American Scientist

Paula Cushing is senior curator of invertebrate zoology at the Denver Museum of Nature and Science. Over the course of her career, she has studied a variety of arachnids—not only spiders, but also some of the stranger lineages such as solifugids, more commonly known as camel spiders (although Cushing notes that the name spider is a misleading misnomer in this case). Cushing began her career studying the antipredator behaviors of spiders. As a master’s student, she received two Sigma Xi Grants in Aid of Research to complete this research. Cushing spoke with American Scientist editor-in-chief Fenella Saunders earlier this year. This interview has been edited for length and clarity.

How do you describe how you fit in your field of study?

I’m a generalist arachnologist. I’ve published on spiders, scorpions, and several groups of arachnids. I’ve published a lot of taxonomic work, some biogenetic research, some behavior research, as well as morphological work. I’m a generalist in that sense. Since my master’s work, I’ve branched out to a bunch of different subdisciplines beyond the behavioral aspects that I researched back then.

What got you interested in biology?

I remember distinctly as a little girl thinking how weird it was that we live on this planet that has all this life on it, and yet as far as we know it’s the only planet in our Solar System, maybe the universe, with life. I started thinking, How did that come to be? In my very logical little girl brain, I thought, 'Well, if I want to understand why life exists on this planet, then maybe I’ll study nature.' I went to one of my older siblings and asked if there was a word for a person who wanted to study nature. My wise older sibling said, 'Why yes, a naturalist.' They pulled out a dictionary, because back in the day you had dictionaries, and opened it to naturalist. I was amazed that somebody else had thought of that before me. And I thought, okay, that’s what I want to do.

How did you start studying spiders, specifically?

I grew up in the Washington, DC, area. In high school I became a volunteer naturalist at a park. And then I interned at the Smithsonian National Museum of Natural History. I realized that I was more interested in animals than plants. And in terms of animals, somehow I knew that there were a lot more interesting questions to ask about arthropods than there were about vertebrates: There’s a lot more species diversity. And so I applied and got to be an intern at the Smithsonian’s insect zoo. I gave my first spider program at a park as a high school student.

I went to college at Virginia Tech, and I found out from a friend that as a college student, you could volunteer to do research in a professor’s lab. I thought that would be a great way to see if I liked it and if I had the skill set to be a researcher and a biologist. There were two professors in the biology department who did research on arthropods. One was a butterfly person, and I realized that butterflies are like the birds of the arthropod universe, in that they are very popular. So lots of people wanted to study butterflies. I wanted to stay away from that. The other professor was Brent Opell, who studied spiders. I made an appointment and met with Brent. He was very open to having this great discussion with me and welcomed me into his lab. He became my master’s degree mentor. He was a brilliant mentor, gentle and supportive. That’s how I got started in the field of arachnology. He took me to my first meeting as a freshman where I gave my first presentation.

What drew you to the field of arachnology?

The field of arachnology is a very small, intimate field. The field is very supportive of amateurs, or what we would call non-PhD researchers. They’re very supportive of students and anyone who’s interested in the field. I really liked that environment. I’ve always felt like science moves forward faster in that kind of a supportive environment than it does when everyone is competing against one another. Being at that meeting made me happy about my choice of field.

So it was both a fascination with the animals, but also just feeling welcomed into this subdiscipline of biology [that drew me to this field of study]. I think that’s a super important message to get across. Science still suffers from a lack of diversity. We talk a lot about DEAI [diversity, equity, access, and inclusion], but sometimes improving and increasing diversity is as simple as making people feel welcome, making them feel like their ideas are being listened to.

It's interesting that your GIAR grant was related specifically to predator response behavior in spiders, because people usually think of spiders as predators, not as needing defensive mechanisms.

Lots of birds that eat arthropods, even hummingbirds, will eat spiders readily, as will lots of mammals and reptiles. It was intriguing to me to flip the question. Instead of focusing on how spiders hunt their prey, I thought about how they escape predation.

In this case, when I was a master’s student, I noticed that when you would brush past the webs of certain spiders, they would jump out of the webs. That seemed like an odd response. It was probably an antipredator response, but no one had tested that obvious hypothesis. It was intriguing to me to try to figure out ways to test it. I did test it, and for me, that was really satisfying, from the very beginning of deriving the question to the end product.

