WRIT 340 Illumin Article
Prof. Harlynn Ramsey
Name: Shakya Sur
Bio: A junior majoring in mechanical engineering with a passion for everything wild and natural,
particularly creepy-crawlies.
Keywords: nature, structure, construction, materials science, mechanics, spiders, spider web, silk,
biomimetics, civil engineering
Multimedia Suggestions:

‘A Typical Spider Web’ (Flash animation) http://science.howstuffworks.com/zoology/insectsarachnids/spider5.htm

‘BBC Invisible World – Spider, The Genius Architect’
http://www.youtube.com/watch?v=1JlLLpaCkI4
Abstract
Although spider silk is widely celebrated amongst material scientists for its exceptional tensile strength
and diverse engineering applications, only recently has significant research been done on mechanical
characteristics of the spider’s web. After examining the architecture of a common orb-weaver’s web, this
article looks at the micro-scale structure of spider silk and its role in the construction of the web. It shows
that both the silk as well as its intricate arrangement are responsible for the spider web’s remarkable
response to various kinds of external loading. In addition, there are valuable lessons to be learned from
the spider’s construction, design and maintenance of its web that can be applied to improve our own
engineering techniques.
Webs That Inspire
When King Bruce of Scotland sat moping in his cave after a string of defeats, little did he know
that the same eight-legged creature that gave him his famous morale boost would continue to fascinate
scientists and engineers many centuries later1. Spider silk has been extensively studied for its remarkable
material properties, and numerous uses of the material have been proposed in engineering, military and
manufacturing industries. Only recently, however, have spiders’ webs begun to attract attention
themselves. As this article reveals, it is a marriage of the unusual characteristics of spider silk and its
creator’s incredible craftsmanship that makes the web a highly optimized design. They are truly as unique
1
According to legend, after being defeated by the King of England six times in the 13th Century AD, King Bruce went into hiding in a cave while
his army retreated. It was in this cave that he watched a spider trying to spin its web across the roof. After failing six times, the spider
succeeded on its seventh attempt. This was supposed to have inspired King Bruce to rally his men to war once more. He finally emerged
victorious against the King of England on his seventh attempt, and was immortalized as a hero both on the battlefield and in childrens’ stories.
as the material they are made of, and carry enormous potential for creating new advances in the rapidly
growing field of biomimetic engineering.
Silk Production
The spider’s web begins its journey in a set of glands in its abdomen, each dedicated to producing
a specific kind of silk. Dragline silk, usually referred to as ‘spider silk’ in common parlance, is known to
be secreted as a liquid in the major ampullate gland. It is the type used by spiders to construct the support
structure for the web – the frame, radial spokes and quick escape paths to avoid predation [1]. Scientists
speculate that it is also the most primitive variety of spider silk. Prehistoric spiders probably used it to
find their way back to their dens, until their eureka moment arrived: they spotted the first unlucky insect
trip across a stray dragline. As they began to construct elaborate webs, additional kinds of silk evolved,
such as the special adhesive silk used to trap prey on webs, as well as silks used to wrap egg sacs [2].
Dragline silk is the strongest of them all, and consequently, the most commonly studied. It has a
composite structure, made up of crystalline ‘blocks’ of one kind of protein embedded in an amorphous
matrix of another type. Its celebrated material properties arise from an interplay between these two types
of protein configurations when subjected to mechanical stresses (discussed later in detail). Before
emerging through the abdomen, the silk undergoes several stages of chemical treatments in ducts called
spigots that convert it into solid phase from an aqueous solution. It then emerges as streams of tiny
threads, called ‘fibrils’, from the each spigot. Multiple fibrils are then twisted together to make a single
strand of silk [5]. When it senses the presence of sufficient material, the spider coaxes the silk out with
tiny claws on its hind legs.
