Appliance of Science: How and why do spiders make webs?

Love or hate them, there is a lot to be admired in these eight-legged creatures and those intricate webs they weave can help keep the flies at bay; but how do they make their webs and are they just to catch their dinner?

There are more than 45,000 known species of spiders on Earth and while all of them can spin silk only about a quarter of these species weave those elaborate, spiral webs we are familiar with. They are referred to as orb-weaver spiders.

Spiders don’t just make one type of silk, they can make up to seven different types. Spiders silk is strong, tensile and flexible. This amazing material is about twice as strong as steel (when comparing the same mass of both).

Spiders silk starts off in liquid form, made in specialised internal glands. It is then converted into a solid form and spun into silk by the spider’s spinnerets (silk spinning organs), located on its abdomen. When spiders are finished with their web, some eat them, replenishing their silk supply material.

Of course many insects produce silk too, some make cocoons to hide inside as they transform into adults. Honeybees use silk to strengthen the inside of their wax cells.

The spider starts by creating a length of sticky silk which it dangles from a point. It is so light it gets caught by the breeze and sticks to a second point, the anchor point. This first strand of silk is called the bridge thread.

Once the bridge thread is firmly anchored in place the spider crosses this thread, creating a second thread and eventually the weight of the spider pulls it down, pulling this second thread with it and the main building of the web can begin. This becomes the centre point from which the spider creates the radial spokes and anchors them to any surrounding vegetation or solid structures.

Next the spider makes a spiral, starting from the inside and moving out. This is the auxiliary spiral and it creates a structure to bind all the radial spokes together. It is made from a dry type of silk that is not sticky.

From this the spider spins a final spiral web, building this one from the outside in. This one is made of sticky silk and is called the capture web. Most spiders cuts away the auxiliary web while weaving the capture web.

So why do spiders spin webs?The do it for a number of reasons. They can make webs to catch a meal and to protect themselves from prey. Webs come in lots of different shapes too. From fluffy triangular shaped webs that smother their captors to funnel shaped webs with front and back exits in case the spider needs to make a quick getaway.

Spiders spin webs to protect their eggs or as a hunting tool, a small silk net that they throw over their prey.

They can also use webs for transport, climbing to a high point and then release one or more sail-like strands of silk which get caught by the breeze and carries them away. This process is called ballooning or kiting.

Young spiders often use this method of transport to disperse themselves after hatching but certain adult spiders use it too. This method usually brings them wherever the wind blows but spiders may use this to travel on jet streams that carry them right across oceans.