically requires the ability to live through colder than optimal temperatures. And there are many different strategies for that. But freeze tolerance has kind of a specific meaning of tolerating below freezing temperatures at which water forms ice crystals inside of the body because these things take different kinds of strategies to deal with you can avoid freezing temperatures and just not have to deal with it you can super cool where you were able to get to well below the freezing point of water and then freeze tolerance is being able to come back from that physical freezing actually happening and all three of these we see in insects in different places. So we’ve established why cold temperature is a problem, how it affects insects, how it affects humans. So that’s the question where are the insects go when it’s cold? And I’m so glad you asked even though you didn’t ask because now I’m gonna answer…
But before I actually answer that question, I wanted to talk a little bit about insects that are always cold because here’s the thing Here’s the thing, there are many different ways that insects deal with cool look very cold temperatures, well below optimal temperatures for most insects, and a lot of them involve long periods of inactivity in various stages of life. But there are some insects that are not only still active when it’s cold, but they are only active when it’s cold. And my favorite example of this is, well, it’s hard to know exactly what to call them, because they were originally described, I think, as a family within Orthoptera, back in 1914. And then eventually, they got their own ordinal status as grilled latonia. And now in the past two decades, people, at least a couple of people have pushed for the downgrading of it from an order to a sub order, alongside Mantophasmatodea, which was described as an order into 2002 and have them both be a single order, Notoptera, and I’m not going to get into this in more detail, because I think it’s probably extremely boring for everybody except, maybe just me. But it would would absolutely kill me as a taxonomist by nature, not to mention that there is disagreement on what rank it should have and what its name should be.
But I’m going to refer to them as Grylloblattodea. Because I think that’s how you’re going to find if you’re curious about them. That’s how you’re going to find most other information that is available about them. So they’re sort of name in the tradition within the tradition of Linnaean taxonomy is Grylloblattodea. And their common name is sometimes given as rock crawlers or as ice crawlers. Do you know about these guys at all?
I don’t think so.
I’m thrilled that today’s the day that you learn about them. So they are basically omnivorous, or rooftop droid insects, or fibroid meaning that they are in the the same general area as orthoptic row which crickets, grasshoppers, kids, etc. And that’s sort of the major group within a clade of insects known as polini. After again, this is a lot of like inside baseball, insect taxonomies stuff that I know, I know that most people don’t care about. But that’s not you know, that’s not going to stop me.
Let’s face it, the people who don’t care aren’t the people who listen to this podcast.
I mean, hopefully, right? And so they basically, in your mind, I want you to picture what would happen if you had a transporter accident where you combined a cricket and a cockroach.
Are you picturing it?
And they’re kind of orangey and they don’t have any wings. And they live in cool areas. And that’s Grylloblattodea, basically.
And they are generally known as a relic group, or they have one of the most narrow ranges of any order or sub order of insects, where they are found only at, quote, high elevations in the mountains of China, Siberia, Japan, and the western United States and Canada. So the idea behind Grylloblattodea is that at some point in the past, they would have probably had a bit of a broader distribution. But because they evolved to have a very narrow temperature tolerance range, they have essentially gotten stuck in the very small areas that they’re still found in because there’s no way for them to disperse out of them without moving through unbearable environmental conditions.
So they’re environmentally isolated.
Yes. And so basically, Grylloblattodea is extremely highly threatened by climate change, not because of, not because of human problems, really, but just because they only live in very cold places. And as those cold places get smaller and smaller, they look like they can’t migrate northward, or to higher elevations, because they’re already in the few places that they can still survive and the places that you find them. Unlike many insects, if their ideal temperature is not warm, they can tolerate like negative eight Celsius up to like four Celsius.
This reminds me a little bit of the ice worms they find in Alaska, which are not insects are different class of invertebrates, but they are obligate sacrifices, and that if the temperature gets above about, I think five or 10 degrees of Celsius, their enzymes will auto catalyze and basically dissolve them.
I, I don’t know if that is the exact case with grilled zodia. Even the temperature of like, if you held one in your hand, it would be too hot for them and they would die.
