Oh, the nerdity of it all. I just read about a discussion of how spider-aliens would survive in space. This is my kind of thought-experiment!

In my stellar empire, the sapient life of the home world are arachnids. Due to an oxygen-heavy world with certain evolutionary characteristics, spider-like beings developed intelligence and formed society, leading them (eventually) to start looking toward the stars. This led to the development of space suits for the pioneering arachnid astronauts.

What would these look like? How would space suits be differently designed to support arachnids?

Let us posit that the arachnids are roughly 4 feet from “spinneret” to fangs. Their legs are large enough to support them (I don’t know what that is). They’re light compared to us (maybe 25 pounds at the heaviest – bear with me on the whole square cube law deal). They have roughly equivalent technology levels to ourselves at the time of our first missions into the stars.

Just by coincidence, I’ve been reading up on spider physiology recently, so this piqued my interest. Most of the answers in that thread are pretty good.

I’d first have to state a caveat: multi-legged alien beings evolving on a distant planet will not be spiders. It’s unfair to compare limitations and abilities that they have to those of terrestrial spiders — they aren’t related! Just blowing up an Earth spider to 1.2 meters long is not a valid comparison. But OK, let’s play a game and imagine an alien “spider” that evolved from an ancestor living in a similar niche to that of our spiders. Traits that are probably relevant are:

• Obligate carnivory. It’s a predator with a need for live food. Unless there’s a way to store frozen bug juice shakes that this species will willingly eat, this sounds like a big problem — space-faring “spiders” are going to have to bring along along bug ranches to maintain livestock. And food for said livestock. Forget the spacesuits, there are going to be some demanding requirements for life support on space ships.
• External digestion. Why, you might ask, would they need live food? Spiders inject enzymes into their prey which break down the guts of the animal into a liquid soup that they can slurp up. The food is both the container and the meal. “Spider” Tang is going to be a tough recipe.
• Ambush predator. This has pluses and minuses: most spiders have notably lower metabolic rates than other poikilotherms, about 70% of what is expected in animals of equivalent size. So, lower oxygen consumption — that’s great for spacecraft/spacesuit design. On the other hand, they can have extreme surges in activity, for instance, when prey is captured. Oxygen consumption is going to be somewhat unpredictable.
• Pulmonary system. The linked discussion mentions how spiders often have a dual respiration: they have two or four book lungs in the ventral abdomen, but also many of them have a trachaeal system, an array of small tubes that penetrate the cuticle and permit atmospheric gases to flow in to the tissues. This one might not be a problem, though: trachaeal systems are less and less effective the larger the animal, and they rely more on discrete respiratory organs, like lungs, as they grow to a larger size. At 1.2 meters long, these alien “spiders” are almost certainly going to have “lungs” and “nostrils” — it’s just that if they’re like our spiders, they won’t be on the face, but low on the abdomen.
• Respiratory pigments. This is where it becomes obvious that you can’t simply extrapolate from Earth spiders to space “spiders”. Our spiders use hemocyanin to carry oxygen, which has a significantly lower O₂ carrying capacity than our hemoglobin. You’d think along the way to evolving larger size and metabolically demanding intelligence they’d have to be using a more efficient respiratory pigment…although blue-green blood is kind of cool.
• Sensory apparatus. Look closely at a spider sometime: they are covered with hairs. These aren’t dead insulators like our hairs, either, but are all invested with nerves for mechanical and chemical sensory functions. Putting them in a suit that limits them to only visual input is going to be like dumping them into a sensory deprivation tank.
• Motility. There is a suggestion that they could use “an enclosed pod that the spider can sit inside with legs folded and have mechanical arms/legs that support the pod and allow it to walk around.” Maybe. But that throws away many of the virtues of a space-walking “spider”. They are incredibly agile, are accustomed to maneuvering in 3 dimensions, and are beautifully adept at manipulating objects in their environment. Watch one wrap a prey item in a web — they are weaving with all 8 legs simultaneously, flipping it around and dancing about delicately. It would be a shame to limit that by stuffing the “spider” astronaut in a barrel and making it manipulate its environment with a few waldos.