Constructing the web
After accumulating enough dragline silk from its abdomen, the spider literally throws its chances
into the wind. If it gets lucky, the first strand catches a branch and the spider quickly scurries over to
secure the web’s first anchor. It then searches for a third node, such as a tree stump on the ground or a
lower branch, to form the primary Y-shaped backbone for the web. The spider then connects the endpoints of the ‘Y’ structure and adds additional reinforcements to complete the framework. Next it begins
work on the first of its two spirals reaching out from the
hub to the outer fringes of the web. Using its non-adhesive
silk for the last time, it creates a widely spaced ‘scaffolding’
spiral. This gives the spider a secure footing before it lays
the sticky silky spiral, the killer trap, beginning at the outer
edges of the web. The sequence of steps followed in the
creation of the web are summarized in Fig 1. The spider
truly justifies its reputation as a master craftsman during
this last and most important phase of construction. Despite
having poor vision and relying solely on its hind legs, the
spider simultaneously combs strands of silk out from its Fig 1: The different steps in the construction of a
spider’s web: (i) the first line is cast; (ii) The Y-
spinnerets while measuring the distance to the previous frame takes shape (iii) The radial spokes and
segment of sticky glue. Once satisfied, it carefully pins the
free end of the strand to a point on the next radial arm. It
scaffolding spiral are added (iv)The final sticky
spiral is completed
Adapted from: ‘How Spiders Work’ (Harris, T),
howstuffworks.com
often switches between clockwise to counter-clockwise directions to correct any missteps, but never
strays from its scaffolding [2] [6]. A typical orb weaver spider meticulously prepares its sticky death-trap
for twenty minutes, before finally settling down at the hub. Following completion, some spiders are
known to spin a ‘stabilimentum’ across its web. Much debate prevails over the exact purpose of this
apparently decorative zig-zagging element, but one study suggests that it signals birds to steer clear of
their laborious handiwork [4]. This carries a trade-off, however, because potential prey are equally likely
to detect the web’s presence. Field and laboratory observations conclude that spiders starving for a meal
usually do not invest the time and effort needed for building stabilimenta, while their more successful
brethren can afford to do otherwise [4]. Stabilimentum or no stabilimentum, the web is now complete,
and the spider can let the waiting game begin.
Time to feast
Carrying out a task as sophisticated and labor-intensive such as web-building may speak volumes
about the spider’s prowess, yet the true genius behind the web’s design is observed when it actually
begins to perform its intended purpose. When an unwary insect flies into the sticky silk spiral, vibrations
from the point of impact travel to the spider’s sensitive claws [6]. Guided by touch and knowing the web
like the back of its – well – feet, the spider eagerly rushes to the source of disturbance along the nonsticky dragline members. Taking no chances with possible retaliation from its prey, the spider shrouds its
helpless victim in even more silk before tucking in to its well-deserved feast.
Unraveling the secrets
Apart from the strong adhesive of the sticky spiral that helped capture the insect, researchers have
identified two key aspects of the web’s design that proved crucial to the spider’s success. The web has a
unique property of yielding slowly after impact, before quickly becoming stiff [1]. In addition, they limit
distribution of stress only to those members in the immediate
vicinity of the point of impact [1]. The first observation is a
characteristic of a non-linear elastic response as illustrated in Fig
2, one that is not commonly encountered in everyday materials.
Most ordinary materials can be modeled as a simple spring
obeying Hooke’s Law, which after elongating under stress,
spontaneously return to a value close to its original length [1]. If
Fig 2: The characteristic non-linear response of spider silk.
