All in a spin (Part 1)

What creature springs to mind when you think of an animal that produces silk? A spider? A silkworm?

Here is an image (below) of a gorgeous spider in a silk web. It’s an orb-weaving spider in the genus Argiope from the Kimberley region in Western Australia.

Spider webs are made from spider silk which is produced from a number of silk glands located in the spider’s abdomen. The silk at this stage is a viscous liquid consisting of about 50% proteins and a mixture of other substances. The silk is extracted through special finger-like organs called spinnerets, which cause the protein molecules to align and form solid silk.

Modern web-spinning spiders have three pairs of spinnerets and each pair does a different job. The silk from the front spinnerets is used for attachment points, drag lines, and in the construction of the main frame threads of a web. The middle pair of spinnerets produce swathing silk used to wrap up prey and spin egg sacks. The rear spinnerets produce the gluey silk used in orb web construction. Spider silk is noted for being ‘five times stronger than steel’ – you can read more fun spider silk facts in my earlier post Said the Spider to the Fly.

One of my favourite spiders is the Australian Golden Orb-weaver (Nephila edulis). This spider is very widespread in Australia – it is found in tropical regions as well as temperate areas. The common name, Golden Orb-weaver, comes from the colour of the web rather than the colour of the spider – as you can see below. Gotta love those hairy knees too!

A silkworm is the larva of the domestic silk moth Bombyx mori. These insects were domesticated thousands of years ago. Since then the adult moths have lost their ability to fly, and most of their colour, but still bear some resemblance to their wild cousin Bombyx mandarina.

Silkworms are quite easy to keep – all you need is a good supply of mulberry leaves (Morus spp.) to feed the larvae. I kept some silk moths for a few generations which enabled me to photograph their life cycle.

It is the larva which spins the silk – into a cocoon so it can pupate inside. The silk comes from a single spinneret near the larva’s mandibles. Silk moths are one of many moths which spin silk cocoons as protection while pupating – lepidopteran larvae are immobile and very vulnerable at this stage. However, not all moth species larvae spin silk cocoons when pupating nor do butterfly larvae.

Some moth and butterfly larvae which don’t spin cocoons produce silk for other purposes, such as creating anchors and silk girdles for chrysalids like the Cairns Birdwing (Ornithoptera euphorion) below.

Some moth larvae, such as diamondback moth aka cabbage moth (Plutella xylostella) larvae, spin net-like cocoons which don’t seem to provide much protection at all (below).

The larvae of several moth families use silk to join leaves together. Some make shelters individually, while others like  the web moth (Pyralidae) larvae pictured below are gregarious and create communal shelters. The larvae can now shelter and pupate in safety. They venture out of their shelters at night to feed on leaves of the plant.

Juvenile spiders of some spider families and the first-instar larvae of some lepidopteran families utilise silk to disperse by ‘ballooning’. This occurs when a fine silk strand catches a breeze and the tiny spiders or larvae are dispersed by air currents. Some spiders of the families Linyphiidae (sheet-weaver spiders), Araneidae (orb-weaving spiders), Lycosidae (wolf spiders), and Thomisidae (crab spiders) use this method to disperse as spiderlings. The first-instar larvae of some Cossidae (wood moths), Psychidae (case moths), Geometridae (loopers) and Lymantriidae (tussock moths) are also known to disperse by ballooning.

Moth larvae of several families use a silk thread to escape from predators. Larvae of moths in the family Tortricidae are well-known bungee jumpers. During my time working for an Integrated Pest Management consultancy I often saw larvae of the light brown apple moth (Epiphyas postvittana) escaping rapidly on silk threads just as I was trying to count them! Codling moth (Cydia pomonella) larvae may also use this method to lower themselves to the ground prior to pupation.

Case moths (Psychidae) create their cases  from silk to which twigs, sand, moss, lichen, leaves or bark are attached. You can read more about them in my post Insect Architects.

This post has outlined how just a few types of insects use silk. Which other insects use silk and what do they use it for? All will be revealed in Part 2!