One of the few videos that you can find that somebody has recorded of them like walking around, they’re holding like an ice chunk in their hand and they’re rotating the ice chunk as the grylloblattid moves along it because they have to hold the ice chunk instead of holding the insect because if they held the insect, they would kill it. They’re known to have pretty low metabolism, which gives them kind of an unusually long lifespan for an insect with estimate between five and up to 10 years and I saw one source that even claimed that they didn’t fully reach maturity until they were seven years old, which is very unusual for insects…
I mean that’s very unusual even for mammals.
Yeah, yeah. And they’re relatively large. And by relatively large, it’s still pretty small but relatively large for cold dwelling insects because you often see cold dwelling insects, like if you go up into the Arctic Circle in North America even you’ll see a lot of midges and very small flies and stuff, but not a lot of things that like you could hold in your hand, theoretically, if you were Mr. Freeze and be able to see details about its morphology, right. And so the actual mechanism for cold tolerance not known again, because there’s just not a lot of research on them, but it seems like their primary adaptation for dealing with cold temperature is to be relatively transient so that they can stay at a stable temperature so as it cools down they go find slightly warmer microhabitats. As it warms up they go find slightly cooler, microhabitats. So they’re moving around a lot so that they don’t have to deal with variation in temperature. They can just kind of stay at their ideal right around the freezing temperature or water. I love these guys though. They’re great. It’s a real it’s a true dream of mine to like, go on a pilgrimage go find some because you’re just so unlikely to see them. I have a bunch of places that I want to go specifically to see specific cool insects
That makes sense. I mean, birders have a life list. It’s just kind of the same thing.
Sorry to birders, but come on, insects, insects. Also, on the topic of super cooling, I found… I’m including this specifically because I was reading today and I found an example in cockroaches. And I was so excited about it because cockroaches never get to have any fun in terms of being held up as examples of cool stuff. And so in terms of insects that produce ice nucleating agents where basically they induce freezing in a controlled way instead of just like it happening haphazardly, right, these cockroaches as an example of insects that have like anti freezing agents in their bodies, right.
And so it’s the specific species Cryptocercus punctulatus – the context for Cryptocerus is that Cryptocercus is the only genus of the family Cryptocercidae, which is now known as the sister group to all termites. I think we’ve mentioned this before. But Tessa, you know that termites phylogenetically speaking are…
They’re eusocial cockroaches, basically, right?
Yes, hyper specialized eusocial cockroaches, which was actually suggested multiple times over the 20th century in different places. But it didn’t really stick definitively until about the past two decades, when there were increasing molecular phylogenetic studies, which is another topic that we’ll definitely do an episode on, at some point, but not today. Today, the only point is that is important context, because crypto circus are wood roaches, right? And so they live in variably, rotting wood, and they have cellulose digesting protozoa in their gut that enable them to digest and process cellulose, right, which is in wood. This is the thing that that basically categorizes termites also. And this similarity between the two of them is actually one of the points on which people were very confused about carpet to staircase through a lot of the 20th century on the basis that why would they share this with termites if they weren’t closely related to termites? But if termites are the sister group to cockroaches and most cockroaches don’t have cellulose digesting protozoa, what are we doing?
A lot of great articles that are extremely exciting to me, probably pretty boring to most people. But this is my podcast.
I imagine there was a lot of quiet drama involved in the back and forth out there.
Yeah, yeah. And so this is important in that they’re basically the only research that I could found on this like original research was from a thesis that was published in 1985 on Cryptocercus punctulatus, where they found ribortol, which I believe is how it’s pronounced. This is a polyol which is, with sugars and proteins, categories of what are regularly present in insects as anti freezing agents. The rabbit hole is like a five carbon alcohol, which they found present in crypto circus punk geladas. And the suggested explanation for this is that microbes produce significantly more five carbon sugars than animals. And so because there are these cellulose digesting protozoa microbial endosymbionts Inside the cockroach, they suggested that the presence of these symbionts was the origin point that they might produce the alcohol or its precursors in the body. And I thought that this was very cool.
