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Many fantasy stories involve large spiders like in Harry Potter. These creatures behave almost exactly like normal spiders despite their size. My question is: can large spiders still walk upside down on flat surfaces? And if so, how massive could they get while still keeping this ability?

The giant spider would be about the size of a large dog breed like Newfoundland or Saint-Bernard : between 60-70 kg

I'm using the Hard-science tag (it's my first try). I'm looking for answers citing scientific papers or equations.

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    $\begingroup$ An answer would require an extrapolation on existing science, which I believe falls into science-based not hard-science. $\endgroup$ – Samuel Aug 18 '15 at 1:38
  • $\begingroup$ you ask, I googled $\endgroup$ – user6760 Aug 18 '15 at 1:54
  • $\begingroup$ Geko Glove can hold a 68Kg human, but this is a different system than what spiders have evolved: smh.com.au/technology/sci-tech/… $\endgroup$ – Thucydides Aug 18 '15 at 5:14
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In the real world, the adaptations that allow spiders to walk upside down would be insufficient to support the weight of a giant spider.

Spiders walk upside down by the use of tiny, sticky hairs on their legs.

enter image description here

In case you’re dreaming of someday climbing walls, Wolff added that it’s unlikely we’ll have any real-life Spider-Mans anytime soon: Even if we donned a suit of sticky hairs, people are simply too heavy for it to work.
- National Geographic

Since a normal size adult human weighs about as much (ideally and very roughly between 100-200 lbs) as your giant spiders (132-154 lbs), it seems that the giant spiders couldn't walk upside down either.

Someone who is good with mathematics might be able to interpret the following paragraph from the study cited above and determine just how much weight can be born by spider leg adhesive:

For all eight legs in contact, an average force of 97 mN was measured, which is three times higher than the average spider body weight. With the decreased number of intact legs, attachment force decreased more rapidly than would be predicted due only to the loss of available adhesive pad area (Fig. 1). If the adhesive surface of the first pair of legs was disabled, the mean force was reduced to 74% of its original value (77% predicted). Interestingly, when the fourth pair of legs did not attach to the substrate, the mean force was reduced to 27% (71% predicted). For two pairs of legs with disabled adhesive surfaces, the attachment forces were reduced to 27% of their original value for disabled front legs (53% predicted) and 9% for disabled hindlegs (47% predicted). With only the first leg pair remaining intact, initial forces dropped to 2% (23% predicted) and for the last pair of legs remaining intact they dropped to 6% (28% predicted) of the attachment force obtained with untreated animals.

The good news is that we're having more luck with studying the adhesive properties of gecko feet.

Further reading:

http://jeb.biologists.org/content/217/2/158

http://www.researchgate.net/publication/264459486_Adhesive_foot_padsan_adaptation_to_climbing_An_ecological_survey_in_hunting_spiders

http://www.researchgate.net/publication/235368901_Radial_arrangement_of_Janus-like_setae_permits_friction_control_in_spiders

http://www.researchgate.net/publication/51872362_Surface_roughness_effects_on_attachment_ability_of_the_spider_Philodromus_dispar_%28Araneae_Philodromidae%29

http://www.researchgate.net/publication/51147062_The_influence_of_humidity_on_the_attachment_ability_of_the_spider_Philodromus_dispar_%28Araneae_Philodromidae%29

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Have you ever tried to pull a creeper (i.e. plant) off a wall and noticed that it sometimes pulls away a lot of the paint and plaster? Or even posters stuck up with blu-tack and you peal off paint at the same time? I suspect that even if you managed to get something which can stick to surfaces and hold your desired weight, a lot of surfaces themselves cannot support the weight due to the way they are composed/constructed.

According to this blog article from Cornell, spiders walk by lifting two alternating pairs of legs (i.e. 4 legs) and leaving the other 2 pairs down. So a walking giant spider is supporting its 60-70kg mass via 4 surface points at a time. You'll need to work out how big each "foot" is, but in order for the material in question to support your giant spider, that surface area needs to support at least 15-17.5kg without de-laminating. Quite a few surface materials would support that happily, but quite a few would not.

Your spider would have to be very picky about where it walks and very careful with its gait to make sure it doesn't lift any of its anchor legs early.

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There are good reasons we don't see land dwelling arthropods any larger than 15-20 centimeter leg span (for insects and spiders) or up to about 50% above that (for exceptional crabs, like tree-climbing coconut crabs).

That reason is the square-cube law.

If you double the size (linear dimension -- height, leg span, etc.) of an arthropod, you quadruple its strength, but you octuple its weight -- and with the muscles trapped inside the exoskeleton, they can only get so strong. Worse, the exoskeleton is an inefficient way to get bone strength; you gain more weight for a given amount of added cross section area (=> strength) than you would for an internal skeleton like those of vertebrates.

Still worse, breathing apparatus gains effect on the square (the area exposed to air), while oxygen requirement goes on the cube (volume/mass of flesh to supply).

By the physics, it's my understanding that an arthropod dwelling largely on land simply can't get any bigger than about the size of a coconut crab, blue crab, or at most a dungeness crab (which, however, live in deep ocean). Horseshoe crabs get somewhat larger, but they don't leave the water often and have many more legs (and aren't really crabs at all).

So, far from being able to walk upside down on a suitably strong ceiling (cave roof?), your 60+ kg spider wouldn't even be able to walk upright on the ground -- it might not even be able to breathe in order to stay alive.

Now, move everything underwater, where the displaced water supports most of the animal's weight (and, surprisingly, breathing may actually be easier relative to size -- cold water can carry a lot of dissolved gases), and things get a lot more likely...

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  • $\begingroup$ When talking about square-cube law, "doubling the size" is always taken as doubling the linear dimensions. Also, talking about 4x area and 8x volume makes this obvious. $\endgroup$ – Zeiss Ikon Sep 16 at 11:07
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    $\begingroup$ @DKNguyen Edited. $\endgroup$ – Zeiss Ikon Sep 16 at 13:36
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A 60-70 kg spider is probably impossible. The spider Beregama aurea is a large spider with a legspan of 16.5 cm, a body length of 4.8 cm and a weight of 5.5 grams. At 60-70 kg, the same spider would be 3.66 meters in legspan and 1.06 meters in length. A weight of 0.175 to 4.1 kg is much more plausible. A spider that weight would be 52-150 cm in legspan and 15-44 cm in length. At this weight it shouldn't have much difficulty traversing vertical surfaces, although probably couldn't climb glass or plastic or other smooth materials as easily.

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