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I think I understand how legs of humans work. We have bones to which muscles are attached. Muscles can only contract / relax. By the combination of many different muscles we can make complex movements:

enter image description here

Image source: https://commons.wikimedia.org/wiki/File:1015_Types_of_Contraction_new.jpg

However, I have no idea how this works for insects / spiders. On the one hand, I'm not sure where the muscles should be attached to due to the exoskeleton. On the other hand, I have difficulties imagining that the same system works for legs which are this tiny:

enter image description here

Image source: https://commons.wikimedia.org/wiki/File:Spider-IMG_1630.jpg

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  • $\begingroup$ insects too have muscles $\endgroup$ – Always Confused Sep 11 '16 at 8:55
  • $\begingroup$ Probably the OP is asking is there any muscles at terminal, stick-like fragments and their joints. $\endgroup$ – Always Confused Sep 11 '16 at 12:30
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    $\begingroup$ Moskito... did you mean 'mosQUito'? $\endgroup$ – Always Confused Sep 11 '16 at 13:20
  • $\begingroup$ @AlwaysConfused Yes, thank you. I fixed the spelling. $\endgroup$ – Martin Thoma Sep 11 '16 at 19:22
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Arthropods use basically the same system to operate their legs. Yes their legs are tiny, but so are their muscles.(Is it so hard to imagine everything is downscaled? Their digestive system is also much smaller, but they don't need to eat the same amount as we do either...) In general it's just a kind of smaller-sized mechanism that is based on the same principles: use a muscle to create tension over a joint, and so move the appendages. The big difference is off course that in mammals the muscles work against an internal skeleton, and in arthropods the muscles attach to the inner surface of an external skeleton.

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  • $\begingroup$ I try to save daddy-long-legs spiders that can't climb out of my bathtub. I do so by grabbing one of their legs. Sometimes the legs detaches. If I squeeze the end of the leg that was attached to the body of the spider, the leg extends. How does this work? $\endgroup$ – CJ Dennis Sep 11 '16 at 21:37
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Well the subject of how insects move their limbs is a bit more complicated than suggested by Kasper.

Insects do have flexors and extensors muscles to move their legs. But there are other options too.

Spider legs work by hydraulic fluid. The spider has flexor.. but extension is done by pumping hemolymph into the leg.

Also it has been discovered that certain movements of insect legs is due to the chitin skeleton flexing and join structure. ie energy is stored in the chitin, and the structure of the joint cause the legs to return to the rest position.

https://www.sciencedaily.com/releases/2013/07/130718130524.htm

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