Those are among sort of some examples, some specific examples of insects that are still active when it is very cold. sort of the best known. Other examples are snow fleas which are not insects. They are Collembolans, which are non insect hexapods. So they are arthropods, they have six legs, but they are not insects specifically, and then also members of the family Boreidae in Mecoptera. And if you think that I’ve mispronounced either of those, you can go tell your mom and not me, because I don’t care. The Mecoptera are an order of holometabolous insects closely related to… I believe their sister group is is still believed to be Siphonaptera which are the fleas.
So Mecoptera are known as scorpionflies, because the male’s, their genital structure, they have like a bulbous general structure at the end of their abdomen that sort of curls over their abdomen and makes it look like they have a scorpion tail. And that’s how you can tell males from females and scorpion flies because females don’t have that they just have like a normal tapering abdomen like real losers. And both of these are known to be active on as you can probably tell from their common names, snow and ice and cold conditions.
So all that said, now we’re getting to the actual answer to the question, Where do the insects go when it’s cold. And there are a lot of answers to that in sort of, conceptually, a lot of overwintering strategy where overwintering just refers to what happens over the winter.
Right, as the name suggests.
It’s pretty straightforward. A lot of overwintering strategy comes down to control controlling whether water freezes in their bodies, how much and under what conditions and so insects overwinter in any stage of life that they spend any other part of the urine, so insects will overwinter as eggs, they’ll overwinter as nymphs or larvae, they’ll overwinter as pupae, and they will overwinter as fully mature adults, although that one is relatively uncommon.
The first strategy that I wanted to mention just very briefly, which you actually mentioned earlier, is just leaving not dealing with cold temperatures at all. And sort of the classic example of this here in North America is with monarch butterflies, which fully migrate into a warmer temperature, and then they migrate back. And they’re one of maybe I don’t think the only example but certainly the best known example.
There aren’t a whole lot. But there are other butterfly species in Europe that do it as well.
Yeah, it’s… I don’t… No offense to lepidopterists, again, no offense to Erin, friend of the pod, but I just, I just can’t be bothered with most butterflies most of the time. But it is interesting that they have the same kind of two way migration that a lot of birds have were two way migration, meaning they start out in one place, they go to another place, and then they go back to the first place. And there’s all kinds of really fascinating research on like, what are the cues that butterflies use? How do they manage this migration, like, et cetera, et cetera. But we’re not going to get into it right now. Because I refuse to… I refuse, and I’m not going to do it.
So the next thing is just going through, you know, life stages, from earliest to latest possible overwintering as eggs is a relatively common where adults will physically mature, they’ll mate in the fall, they’ll lay eggs, and then the adults will often in this strategy, just die. And they don’t survive the winter. But their egg cases do and sort of the one of the most classic examples of this is mantises. So members of the order Mantodea. If you’re listening to this, you certainly have seen a mantis, if not in person then a picture. Mantises are the sister group to cockroaches and termites together and then all three of them former group frequently known as Dictyoptera.
Mantises… one of the sort of the key characteristics that mantises share with cockroaches is that both groups make ootheca – basically just egg cases. And cockroach egg cases tend to be like, they kind of look like cute little leather clutch purses. Does that – do you know what I’m talking about?
They they sort of they have basically just… imagine like a leather clutch. And then imagine it coming out of the abdomen of a cockroach. And you’re basically there.
Okay, I can picture that.
And if you need a visual reference, I would encourage you to think of the cover of the Animorphs book where they first morph cockroaches and much love to the guy who did the cover art for the original American copies. But it doesn’t make any sense for the cockroach that Marco Merson to to have a protruding egg case based on the logic established in the lore of morphing but all forgive him. The guy who did those covers also sells them on Etsy signed there actually I bought one of that cover. They’re really nice quality prints. I really recommend it. Anyway.
