(Araneus diadematus) with lunch!
© Roger Key
Invertebrate silk is incredibly versatile. Flexible, elastic and seven times stronger than steel, silk is used by a range of invertebrates for hunting, protection and shelter. You will know that spiders use silk, and we will come to those later, but first here are some other invertebrate masters of silk.
Silk and Lace
Female Green lacewings use silk to protect their vulnerable eggs during development. The eggs are laid at the end of stiff silk stalks placed on the underside of plant leaves and stems; this prevents heavy predators such as ants from reaching and eating the eggs.
Lacewings belong to the insect order neuroptera, neuro=sinew ptera=wing. Adults typically have wings with numerous cross-veins, large eyes and long neck with piercing sucking mouthparts. Of the 5000 species many are predatory however some species consume pollen, nectar and honeydew (the sugary water excreted by sap sucking bugs like aphids). Lacewings undergo complete metamorphosis; hatching as larvae which look nothing like adults they are usually specialised predators consuming things like spider egg masses. Once satiated the larvae settle and form a cocoon, inside they reform their body and emerge as an adult lacewing.
Green lacewing eggs © Dragiša Savic
Some moths and butterflies use silk. Caterpillars of the Marsh Fritillary butterfly live in groups and between them produce silken larval webs which protect them from predators. The silk worm is in fact the caterpillar of the silkworm moth (Bombyx mori). When the caterpillar is ready to pupate into an adult moth it spins a protective cocoon of silk. The caterpillars are commercially farmed, and the cocoons are collected to produce the silk which is used to make clothes.
Marsh fritillary butterfly © Roger Key
Silk is also used under water; the larvae of caddis flies live in rivers and build themselves cylindrical homes by binding small particles of sand or bits of leaves together with silk. Some even spin mesh nets to trap food carried on the river currents.
Weaver ants are found across Africa and Australasia, they use silk is to sew leaves together to provide shelter for their colony. Adult ants can not produce silk themselves, they instead use their larvae. The ants carry the larva in their front legs, weaving them back and forth between leaves to join them together. The larvae can produce silk on demand with a light stroke from the adult handler.
© Ben Hamers
What are spiders?
Spiders belong to the group of invertebrates called the Arachnids. Arachnids all have eight legs; the group also includes harvestmen, scorpions, mites and ticks.
Spiders have two body parts – the cephalothorax (a fused head and thorax) and the abdomen. Most spiders in the UK have eight eyes, a few species possess only six.
There are about 650 different species of spider in the UK, these are split into 33 families of similar species. One of the largest families is the money spiders (Linyphiidae) which make up nearly half the total number of species.
Spider silk is produced as a liquid protein in the spider’s abdomen. At the tip of the abdomen are two or three pairs of spinnerets, these spin the silk which immediately solidifies to form threads. Spider silk is incredibly strong and elastic – it can be stretched to one third longer than its original length without breaking. Spiders use their silk for different uses, it can be used to construct webs to catch prey, to protect eggs, to wrap prey, or to weave a shelter.
Some spiders use silk to colonise new areas. On fine days in late summer or autumn tens of thousands of small money spiders climb to the tops of blades of grass or fence posts and spin strands of silk. As the wind catches the silk the spiders become airborne and drift along with the breeze. This ‘ballooning’ allows the spiders to travel huge distances and up to astounding heights. Weather balloons over 1000m up in the atmosphere have caught ballooning spiders!
Linyphid spider © Jon Mold
STAR FILE: Wolf spider
Wolf spiders (Lycosidae) are featured in programme 3 of Life in the Undergrowth. They do not spin webs, they hunt and chase their prey on the ground. Common wolf spiders (Pardosa pullata) can often be seen running around or basking in the sun on bare surfaces. You can find them in your back garden or a local park. Female wolf spiders are excellent mothers; they carry their eggs around in a silk sac which is attached to their abdomen. When the eggs hatch the young are carried on their mother’s back for a few days until they are large enough to fend for themselves.
Female Wolf spider (Pardosa sp) carrying her young
© Roger Key
Many spiders are very small and often go un-noticed. It has been estimated that an ungrazed meadow may support as many as 4,000,000 spiders per hectare!
The web of an average Garden spider (Araneus diadematus) contains up to 30 metres of silk!
Garden spider web with frost © Roger Key
The world record for the largest outdoor spiders’ web is held by British spiders. In October 1998, a cobweb that covered the entire 4.54 hectare (11.2 acre) playing field at Kineton High School, Warwick was discovered by Ken Thompson - the school’s caretaker. It had been created by thousands of money spiders. (Guiness World Records)
Would you like to know more?
Other pages on the Buglife website:
British Arachnological Society