Spiders control their leg movement with the help of flexor muscle (which contracts the legs inwards) and hydraulic pressure of hemolymph fluid (that helps in extending the legs). In the default position, the spider’s legs are flexed inwards due to flexor muscles. During locomotion, spiders increase their hemolymph pressure to extend their leg. When they die, this hemolymph pressure decreases, and the spider’s legs are forced to their default position. Therefore, the spider’s legs curl up in a ball when they die.
Why do spider’s legs curl up when they die?
Have you ever noticed the spider’s leg getting curl up when they die?
Well, it is a great question that has an interesting answer that might surprise you. To understand why spiders curl up in a ball, we need to learn how spider’s legs work.
Hydraulic legs of Spiders
You might have noticed that spiders usually move in an unnatural way. The reason behind their curl up and unusual locomotion lies in their hydraulic legs. Yes, you heard that right!
Spiders have 8 walking legs and each leg has 7 segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus.
Spiders generally have muscles to flex their spindly legs inwards but do not have muscles to extend them outwards. Almost all animals including humans, have both extensor and flexor muscles to increase and decrease the angle between bones on two sides of a joint respectively. Spiders lack these extensor muscles which can help them in straightening the knee. Therefore, in order to compensate for these muscles, the femur-patella joint and the tibia-metatarsus joint in spiders are extended by hydraulic pressure.
Spiders use their blood pressure to control their legs. They increase their blood pressure to extend their legs. Naturally, the legs of spiders have a tendency to contract due to flexor muscles. To resist this contraction and extend legs, spiders increase their hemolymph fluid (similar to blood in vertebrates) pressure. Due to an increase in hemolymph pressure, the lower side of the femur-patella and tibia-metatarsus joints get filled with hemolymph fluid which forces them to extend the leg.
For locomotion, Spiders use their flexor muscles as well as hydraulic pressure of hemolymph fluid. Spiders such as jumping spiders can regulate their hemolymph pressure so fast that they can jump up to 30-40 times their own length.
Why do spiders curl up in a ball?
We know that the spider’s leg uses hydraulic pressure. Spiders continuously regulate their hemolymph pressure for the movement of their legs. When spiders die, this hemolymph fluid pressure decreases and the extended legs cannot resist the contraction pressure by the flexor muscles. This forces spider to curl up in a ball which is their default position if no hydraulic pressure is applied. Therefore, whenever you see a dead spider, it is curl up in a ball.
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