So those are cockroaches but mantis egg cases they don’t carry them around. And they basically extrude them onto twigs, or branches or to the sides of buildings, some strong substrate and they they make a very foamy substance that in cases the eggs, essentially it does foam up and it hardens around the eggs. And I tried to find more details on like mechanistically how this works like what is the insulation made out of basically just saying that it is foamy, and it surrounds the eggs as as far as you can get without getting really into the weeds of specific proteins that don’t even have a common name. Essentially, that foamy material around the eggs works as insulation and it protects them as they go even in sub freezing temperatures over the winter. And then in the spring, various cues cause them to hatch, they usually hatch all at once. Often they will eat a lot of each other because they’re generalist predators and then they’ll just develop you know step by step through the rest of the year until they get into the fall when they reach sexual maturity. And in their final molt. The final molt that insects do they get genitals and they get wings as they have wings, and they use those genitals to connect them up with each other they meet female mantises will often grow the before mentioned voluminous ovaries full up of eggs, and then the lay egg cases. And then when it gets to be too cold for the adults, they’ll just die. And, but the eggs won’t. And then the cycle continues again, where they only have just the single generation that happens every year.
So that’s sort of the classic but not the only example of overwintering is eggs. And so then if we step into the next sort of stage of life, overwintering as nymphs, or larvae, where the difference between nymphs and larvae is really whether an insect has hollow metabolites are not word typically we use nymph to refer to Hemi metabolites and comparable insects and then larvae to refer to that specific image or phase of holo metabolism insects. A lot of insects will overwinter as larvae, particularly because the larvae will live inside relatively warm environments. So like a lot of grubs, I think will overwinter deep in the soil where you know, it is not freezing as much as it would be above the soil or the European corn borer, which is like a really established pest on corn crops. They will overwinter at the last instar the last stage of being a larvae inside of corn stalks. They are freeze tolerant, they can survive freezing of the water in their body, but not within cells, which is kind of a common theme. And so caterpillars, grubs, etc. will often they’ll live inside these different micro habitats which are more stable and more warm than sort of surrounding habitat. But I have two sort of specific things that I wanted to talk about.
One is a lot of freshwater like aquatic insects will overwinter in their nymphal stage in the water. And the reason that they can do this is that water, as we generally know, is much more sort of temperature stable…
Right, has a higher heat capacity.
So yes, yeah. And so basically a lot of these insects unless they’re living in very high altitude, or like very, very cold areas, the water will continue, the water will get colder, but it won’t freeze. And even if it freezes at the top, it’s not going to freeze all the way through. So they’re protected from the dangers of sub freezing temperatures by being in the water, because they aren’t actually having to deal with as high of a variation in temperature as a fully terrestrial. As we talked about mantises those adults are, they’re just dying.
But there is at least one stone flying them which is known to be freeze tolerant, ie it can come back from fully itself freezing and it’s this species name is Nomoura artica, which is widely distributed through what the US government recognizes as Alaska and its freeze tolerant strategy is most likely due to glycerol where according to the sort of the primer article that I found on a quote, :many freeze tolerant insects prevent the formation of intracellular ice by producing extracellular ice nucleators that promote ice formation at high subzero temperatures, reducing both rate of ice growth and the probability of intracellular Freezing Ice growth in the extracellular fluid excludes solute increasing the osmotic pressure. This causes an osmotic flux of water out of the cells and results in intracellular desiccation that may eventually pose challenges rival to mitigate the effects of intracellular dehydration freeze tolerant insects produce low molecular mass cryoprotectants, such as glycerol these cryoprotectants reduce the proportion of frozen water and have a stabilizing effect on proteins and membrane.”
… which is to say if you are a freeze tolerant insect off then you will have these ice nucleators outside of yourselves that instead of being like, Oh no, it’s cold, how do we avoid freezing temperatures? How do we avoid the water freezing in our bodies, it’s sort of an acceptance of, of environmental like I’m here, and it’s gonna freeze. And so then exerting control over how and how much water freezes in the body. And part of that is using cryoprotectant, such as glycerol, which reduce the proportion of frozen water and thus, sort of stabilize the body overall, which is not the same as but not 100% dissimilar to sort of what we talked about in terms of actual effective methods of protecting, for instance, frozen human organs through like vitrification, where you are still freezing, but by preventing the formation of just willy nilly ice crystals, you are keeping things much more stable, so that there is not that strange, sane, like mechanical stress on and around cells of things getting bigger and smaller and bigger and smaller, or at least that is my impression as somebody who is not in this area.