It’s a fun exercise, but if we ever find such a creature I suspect it will have less similarity to our spiders than we humans do to an acorn worm, so much of the speculation is moot.

Real spiders are more interesting. As I said, I’ve been reading up on spider physiology, so here’s a diagram of the main elements of the spider circulatory system (“h” marks the heart, on the dorsal side of the abdomen.)

That’s the cartoon version — here’s a resin cast of the full circulatory network of the opisthosoma of Cupiennius. It blows me away that they were able to do this — Cupiennius is fairly large as spiders go, but still pretty tiny.

Even more impressively, people have measured the blood pressure in the spider circulatory system — no, not with an itty-bitty sphygmomanometer. I suspect they used optical methods to visualize pressure changes, but that’s a paper I haven’t tracked down yet.

Systole and diastole are still valid concepts in a spider. In case you were interested, their hearts beat at a rate of 9 to 125 beats per minute. That’s quite a range, but as I mentioned above, one of the challenges is a highly variable metabolic rate.

Ultimately, though, if you’re going to design an SF “spider”-like alien, you shouldn’t be constrained by the form and physiology of terrestrial spiders. A homeothermic creature with an endoskeleton, but with a bunch of limbs and eyes and a sclerotized cuticle, and maybe some funky spiky complex mouthparts, is going to look enough like a spider to Zapp Brannigan that that’s what he’s going to call it anyway.

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Schmitz A (2016) Respiration in spiders (Araneae). Journal of Comparative Physiology B 186(4):403–415.

Wirkner CS, Huckstorf K (2013) The Circulatory System of Spiders. In: Nentwig W. (eds) Spider Ecophysiology. Springer, Berlin, Heidelberg.

There are these weird salticid spiders that have evolved a radically different morphology — they live in ant nests, and physically mimic the ants. Look at this ant-spider. Isn’t this amazing enough?

That’s a spider? Yeah, count the legs. It’s trying so hard to fit in with tunnel-dwelling insects with three body segments, you just have to applaud the effort.

What’s more, they’ve acquired another evolutionary novelty: they secrete ‘milk’ to feed their young, and have extended parental care. The necessity of milk production was tested with the cruel experiment of painting over the epigastric furrow (the site of secretion) with White-Out, and what happened? All the spiderlings starved to death. The utter bastards. There are things you can get away with when working with invertebrates that you couldn’t do with cute fuzzies with bones. Try doing that experiment with bunnies, just be prepared for torches and pitchforks.

There’s another revelation in this figure caption.

Did you get that? They are milking spiders. I come from a long line of Norwegian dairy farmers in Minnesota, so you can guess where my mind went from here. Can I get state and federal subsidies for my spider farm? I’ll have to look into it.

The study is primarily about the life history of this spider species, with some experimental manipulation, and it does a thorough job of that.

It’s missing one thing, though: any analysis of the chemical make-up of spider milk. I’m going to take a wild guess that unlike mammal milk, which is rich in fats and carbohydrates, spider milk is going to be more like a protein shake — that it’s going to be in many ways similar in composition to the dissolved bug guts that spider adults live on, to simplify the transition from an independent hunting spiderling to a spiderling with an obligate dependency on parental care. Which means a) humans can probably synthesize it by homogenizing masses of fruit flies in a blender with some digestive enzymes, and filtering out the chitin, and b) there’s not going to be much of a human market for it. Alternatively, they suggest that spider milk may have evolved from the breakdown of trophic eggs — that is, eggs produced that do not develop, but provide a food source for other members of the brood. In that case, it may be a soup of phospholipoglycoproteins, similar to the vitellogenins of other arthropods, and its closest vertebrate analog would be egg yolks.

Inquiring minds want to know. They’re going to have to milk a lot of spiders to get enough to analyze, though!

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Chen Z, Corlett RT, et al. (2018) Prolonged milk provisioning in a jumping spider. Science 362(6418):1052-1055. DOI: 10.1126/science.aat3692

P.S. There is a Minnesota milk song. They might have to change some of the hand gestures.