Why can insects/spiders crawl on walls and ceilings but not larger species?

Question

What role does gravity play into these circumstances? Does mass? (if so, how much would something have to weigh in order to achieve this). Any other forces? Do all these animals have specially designed feet? Please answer seriously if you know the answer.

when I say designed, I mean by nature not a deity.

Answer 1

The ratio surface/mass of an organism decreases as we move toward larger size. This means that the force due to gravity increases more than the surface of contact does when we move toward larger size. Also, there are small irregularities in the wall. It is not really flat. The smaller organisms can take advantage of these two facts.

Edit: Let us consider what would happen with an animal like a spider, but 1 000 times bigger. Of course, no such animal exists. This is just to explain the principle. Such an animal would probably fall under its own weight. Its legs would be 1 000 000 bigger (in terms of their cross section, which is what matters), but the weight would be 1 000 000 000 bigger. So, the natural constraints imposed by physics aren't the same at different scales. In fact, because of this, researchers have proposed theoretical limit on the size of animals on earth. Beyond that limit, they cannot even walk and certainly cannot climb anything.

The fact that the wall has irregularities which small insects can use is an important part of the explanation, but the surface/mass ratio is more important. If the surface/mass ratio was the same at different scales, the same technology (little tiny hairs that adhere to the wall) could be used by a animal as big as a bear to climb the wall in your house. The force per unit of surface on the wall would be the same, the paint on the wall would resist, etc. It would work. Of course, in reality, the same technology cannot work because the surface/mass ratio is too small for a bear.

Edit 2: It is interesting that a gecko can use some special technology to adhere to a wall, but a gecko is still a relatively small animal. Moreover, it must use that special technology on relatively large toes, whereas an insect only needs to use a less sophisticated technology in the lower part of its tiny legs, and this can only be explained by the smaller surface/mass ratio at larger scale.

Edit 3: Since you asked about force, it should be pointed out that researchers have studied how a gecko can adhere to a wall. It has been proposed that the chemistry at the surface of the toes is such the van der Waals force, the same force that hold the molecules together in ice or other substances, is put in contribution.

Answer 2

Gravity plays a major role in trying to pull whatever it is off the wall or ceiling, the heavier a thing is the better grip it needs on the little rough bits that stick out of the walls and ceilings and that you need a good magnifying glass to see. If you are a big heavy thing then you need really good specialist feet to still crawl up walls and even then there is a risk that you will pull bits of flaky paint and plaster off and fall down - just because you need to exert so much force on the surface to keep your big heavy body up.

Bigger specied than insects/spiders do manage it though - geckos are famous for it.

Leaving aside whether any animals feet have been 'designed'
it's certainly true that gecko's have some very neat feet!

and so have some insects and spiders of course

Answer 3

spiders and other insects do have specially "designed" feet suited for wall climbing and the same..

the tip of their feet have claw like "fingers" with which the insect gets a grip on the surface

it is all a thing of scale and perception. same thing is done by cats or even bears while they climb trees. from a spiders point of view, there is no wall that flat so it can't climb it..

Answer 4

They have microscopic 'hairs' all over their feet. These hairs will 'catch' on even the most invisible of cracks and surface irregularities to provide a 'grip'. But the available 'holding force' is very small so larger species have never evolved such capabilities.

Doug