And then another very interesting sort of overweight, like freeze tolerant in a larval stage is the Antarctic Midge or Mobdro Murphy, which is in fact one of several invasive invertebrates in Antarctica, it has established an invasive presence in Antarctica. And you’d think if there was one place on earth that didn’t have to worry about invasive invertebrates you would be Antarctica, but you would be incorrect based. And basically what this allows itself to do it is freeze tolerant as both are they are, quote, desiccation tolerant, as both eggs and larvae. And what they do is that they essentially, as far as I can tell, fill up its body again with sort of antifreeze sugars and allows itself to be dehydrated by the surrounding soil. So it fills itself up with a replacement, you know, fluid that is not going to freeze and then the actual water gets pulled out of its body and it gets desiccated. And what’s really interesting about this is that that is part of how it exists.
That is part of how it is freeze tolerant, but in sort of a broader, like, not ecological is sort of a lifetime, sort of the overall life strategy of how it is able to continue to thrive even in such an inhospitable environment is that it has parthenogenetic reproduction. So it doesn’t need to worry about synchronizing the timing of adult emergence, which is a big problem with a lot of insects where if they are emerging as adults all at once that timing is really critical. If they’re, if they emerge too late for everybody else emerging, they’re just completely out of luck. And they don’t they can’t meet with anybody. But if you are Perth into genetically reproducing, ie what sometimes people like to call virgin birth, which come on but a sort of an evocative for his, if you don’t need to meet with anybody, you can just reproduce totally on your own schedule, which means that they can continue on making more of themselves without having to worry about timing themselves correctly with other adults. I thought that was very interesting.
And then I don’t really have any notes for overwintering as pupae, because as I’ve specifically written in my notes, “either because of the outsized popularity of Lepidoptera, or because it’s most typical in them many sources on overwintering as pupae are about butterflies and moths,” specifically, and again, no offense to Erin, friend of the pod, but I can’t be bothered.
But it’s kind of you know, it’s the same idea of pupation is, is often a pretty inactive state inactive and literally, you are not going around doing stuff. There are some pupi that are very active, for instance, mosquito pupi you know, often pupi are like it’s more of an internal journey than a walking around and feeding on things kind of a journey. And so it’s not really that remarkable that you would then be able to overwinter is something that is already not doing a whole lot.
And then finally we get to overwintering as adults, which we don’t see as much from as far as I can tell, there are still a lot of insects that overwinter as adults, for instance, leaf beetles, aphids, leafhoppers, many other kinds of beetles and similar to a lot of larvae. A lot of the strategy of overwintering as an adult is just finding a relatively warm protective micro habitat, and then just sort of hunkering in place until it gets warm again. So a couple examples of these microhabitats may be like inside layers of leaf litter inside logs inside on one side of a rock that is going to be protected from getting In a lot of snow, so you will still might be under snow. But you’re not going to be as bothered about like just finding relatively warm places and then going into a state of inactivity until it’s warm enough to be active again.
And I do want to draw a distinction there is something known in insects known as diapause, which is often compared to hibernation. And I’ve also seen other people being like diapause is not hibernation. And I think it really depends on how you probably want to use the term hibernation to begin with. But according to at least one book that I read, diapause is specifically for pre adult insects, where it is a cessation of growth and development, not just a relative inactivity, because insects are among the arthropods that don’t, some arthropods just keep molting forever until they finally die. But insects don’t do that they have a set number of malts that they will go through in their life. And then when they get to the final molt, they develop if they have the wings and genitals, which basically is like it’s party time, you can disperse, you can go find each other, you can connect your genitals up and make more little insects right go have fun.
And in fact, there are a lot of insects where they will their final molt will have genitals and not mouthparts. And they are they they don’t eat at all, they are only adults so that they can mate and then die. For instance, bought flies, which I also love, which makes them look really cute, like they don’t have any mouthparts. So they just have these perfectly round heads. I love butterflies so much anyway.