The Grant in Aid of Research often is what gives you the self-confidence to know that your ideas are valid. I think that alone is such a treasure and such a powerful message.

What’s your pet peeve, the thing that people believe about spiders that you wish they would stop believing?

As a museum curator I answer pretty much a question a day, and almost all of them are, 'I found a spider in my house. Is it poisonous? Is it going to kill me? I have a child! Is my child going to die?' So my pet peeve is that people have this irrational fear. We are giants compared to these tiny little organisms.

I have this hypothesis that you see this irrational fear of spiders largely in well-developed nations like the United States and western Europe, where nature is separated from our lives by concrete and asphalt. Because of that disconnect between our lives and nature, humans in these countries develop this irrational fear of organisms, 99 percent of which cannot hurt us. When I travel to tropical countries you don’t see that kind of overwhelming, irrational fear of arthropods, spiders, and other organisms, because nature is much more a part of day-to-day life in those tropical countries. Often homes don’t even have screens on their windows. Nature really does come inside. I could be totally wrong, but that’s just a pattern that I’ve noticed by traveling around the world.

Spiders are themselves top predators of insects, offering an enormous benefit by controlling insect populations.

There’s a lot of studies about how most people get their science knowledge from informal learning, such as at museums. How did your museum experience differ from working in academia, and how has it affected your career?

It seems like there’s a disconnect between academics and the general public. That’s the role that I think natural history museums are ideally suited to play, to connect the academics, the scientific message, with the general public. These informal mechanisms for science education are ideally suited to take very complex scientific concepts and translate them in a way that the general public can understand and appreciate—and hopefully understand how those concepts are connecting with their everyday lives.

So throughout my career, I’d been doing work in museum collections. When this museum job came up, I felt like this job was a perfect fit for me.

Do you manage a collection?

Yes, two collections. When I started at the museum in 1998, I was hired as the entomologist. I very quickly convinced them that I should add curator of insects and spiders to my title. Then I became department chair for a few years and was able to hire an entomologist. The year after I joined the museum, I started up the arachnology collection. We didn’t have an arachnology collection in 1998. We’ve grown to about 50,000 or 60,000 vials. The data from 40,000 of them are online and widely accessible. At one point I was curating that collection and the entomology collection, as well as a marine invertebrate collection, which was mostly a shell collection. That was a bit too much. I hired an entomologist. But now I curate the arachnology collection and the marine invert collection. And from the very beginning I made sure that all the data from both those collections is accessible online. They’re published and pushed out to the Global Biodiversity Information Facility so the world can see our data, which has helped in terms of increasing access and loan activity.

How are camel spiders, or Solifugae, different from other spiders?

They are arachnids, meaning they’re in the class Arachnidae but in their own order. There are 12 orders within that class. Spiders are the order Aranea. Solifugae are their own order. There are 12 different families. My lab is focused entirely on one of those 12 families that’s found from southern Canada all the way down through southern Mexico, the Eremobatidae. There’s probably four families that are found in North and South America. They are found primarily in xeric [dry], semi-xeric, or desert environments. They’re top predators in those environments. Because they’re restricted to those habitats, they’re often an indicator of the health of those environments whether that’s true or not. Camel spiders likely diversified as these desert environments themselves came to be. As the desert environments diversified, you see a diversification of these desert-adapted species.

By understanding the diversity and natural history of Solifugae, you can often translate that into a better understanding of how these desert environments evolved. I work closely with another colleague, Matt Graham, who’s a biogeographer at Eastern Connecticut State University, and he’s looking at this exact marriage between solifugid diversity and the evolution of these desert habitats. My lab is doing the molecular work and the morphological work to find out where the species boundaries are.

Solifugids don’t look like typical spiders. How do you describe them?

The reason that they’re in the class Arachnida is they have certain morphological features in common with other arachnids. Their jaws are called chelicerae. The structure of the jaws is very similar to what you see in all the other arachnid orders. Like all arachnids they have four pairs of legs. They have a pair of front appendages called pedipalps. They have two major body parts, not three. They have no antennae. All of those characteristics unite them with other orders of the class Arachnida.

