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Roughly speaking, a small, complex electronic circuit or IC might sit in "sleep mode" using a current of roughly 1 µA (e.g. 1, 2), thereby using roughly $3\times 10^{-6}$ Watts, and that converts to roundly $2.5\times 10^{-6}$ kcal/hour or $62\times 10^{-6}$ kcal/day.

I suppose you could call that 62 micro-kcal/day or 62 milliCalories/day.

One day I noticed a small spider in my home, sitting in its small web, and kept an eye on it. After several weeks of watching I hadn't seen it catch anything. I slightly perturbed the web and it reacted. It was still alive.

Question: I'm curious to know roughly how much energy a small spider needs to sit and wait. Might this be termed roughly the spiders Basal metabolic rate? It could be for any small spider that spends much of its time waiting for prey. As long as an approximate size or mass is available then it could be expressed as kcal/day/kg or some similar unit.

I have read about Kleiber's law in this answer, but I don't think it is meant to extend down to small spiders.

note: I'm just asking for the energy expended during resting periods. Of course during a day the spider may do web maintenance, catch or eat prey, but it's the resting rate of energy consumption, on a daily basis, that I'm asking about.

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Short answer
Approximately 240 J on a daily basis.

Background
Ballesteros et al. (2018) modeled basal metabolic rates of insects. They reckoned that endotherms, like insects, basically use energy directly correlated to the number of cells, which is linearly correlated to their body mass. They checked their model with experimental data from Chown et al. (2007) (Fig. 1).

Acknowledging that arachnids aren't insects, but that both are arthropods, let's take an average-sized web making spider, like the black widow, weighing in at 1 g. This yields a basal metabolic rate of 10 J/h, or 240 J/day. However, and given the linear correlation between energy expenditure and mass, a dwarf spider weighing in at 1 mg expends 1000 times less, and a Goliath bird eater approaching 200 g will expend 200 times more.

To convert the unit for the Black widow to a Watt scale, we get 1 J/s that corresponds to 1 W, so 1 J/h corresponds to about 0.3 mW.

enter image description here
Fig. 1. Base metabolic rate for insects. Data from more than 300 species were extracted from Chown et al. (2007). source: Ballesteros et al (2007)

References
- Ballesteros et al., Sci Rep (2018)
- Chown et al., BES (2007); 21(2): 282-90

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    $\begingroup$ Wow, all kinds of goodies here; thank you for the well-sourced answer! $\endgroup$ – uhoh Mar 31 at 11:12
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    $\begingroup$ @uhoh - thanks! You may wish to check the math, however, where needed - I'm just a Biologist and your post seems to radiate quite some proficiency in math :) I added my sources, so that should work out. Cheers. $\endgroup$ – AliceD Mar 31 at 11:32
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    $\begingroup$ @com.prehensible - the answer pertains to basal metabolic rates only, per the question, and as mentioned in the answer's body text. $\endgroup$ – AliceD Mar 31 at 14:32
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    $\begingroup$ For the black widow, you said it would be 10 J/h, and that 1 J/h = 0.3 mW, so black widow would be 3 mW, which is 1000 times larger than the microprocessor from uhoh's calculation. you also said that the dwarf spider would be about 1000 times less based on mass, which brings it to 3 micro watts, or 3*10^-6 W, which was the measurement that uhoh got for his IC. Is my math right? $\endgroup$ – mpprogram6771 Mar 31 at 21:17
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    $\begingroup$ This all works out very nicely and indeed at 1 mg spider would seem to be of the order of 3 uW just like a low power IC in idle mode. Cool result, thanks! $\endgroup$ – uhoh Apr 6 at 11:31
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A 400 pound human can survive a year without food, and an arachnid can manage the same, 3000-7000 hours if it consumes it's body mass equivalent.

A spider consumes about 110 to 1933 μW per gram of body mass, standard metabolic rate at 20'C.

By measuring spider oxygen consumption, scientists have found a range of 21–356 μl O2/g per hr. Its good efficiency compared to other poikilotherms of same size.

A human an adult at rest consumes about 16 liters of oxygen per hour. This gives a nominal basal metabolic rate of 75 kcal/hr which translates to 87 watts(=1.2 milliwats per gram).

1 liter of oxygen gives 5.43 watts per liter... 1 microliter = 5.43 microwatts. 21-456 *5.43 = 110 to 1933 microwatts per gram.

Big garden spiders from temperate regions are about 1g, so a big spider from the garden can use 0.1 - 2 milliwatts at 20'C.

1 gram of fat is about 7kcal, that's 8 Watts. So, if a 1gram spider has a 0.5 gram meal, they can keep going for about 10,000 hours, perhaps 3000-7000 hours at 30/70% efficiency.

https://jeb.biologists.org/content/214/13/2175

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  • $\begingroup$ Interesting answer. But... How can a 1-gram spider eat half of its body weight? And, probably related, if it can survive 10,000 hours, i.e., more than a year, with a single meal, why would it even bother catching pray at all? And, related, a 'meal' is hardly pure fat, instead, it will mostly be chitin and other proteins. Importantly, why is exotherm data included in this answer? If that human hourly O2 data is used for the later calculus, it is likely inaccurate, as exotherms use a lot more base-energy than endotherms. $\endgroup$ – AliceD Apr 1 at 18:47
  • $\begingroup$ Lastly, it's best practice to write the references out in full (J. Doe, Journal of something (2020) etc), as web links may grow cold over time. In fact, they quite frequently do. $\endgroup$ – AliceD Apr 1 at 18:56
  • $\begingroup$ Are you saying that respiration and the use of ATP in arthropods and humans generates a different amount of energy? It's the same respiration conversion of oxygen to carbon dioxide, at 20 vs 37 degrees, correct me if I am wrong, it's basic biochemistry with 02 in and C02 out, so the energy maths are the same. Also correct the maths if you see a specific error. If you scale up the black widow to 65 kg and 37 degrees, it consumes almost exactly 1W/gram. Silicon wafers are 50 micrometers, though, so the IC silicon using a 1x1x0.01mm wafer weighs about 0.00023 grams. $\endgroup$ – com.prehensible Apr 1 at 21:39
  • $\begingroup$ It wasn't clear to me why the human data was in there. Your train of thought slipped me, as it's just respiration Biochemistry you were needing the human data for, it was quite confusing. Thanks for clearing that issue up. $\endgroup$ – AliceD Apr 1 at 22:18

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