the web were to be replaced by a linearly-elastic material, spiders ‘Entropic unfolding’ refers to the unfurling of the
crystalline protein molecules under stress
would despise physics. Upon impact, the insect’s kinetic energy Source: ‘Nonlinear material behaviour of spider silk yields
robust webs’ (Buehler et al, 2012)
would be rapidly converted into potential energy of the fabric (and any irrecoverable forms of energy
such as sound or heat) as it slowed the insect down. However, because of its linear behavior, the stored
energy would be reconverted into kinetic energy as the fabric returned to its un-stretched state – thus
giving the insect a better chance of making a quick getaway. Spider webs, on the other hand, convert most
of the impact kinetic energy into forms from which kinetic energy cannot be recovered. The difference
between the two responses can be easily demonstrated by first dropping a bowling ball on a sand pit and
then on a strong trampoline from the same vertical height: both would absorb the same amount of kinetic
energy on impact, but the sand pit would dissipate energy among its countless grains of sand instead of
returning it to the bowling ball. The second characteristic behavior, where distribution of impact forces is
limited by the web, helps retain its structural integrity when struck by a flying object. If they were
allowed to uniformly distribute among various structural elements, a larger portion of the web would be
subjected to deformation. In turn, this elongation could risk being permanent due to the non-linear
response described earlier, and compromise the spider’s future chances of success. The secret to these
unique features of the web can be found in a few tricks up the spider’s eight sleeves and quite literally
entangled in the silk itself
Spider silk under the microscope
The composite structure of spider silk consists of blocks of
crystalline protein embedded in a matrix of amorphous protein as shown
in Fig 3. [5]. Under normal conditions, indicated by (i), these crystalline
protein molecules are randomly dispersed and remain coiled up
throughout the amorphous matrix. Under stress however, they start
clustering and tend to line up alongside each other, as shown in (iii).
During the time it takes for the crystals to unfurl themselves and align,
Fig3: The different stages of uncoiling
of protein crystals (i-iv)
Adapted from: ’A Materiomics
Approach to Spider Silk: Protein
Molecules to Webs’ (Tarakanova et al.,
2012)
the silk fibril starts to deform and yield. When most of the crystals attain
a linear structure, strong hydrogen bonds develop between the protein
molecules and the silk fibril transitions from yielding to stiffening (Fig 2).
Beyond this point, in stage (iv) of Fig 3, increasing applied stress brings these crystalline protein
molecules closer together, thus making hydrogen bonds stronger. In this condition, indicated by the
steeply inclined part of the curve, the silk absorbs a lot of stress while deforming very little. The ultimate
breaking point is reached when the crystalline protein cannot come any closer, and the overall structure
disintegrates when they begin sliding past each other under excessive loads [5]. This response to an
external applied force can explain how the kinetic energy from an insect’s impact cannot be easily
recovered by the web: initial energy gets transferred to composite protein matrix, and pushes crystalline
protein together. Subsequently, with the formation of hydrogen bonds between strands, part of that kinetic
energy is released in the form of heat. The rest is dissipated by other means such as sound or heat, or
damping by air [1].
Magical ‘Globules’
For all its unique material properties, however, the silk would be of
limited use if the spider didn’t fully exploit its merits. When constructing the
web’s support structure, at every intersection between two members of dragline
silk, it placed a globular cluster of silk [6] as shown in Fig 4. These globules, or
droplets, contained additional reserves of the folded composite matrix, waiting to
be uncoiled when something snagged on a connecting strand. Therefore, although
forces from the initial impact do tend to disperse throughout the web, they
Fig4: (i) Under normal conditions,
cannot get far beyond the nearest droplet. These droplets thus bear the brunt of proteins remain coiled inside the
droplets (ii) When subjected to a
impact forces without letting them deform the rest of the structure. These tiny force, they stretch and uncoil
droplets also play an important role in protecting the web when subjected to
stresses other than point impacts [6].
Source: ‘Spider, the Genius
Architect’, Invisible World (BBC),
http://www.youtube.com/watch?
v=1JlLLpaCkI4
Facing the elements
Webs need to be capable of withstanding severe winds and
swaying movements of anchor points, in addition to stresses due to
collisions. These forces are applied to the web taken as a whole and lead to
tensile forces on every strand. In Fig 5, three webs made of different
materials were exposed to winds at varying speeds and corresponding
Fig 3: The web’s response (dragline silk
denoted by A,B) to varying wind speeds
Source: ‘Nonlinear material behaviour of
spider silk yields robust webs’ (Buehler et
al, 2012)
deformations were recorded. Buehler’s data, plotted in Fig 5, reveals that
the web made of spider silk (A, B) deforms the most before failing [1].