And another strategy, a particular strategy that a lot of adults might have for overwintering is finding a particular kind of warm habitat, which may be indeed your home. Cuz if you think about it, it is often kept at a stable warm temperature. But on the flip side of that a lot of insects may still die, even if they you know, are trespassers due to lack of appropriate food sources. So that’s basically kind of the story of overwintering as adults, it’s not that exciting, particularly but when I was looking for information, I did find a very, very interesting specific insect, described in Insects: Structure and Function, fifth edition, of a cynipid, which is a kind of wasp – it’s a gall wasp. Gall wasps… do you know what gall are actually, Tessa?
They’re like cysts, I guess for lack of a better term, that are formed by some insects, in like trees and plants as a place to either pupates or lay their eggs.
I mean, yeah, and sort of the best known example, or at least the most popularly known as far as I can tell, of the gall making insects are gall wasps, which are, I think, all parasitoids, where the distinction between parasite and parasitoid is generally given that parasites will not kill their hosts and parasitoids do kill their host. I think there’s some disagreement about that. But listen, let’s lay all our cards out on the table, there’s disagreement about literally every formal term in biology that you can possibly look at. That’s just the nature of biology, and you just have to accept it and accept it into your heart.
Anyway, so one strategy is having alternating parthenogenetic reproduction with sexual reproduction through alternating generations. So the example given in the text of insects structure and function is a species of snippet goal, wasp neurotherapeutics, Polaris, which forms goals on the underside of oak leaves. And so this species overwinters, as far as I can tell, as adults in the goals on the underside of these leaves, but only the females emerge in the spring, then the females will lay unfertilized eggs on young leaves. And then those eggs give rise to both haploid males and diploid females, then that generation of males and females meet with each other, they lay eggs, which are fertilized, and then those eggs produce the females that will then parthenogenetically reproduce the next spring.
And so the answer ultimately to where the insects go, when it’s called is, in some cases, they just straight up die. And in other cases, they haven’t really left us at all. They’ve just gone, where we can’t find them unless you go rooting around and like leaf litter, or turning over rocks and streams. But the good news is that even though they’ve left us they’re never very far away, and they’ll be back sooner than later. Which I guess is a nightmare for some people. But it’s a real dream for me. All winter long. I just sit at home and I’m I get excited about the return of insect season. God I love those guys.
I mean, understandable.
They’re just the best. I mean, it is sad that they aren’t around but, but on the other hand, that gives me more time to bake. And then when it’s too hot to bake, the insects are out. And in that way God really does provide for us at all stages of our lives. Take that theology straight to church.
Well Tessa, I hope that this has been an illuminating experience.
I’ve learned a lot.
I’m so glad. I can’t believe that we’ve gone so long without an insect episode. My solemn promise to the people of Earth, is that will never go this long without talking about insects for a whole episode ever again. Unless I die, I guess in which case, you can do whatever you want. You can do whatever you want with the podcast. As long as you don’t bad mouth insects.
Oh, of course not. Never.
Yeah. So happy fall, everybody. It’s the season of death. So get out there and think about the profound fragility of life. Well, I’m not going to say to find me on Twitter, because who knows if Twitter will still even exist tomorrow. But Tessa, you can tell… if people want to find you, where should they look?
I mean, you may be able to find me on Twitter, if Twitter still exists. But in the meantime, you can find me at my website tessafisher.com.
If you are a trans, non binary etc, person who is in or around science – for instance, if you are a grad student, researcher, journalist, cockroach breeder – and you would like to be a guest on the podcast, we are looking for guests, we have no more saved up guest episodes. So there is a guest interest form that you can find through our website, or you can email us email@example.com. Our theme song is by Nicole Petkovich, friend of the show and previous guest, and if you’ve liked the show, please tell other people about it. I guess if you think that they would like it word of mouth is really the primary way that podcasts grow. And Tessa?
And until next time, keep on science-ing.