But in terms of what they look like, they’re kind of a cross between a weird-looking spider and a scorpion. But they are their own beasties.

The smallest adult would be maybe a centimeter, up to about six to 10 centimeters. Pretty large. The largest species are found in the Middle East.

Where they fall in terms of which of the other orders is their closest relative is anybody’s guess. There are studies that place them as closely related with pseudo-scorpions. Other studies place them as more closely related with one lineage of acarines, of mites. Different colleagues are looking at the higher level relationships between these different orders of arachnids. That’s not my work at all. But with every different data set that’s used, the placement of solifugids seems to shift. We don’t really know who they’re most closely related to and what their ancestor may have looked like right now. That’s still a moving target.

How did you begin studying the Solifugae, these camel spiders?

When I got my job at the museum in 1998, somehow either I found out that a man named Jack Burkhardt—who’s been publishing on camel spiders since the 1960s—lived in Denver, or he found out that there was an arachnologist at the museum, or both. Neither one of us can figure out exactly how we connected. But he contacted me. I knew his name from the literature. He said he was interested in working in the collection. Of course, I was very welcoming of that, because I knew he was a world expert on this group of arachnids.

Jack is now in his late 80s. He’s a brilliant scientist, very knowledgeable about camel spiders, and a gentle soul. He’s also very persuasive. When he first started coming into my lab, at first he said, 'Well, let’s collaborate. I don’t draw well, so why don’t you draw the diagrams and the morphological figures, so that I can continue my taxonomic work?' And so I started illustrating his publications. Then he said, 'Well, you’re a scientist, so why don’t you review these papers and put in your 25 cents worth of input?' I became a more active collaborator. Then we found out that there was another colleague who was working on a big NSF [U.S. National Science Foundation] grant focused on camel spiders. I wrote to that colleague and said, 'Hey, we’re doing all this taxonomic work. We need to be plugged into this grant.' I became co-PI [co-principal investigator] of that grant, and Jack was instrumental in that grant. That was a five-year NSF grant. I got a four-year NSF grant to continue our work. That second grant I largely got because I was able to say, 'There’s one world expert, Jack Burkhardt. He’s in his 80s. We need to get more people into this field, more students trained up under Jack, before we lose his wealth of knowledge.' That was a large part of why I managed to get the second grant. That’s how I got into camel spiders.

Jack never told me how brutally hard camel spiders were to work on. They’re hard to collect, hard to find, and impossible to keep alive in the lab. We’ve learned that any time we write up a manuscript, we need to explain in the introduction that these are horrible animals to work with. You’re lucky if you get five animals on a night of eight hours of field work. That’s a good night. We have to preface the paper with all of this information. You can’t keep them alive in the lab, and that’s why our N value [sample size] is 10 and that’s it. Because otherwise the peer reviewers, who are largely arthropod people, say, 'Why didn’t they have more?'

You said solifugids are top predators in their environments?

Solifugids think of nothing but food. They have a super high metabolic rate. Mostly they’re nocturnal. They only come out of their underground hiding holes or burrows in the evening after sunset. Then they’re hunting from sunset until sunrise. They move through the environment at very rapid paces. When they encounter an insect or something smaller than them—it could be another solifugid—they grab it with their pedipalps, bring it to their jaws, tear it apart, and then consume it. Then they’re on to find the next thing to eat. They’re really active predators and they eat anything. They eat ants. They eat each other. If they could, they would eat scorpions. There are documented cases of scorpions eating solifugids and solifugids eating scorpions.

So solifugids do not build webs or wait for prey to come to them, as spiders do?

Correct. They’re very active predators. In fact, that high activity level is really unusual for arachnids. Most arachnids are sit-and-wait predators with a very low metabolic rate, even ones that are active hunters like wolf spiders, which can do a quick run to take down prey, but then they have to stop. Metabolically they just can’t sustain that high activity level for very long. Same thing with scorpions: Scorpions come out of their burrows and just wait for something to walk past.