Once again, the numerous droplets scattered across the web were responsible for allowing the strands of
crystalline protein to unfurl, letting the entire web stretch more than it would have otherwise. The other
webs (A’, A”), in contrast, were elongated to their limits earlier, and consequently failed at a smaller
magnitude of strain [1]. Each droplet can thus be thought of as storehouses of extra silk material, which
uncoil immediately when an insect catches a single strand, or in tandem with other droplets when external
forces affect the entire web.
Spider webs have been regarded as perfect examples of optimized engineering design – they
operate ideally, utilizing the full potential of available material resources under a host of loading
conditions. They are subjected to stresses of varying magnitudes and durations, yet they respond in a
manner that is the envy of our structural engineers. Under impact loading, they localize dissipation of
forces and prevent the entire structure from deforming due to a point load, while under forces applied to
the whole surface, they permit maximum elongation for a given stress. Buehler argues that if the concept
of sacrificial elements in man-made structures were altered from ‘the element that fails first’ to one that
‘not only fails first but also prevents other components from suffering excessively large displacements’,
damage to buildings could be significantly limited and confined to a specific region [1]. In trying to
mimic the spiders’ superior building skills, merely substituting the magical spider silk for conventional
building materials won’t suffice, as the critical role played by the droplets proves. The coiled crystalline
protein blocks in those globules essentially provide a secondary means of dissipating kinetic energy,
forming a two-step process of energy dispersal. Perhaps engineers can take the benefits of spider silk a
notch higher and create a three-tier stress-strain response, which will let the material yield in multiple
stages before failing. This would amount to creating a second transition point after the ‘stiffening’ in the
stress-strain curve of Figure 2. Such an improvisation can be carried out at the nanoscale to create a
fundamentally new material, but the same idea can be implemented with relative ease by physically
constructing multiple levels of sacrificial elements in man-made structures. These would fail in a planned
sequence, thereby limiting and ideally preventing damage to the overall structure.
Most of the present knowledge about spider silk and spider webs has been gathered from research
conducted on only a handful of species out of the 40,000 that inhabit an incredible array of habitats across
the world [2]. If the webs of orb weavers can provide such a fascinating insight into how we can learn
from these tiny master engineers, other kinds of webs can only offer so much more. Spider silk shows an
astonishing diversity of material properties not only across different species, but also within a particular
species – depending on the spider’s size, shape and its health, apart from environmental factors [2]. When
such variations of fundamental properties are introduced into the differences in web construction from
one species to another, the resulting combinations are practically endless – and so are the possibilities for
drawing inspiration from these little critters.
Works Cited
[1] Buehler et al., “Nonlinear material behaviour of spider silk yields robust webs” Nature, Vol 482, pp
72-75, February 2012.
[2] Savory, T.H., “Spider Webs”, Scientific American, pp 115-124,1960.
[3] Harris, T., “A Typical Orb Web” http://science.howstuffworks.com/zoology/insectsarachnids/spider5.htm [online] (13 Nov 2013)
[4] Blackledge et al. “Do Stabilimenta in Orb Webs Attract Prey or Defend Spiders?” Behavioral
Ecology, Vol 10, pp 372-376, December 1999
[5] Tarakanova et al. “Materiomics Approach to Spider Silk: Protein Molecules to Webs” JOM, Vol. 64,
No. 2, pp 214-224, 2012
[6] “Spider, the Genius Architect”, Invisible World (BBC)
http://www.youtube.com/watch?v=1JlLLpaCkI4 [online] (13 Nov 2013)