Biomechanics of Spider Silk:
Aggregate Glue
All spiders have spinnerettes they use to dispense silk created in special glands. They use this silk to terraform their world, creating silken retreats and capture webs. All spiders can produce multiple types of silk, each with varying material properties. Orb-weaving spiders can produce upwards of seven different types of silk. The classic pinwheel shape is actually the result of at least 5 different types. My research is interested in the microscopic glue droplets with which Ecribellate Orb-weavers line their capture threads. This silk, known as aggregate glue, is the adhesive agent, sticking to prey and your hair alike.
Hygroscopic Nature of Spider Aggregate Glue
Aggregate glue droplets absorb water from the air around them, getting larger in higher humidity. For most orb-weavers this process is reversible! For our study species Cyrtarachne, it isn't! The glue shrinks irreversibly and dries in low humidity.
Aggregate glue droplets have two distinct regions. The first is a glycoprotein core, the sticky part, which acts as an adhesive. The other is the outside aqueous solution. This connection of Low Molecular Mass Components great a dry environment, pulling in water.
As they draw in water the viscosity of these droplets decreases, increasing spreading but decreasing cohesive forces.
Cyrtarachne (a,c) Larinioides (b,d)
The glue droplets of Cyrtarachne are significantly larger than classic orb-weavers (a vs b). These glue droplets are super-saturated with water. They dry with a core of tendrils, different from that of other orb-weavers, we believe this is the drying of proteins, separating from surrounding salts (c vs d). There appears to be much more water than glue, with glycoproteins only within the center instead of all of it (g vs h)
Cyrtarachne (e,g) Larinioides (f,h)
Our study species, Cyrtarachne akirai, is a moth specialist spider. They are the crab-like creature to the right. They take the classic orb-web design and transfer it to a horizontal orientation. In addition, their capture threads are long and dangling. The white color you see in their webs is the aggregate glue, they make large enough droplets to see with the naked eye! Cyrtarachne only creates webs in above 80% RH. Otherwise, their glue dries irreversibly and becomes useless.
One side of the dangling capture thread is a false joint. It breaks upon contact with prey, the moth, and allows the prey to fall. The creature then bounces on a bungee cord until the spider is able to pull it up and envenomate it. These spiders capture their prey on a single thread! Instead of in a 'net' like other spiders.
In order, to test the strength of silk, the capture threads are collected on cards. We then bring the capture thread into a surface, usually glass or moth wing. We pull the thread from he surface and measure the amount of force necessary to remove it. We can tell how strong it is. The image to the left shows a capture thread being pulled from a surface. The aggregate glue droplets stretch before detatching.
Moths are plentiful in many ecosystems but are very difficult for most spiders to catch. Moths are covered in a sacrificial layer of scales that coats their wings and bodies. These provide the moth many different evolutionary advantages but in this case, their scale flake off when in contact with a spider's aggregate glue. The moth then thrashes which allows it to escape to freedom. We are interested in the material properties necessary to overcome this dirty surface.
To delve further into this system, we have brought glue droplets into contact with various surfaces. By bringing them into contact with glass, we can get an estimate of the glue droplet viscosity. By bringing into contact with moth scales we can quantify the behavior which leads to the increased adhesion strength. We run these experiments by measuring the radius of the glue droplets over time. We can compare these spreading behaviors between species and surfaces. These experiments are done under a microscope at 2000 fps.
When tested on glass the strength of the glue droplets was high but proportionally to its size, the glue droplets were weak! It was only when we tested the glue droplets on Moths did we see the trend. Ordinary orb-weavers lose adhesion strength on moths due to their loss of scales, the adhesion strength is equal to the connect of the scale, not the adhesive strength. When on moth scales Cyrarachne glue is even stronger than on glass. The glue is using the scales to increase strength, using the moth's defense against them. Common orb-weaver's glue only sticks to individual scales. Cyrtarachne glues the scales together into bricks that must be removed together. Increasing strength
Surface tension is what causes water droplets to form. The internal forces of the liquid want to pull themselves together and this creates a droplet.
Viscosity is the resistance to flow. Think of it as liquid inertia. By tracking the glue droplets and their collision with glass we can estimate the viscosity of them. These values can and will be used in our computational modeling of the glues.
Tracking the flow of glues and comparing them to a known standard allows us to determine the important material properties of the glue. Often spreading interactions are dependent on the two factors below:
Our research into the moth-catching ability of these spiders is not over. We have many more questions left to answer. We hope to expand our understanding of these glues by including more species of moth species, from the sub-family Cyrtarachninae. This includes spiders like the bolas spider who only uses a single glue droplet. This minimized web is possible because of the pheromones emitted by the spider. These chemicals attack male moths looking for a mate. Once they come near the spider hits them with the glue droplet. Fishing them out of the air.
A closer look under the microscope shows that the spreadinb behavior of Cyrtarachne glue on moths is very unique! Instead of spreading only radially outward, it curves and turns as it spreads underneath the scales and is pulled along the length of the scales. The most amazing part is this is happening even faster! The glue of Cyrtarachne spreds 3 times a far in the same one second. We also removed the scales to see if it was an interaction with the moths body. Nope! No hyper spreading. The glue is interacting with the topology of the scales specifically.
Cyrtarachne glue droplets have interesting material properties. The viscosity of the glue is extremely low. It functions best at this viscosity where most other spiders fail. This is leading us to question how the various chemicals in the glue are spreading. We find that it spreads 8x its initial diameter vs 2.5 times on glass. This helps explain why its so strong! It spreads further and more optimumly, glue scales together into large groups. The spreading is not as good on moth without the scales. This means the scale themselves are the driving force. We are working to further understand this system through modeling of the glue surface interactions.