All these other arachnids have pretty low metabolic rates, but not solifugids. We’re not really sure why that is. I’m working with another colleague who’s a physiologist to try to understand more about how they can maintain this high metabolic rate and sustain it for so long. I had a colleague, Yael Lubin, who once followed a solifugid in the Negev Desert in Israel. She said she followed it for over an hour to see how long it took before the solifugid stopped and had to rest. It never did. It just continuously moved through that environment. It’s more like a flying insect’s metabolic rate than any other arachnid. Very unusual.

What did you find out about the biogeography of the Solifugae?

In a paper we published in 2015, Matt Graham demonstrated we could look at the timing of when we think different lineages of Solifugae appeared in the environment, and could basically match that to when those habitats were coming on the scene as well. As the glaciers retreated and desert habitats began to appear in North America, you see a concurrent diversification and expansion into those new habitats of these desert-adapted arthropods, including Solifugae.

Matt has also done some field work in the Baja Peninsula with my students last year. They know what the hypothesized evolution of that peninsular habitat is, and that it was inundated by inland seas and divided up into several different islands. Then, as the seas dissipated, the peninsula appeared. He’s been able to show in very recent work that this appearance matches the genetic diversification of some solifugids in these different parts of the peninsular habitat of the Baja Peninsula. That matches pretty well with what we think about how that peninsula itself evolved.

You said they couldn’t live in the lab very well. Is that because of the conditions they normally live in?

No idea. There are a lot of aspects of their natural history that we just don’t understand. It may be that there’s a very narrow range of temperature and humidity that allows them to live. What seems to happen is when you bring them into this artificial lab environment, they just—I don’t know how to say it except anthropomorphically—they just seem to give out. I’ve had ones run around the circumference of a cage until it finally just dies. It had a little heart attack and died. They just do not like being in captivity. Maybe you can keep them alive for a month, and that’s pretty good. But usually they die within a week or two. And that also could be that their life span as adults might just be pretty short. We just don’t know enough about them to know.

To other projects, how did you end up studying orb weaver spiders in space?

There’s this great company in Boulder, Colorado, called BioServe Space Technologies. BioServe works very closely with NASA. It’s a sort of conduit between academics and NASA in developing life science experiments that can be packaged and transported up to the International Space Station (ISS). If a scientist or anybody else has an idea about something they want to ask about space environments that has to do with worms or fruit flies or spiders or any kind of life sciences, BioServe has enough experience that they can work with the scientist, get an understanding of the hypothesis, and then develop the protocol and the equipment required to transport that experiment safely up to the ISS. And so years ago, I think it was when the Columbia explosion happened, BioServe had been working with a high school student who wanted to send orb weaving spiders up to space. This had already been done since Skylab back in the 1970s. They just wanted to repeat this experiment using orb-weaving spiders. They found that there was a spider biologist, an arachnologist, in the state. They had sent the experiment up on Columbia.

They were trying to restructure the habitats and asked to meet with me to go over the plans for the redesign. So, we met, and I went over that spiders need a ready supply of water. They’re water limited, not food limited. As long as they’re well-fed when you send them up, they should be fine for a while, but not if they don’t have water. And so we designed the habitat that would allow them to get a drink of water. Then I pulled in another colleague, but by that time it was November, so they were only able to send juveniles of Colorado species.  But I contacted a colleague who lives in Florida and he was able to send some wild-caught adult spiders, orb weavers of a different species, that are small enough to fit in a very small habitat.

That’s how I got involved, helping them design the habitat. Then I got to go to the launch. I got to be involved in the handoff where we did the final preparation of the habitats, and then the NASA personnel come into the laboratory all dressed in scrubs. The scientist hands off the experiment to the NASA personnel. At that point we can’t touch it anymore because it has to be sterile. That was just so thrilling. Who would have thought I would be involved in that? I got to go to the VIP viewing platform to watch the launch. The idea that something I touched was flying up to the ISS was just&mdasy;oh my God—that was a highlight of my career.

Later, I helped them with another experiment to send jumping spiders up to space, and another experiment to send a different kind of orb-weaving spider up to space, called Nephila. Then I was able to connect BioServe with colleagues who were able to do some data analysis and actually get some of these data from two of the orb-weaving experiments. One of my colleagues, Samuel Zschokke, was able to take the data and get it published. I was on that paper. I was able to connect BioServe with somebody who specializes in spider webs and web construction. Then there’s a professor, a robotics professor at University of Colorado Boulder, Kaushik Jayaram. He has a student who’s really interested in jumping spider behavior and the mechanics of jumping spider behavior. I’ve connected him with BioServe so that his student can analyze the data from the jumping spider experiment. Hopefully someday we’ll be able to get those data out there and get it published as well.

