An insect flies into a spider web and that is it. There are no chances left for him except sitting duck until the return of web’s owner and becoming another delicious nutrition serving for him. These eight legged creepy creatures are there for more than 420 million years and found almost in every continent except the Antarctica (Yes they do not like cold.). These eight legged air breathing arthropods belong to the order Araneae which is further divided into 113 families and consists of more than 45700 species1. Almost every single species of spiders except Bagheera kiplingi discovered in year 1998 are carnivorous2 and that is where spider web comes handy (catching the prey). Although not all spiders build web for catching the prey, even some species do not build web at all, but those build make it so sticky for an insect to fall and get stuck. It is painful even for us, falling in a spider web. This will certainly not enough to make us the dinner of a spider but irritating enough to pick it off from clothes and skin.
If a spider web is so sticky for their prey and even for us to create an issue even for a small time duration, why do these spiders not get stuck in itself? What makes them so special? Is it some design feature of spider web or intelligent moves of spiders making it possible for them to move freely on which is actually a death trap for an insect?
How do they build the one?
Spider webs are existing for at least 100 million years3 and the term is used to define a web which is apparently still in use; i.e. the clean one. The abandoned one are known as cobweb and refer to the dusty one. The cobweb term is also used to describe tangled three dimensional webs formed by some spiders of the theridiidae family. Spiders when came on earth surface from water in early Devonian period they started making silk from specialized organs to protect their bodies and eggs4 but they gradually shifted their silk production ability towards using it a signal lines, guide lines and then as ground and bush webs. This silk web formation eventually shifted in direction of making aerial web; one which we all are familiar, to catch their prey5.
To build a web spiders produce silk from a specific gland found at the tip of abdomen known as spinneret glands. Most of the spiders have three pairs of these glands, each one having its own functions. There are also spider species who have just one pair and others having as many as four pairs. Each gland secretes a special type of silk thread having its specific purpose. For example- a trailed safety line, sticky silk thread for catching the prey and fine silk thread for wrapping the prey. Some spiders are even capable of producing 8 different type of silk threads during their lifetime6.
Spider web construction starts with the forming a bridge thread between two objects using sticky silk thread and their after tightening and strengthening it by repeat threads. When the bridge thread is done spider goes for creating a loosing thread and hanging in center of it. Spiders create another thread from center of loosing thread and attach it to another surface to tighten and create a Y shaped structure which works as the initial base for whole spider web construction. Further other necessary threads in radii were made and finally non sticky and sticky axillary threads are made in spiral to complete the construction and giving birth to a spider web. After the completion of web spider chews off the initial three center spiral threads then sits and waits. It takes about one day to a spider for building a web.
How only insects get stuck not the Spider?7
Spider webs are interesting devices made up of spider silk. They do act like a sticky pad causing insects to stick in that but spiders itself moves on it very easily without getting stucked. What makes these spider webs so selective in gluing ability that only works on insects not on the spider who constructed it? According to old proposed idea the spider do not get stuck in his own web because they coat their pods/ legs in a special sweat which makes it non-sticky on web surface. Some other naturalists believed that spiders only touches the non-sticky thread in web while moving. The idea was good enough to cure the curiosity of human mind until someone not bothered to test the idea. In year 2012 two biologists from Costa Rica decided to test this hypothesis and came out with real mechanism how this happens actually.
Results of their study suggest this not as simple as it looks but a result of a combination of various procedures a spider does while walking over the web-
Not every part is sticky:
The first thing to take into consideration is not every of every spider web is sticky. A spider web contains non-sticky dry silk which is used to make the spokes of web as well as the central part where the spider rests before a prey get caught in web. Spirals are the part of web which are made up of sticky silk threads. Using the central part and spokes of web a spider can move all around the web even get out of it without the fear of getting stuck.
However this behavioral avoidance explanation is not sufficient to explain it because orb weaving spiders are found on video tapes to push the web their hind legs against sticky lines hundreds or thousands of times during construction of each orb and are not trapped
Spiders prefer walking on tiptoes:
It may be possible for spiders to move on dry silk threads to avoid sticky threads but when a prey lands on web and spider goes for taking it as food, it is near to impossible for him to avoid sticky threads. Spider legs have a dense arrays of branched hair like structures called setae. While walking over a web these tiny bristles reduces the area of contact with adhesive material present on sticky silk threads. Also these small bristles are arranged in irregular rows and breaks contact with adhesive on sticky threads one by one, not all at once. It minimizes the adhesive force of multiple adhesive droplets from combining and showing the sticky behavior or gluing the spider on web.
Grooming their legs with non-sticky material:
Carefully walking over web and minimizing the contact area to sticky threads by using setae can do the task but why the setae let a spider moving on a web full of adhesive droplets on thread? Is it only the less surface area or something else? It is found in same study that when researchers washed a detached spider leg and applied it to a sticky thread, the leg got stuck and could not be easily removed. Which means there is something anti-adhesive either a chemical substance or a structural surface layer was there which permitted to not get stuck on the spider web.
Several compounds washed from the legs on analysis identified as oily substances like n-dodecane, n-tridecane, and n-tetradecane. These compounds could act as a non-stick coating. Which means spiders not only do carefully behavioral walking minimizing contact to adhesive but also coat their leg by non-sticky oil like substances making tiny bristles (setae) to easily come out from the adhesive force after getting stucked.
Spiders have their own ingenious way to tackle with sticky nature of spider web so they can easily catch the prey and survive. Combining behavioral walking, minimizing contact with adhesives using setae and coating of oily substances, spiders do avoid getting stucked in his own spider web.
It’s interesting to see how spiders evolved an ingenious way of catching their prey by forming a silk web but more surprising to know they never get caught in it themselves. Flies and insects get stuck so fast in the meticulously designed and engineered structure of a spider but not the spiders itself. There is not a single one step mechanism behind this but a combination of various behavioral as well as evolutionary advantages are there. Most of the time spiders avoid to move on sticky silk threads present in their web however when a prey falls in their web it is almost next to impossible avoid the contact with adhesives present on sticky threads. In that case spider’s carefully walking on web through minimal surface contact using tiny hair like structures present on leg known as setae comes handy. Additionally these setae (bristle like structures) are coated in oily chemical substances which minimizes the adhesive force if any works during moving over spider web.
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- Currently valid spider genera and species”.World Spider Catalog. Natural History Museum Bern. Retrieved 2016-12-19.
- Meehan Christopher J., Olson Eric J., Reudink Matthew W., Kyser T. Kurt, Curry Robert L. (2009). Herbivory in a spider through exploitation of an ant–plant mutualism.Current Biology. 19 (19): R892–3.
- Brasier M., Cotton L., and Yenney I. (2009). First report of amber with spider webs and microbial inclusions from the earliest Cretaceous (c. 140 Ma) of Hastings, Sussex. JOURNAL OF THE GEOLOGICAL SOCIETY, 166 (6): 989-997.
- Kaston B.J. (May 1964). The evolution of spider webs. American Zoologist. 4 (2): 191- 207.
- Blackedge T. A., Scharff N., Coddington J. A., Szuts T., Wenzel W., Hayashi C. Y., Agnarsson I. (2009). Reconstructing web evolution and spider diversification in the molecular era. PNAS 106 (13):5229–5234.
- Craig C. L. (1997). Evolution of arthropod silks. Annual Review of Entomology. 42: 231–267.
- Briceno R., Eberhard W. (2012). T Spiders avoid sticking to their webs: clever leg movements, branched drip-tip setae, and anti-adhesive surfaces. Naturwissenschaften. 99: 337–341.