I feel like I’m more of a connector between BioServe and my colleagues, who I know have the capacity and the capability to do the fine-tuned analysis of behavior. I don’t have that expertise.

Were there any surprises coming out of these studies in space? 

When BioServe sent these habitats up there, in order to take pictures of the habitats, they had to be illuminated. They had it on a 12-hour light, 12-hour dark cycle, and of course the lights were shining down on the web from the same direction every day.

What Samuel was able to document is that on Earth spiders use gravitational cues to know how to orient in their web and position themselves on the web and how to build the web. In space, what they were using is the light cues. The gravitational cues were nonexistent, but the spiders were able to replace the gravity cues with light cues. When the lights were off, the spider was every which way on the web. But when the lights were on, they were always facing downward like they would on Earth. They were able to adapt to this environment. We knew they could adapt to lack of air current and lack of gravity cues and still build fully functional webs.

For jumping spiders, they’re still able to hunt. For the orb weavers we knew they’d be able to do that, because we’d seen that in Skylab. And we knew the structure of the web was more even. On Earth there are more capture spirals below the hub than above the hub. In space it’s more symmetrical. The hub is more or less right in the middle. And there are some other subtle differences. In space the spiders are able to just carry the silk from point A to point B, because they have no air currents to rely on to carry the silk. But what we didn’t know is they could use these light cues in the absence of other cues. They could replace the gravity cues with light cues and know how to orient themselves. That’s pretty cool.

One last question: What has come out in your research on ant-spider symbioses?

I’ve looked at spiders that either are ant specialists, feeding just on ants, or that live in very close association with them, sometimes within ant nests. In my spider line of research, I try to understand how spiders cross that species boundary, how they become integrated with this very complex social dynamic that’s represented by ant colonies.

One of my colleagues, Norm Horner, is an incredible naturalist who’s in his late 70s. Norm has been out there in the Big Bend area of Texas. Years ago, one of his students discovered a spider that none of us could identify. It was just bizarre, a tiny little thing. Bizarre morphology. He sent it to me, as well as several colleagues all over the world. None of us could even identify the family.

A colleague in 2019 finally got some specimens, ran a DNA analysis, and found that it came out as its own lineage. There were many authors on that paper. Martin Ramirez, my colleague who did the molecular work, placed this spider in its own family.

We knew it was a really unusual spider. Norm had only found it in association with ant nests. He and I went down during the pandemic to Big Bend. We were thinking this is a spider—like several other species that have been documented—that’s probably living inside ant colonies. It’s probably eating other little arthropods that live in the ant colonies.

We wanted to find out more about the natural history of this very bizarre spider. We went down to Big Bend and managed to collect two live spiders right next to a nest. We brought those live spiders back to the laboratory in Big Bend. We introduced them to a Petri dish with several ants from that same colony, adding the ants to the dish where the spiders were hiding, and lo and behold, what do we find out? These spiders are ant eaters. They specialize in hunting ants, which no one had known about their biology before. We were able to film them rushing up behind the ants, biting the ant’s rear leg, backing off, waiting until the ant succumbed to their venom, re-approaching, grabbing the ant behind its head, carrying it off, and then if a live ant approached the spider that was holding on to the dead ant, the spider would maneuver its body and twist around so that it was presenting the dead ant to the living ant as a shield. This kind of behavior had been documented for other specialist ant predator spiders, spiders in a family called Zodariidae, which also specialize in hunting ants. Almost identical hunting behavior. It’s clear that this very different lineage has evolved very similar hunting strategies when hunting such a dangerous prey. We were able to get enough data to analyze it and to submit it to the Journal of Arachnology.

Now we want to find out if the spider is living in the vicinity of the nest, which is what I think is happening. But Norm thinks they actually live inside the colonies. If that’s the case, that’s kind of bizarre. You’re talking about basically an ant murderer that lives amongst its own prey. That would be totally bizarre.