Evolution of Spider Webs
Bill Shear (Hampden-Sydney College, Hampden-Sydney, Virginia, USA)
Department of Entomology & Department of Biology, Ohio State University
22 November 1999
The spider body has 2 parts, an anterior cephalothorax, and a posterior abdomen, which contains most of the vital organs as well as the silk glands. Modified appendages (spinnerets) handle the issued silk from the posterior end of the abdomen.
Not all spiders make webs. Crab spiders are sit-and-wait predators. Spiders are generalized predators. Other sit-and-wait spiders include a camouflaged form - can mimic lichen-covered bark - can lie so low as to cast no shadow even in oblique light.
Another spider strategy is seen in jumping spiders - actively going after prey. Many spiders are quite colorful.
For those spiders who don’t use their silk to capture prey, the question becomes: did web building appear 1st or not? Non-web building now appears to be a secondarily derived behaviour. Orb weavers spiders - make a standard circular web consisting of an outer frame, a hub (radii from the center out to the frame), and a catching spiral (1 continuous thread that is attached to the radii). Silk is used to make webs and is used for other purposes as well.
Spinnerets are usually 6 in number. The primitive condition is 8 spinnerets (4 pairs). The dragline is a 4-stranded cable of silk. Other ways silk is made includes ribbons of silk to wrap up prey. Cribellate silk - very tangled core threads with puffy tangled threads around, adding to the overall line’s elasticity. Core lines can also be coiled for elasticity purposes. Can have exceedingly complex overall lines - the irregularities of the thread helps capture insect prey by snagging onto cuticular spines or setae or hairs. Some webs use electomagnetic static to help attract insect cuticle.
Dusty silk is less sticky.
Carbohydrate glue can be applied to the overall line - either as a continuous tube of glue or as droplets along the line. This glue dries out in about 12 hourse, but in wet environments, the glue may stay wet and sticky for a long time.
Cribellum-bearing spiders are the primitive state.
Use of silk - catching prey, attracting mates by vibrating a web; silk used to wrap and protect eggs - females may stand guard over egg cases; silk used by hatchlings to make a web for mass huddling; silk used to wrap prey.
Some webs are up to 2 meters in diameter. Some spiders are known to capture birds and bats and sucessfully feed on them. Spiders construct their webs in such a way as to economize on silk. Many spiders abandoned web building because silk production is metabolically expensive.
A silk-lined tunnel with or without a closing structure (trap door spiders) may be the primitive condition of spider webs. Trap door spiders use substrate vibrations to detect prey proximity.
Extending the range of sensory capability from a silk-lined tunnel would include an extension of the tunnel into a turret using long leaves, serving as trip lines for approaching prey. Other tunnel-making forms have silk trip lines in a radiating pattern from the lair entrance; other lines of sticky cribellate silk are laid cross-ways on top of the radiating pattern. This slows prey down so the spider can emerge, capture, and bring back prey into the tunnel, where the prey can’t maneuver around easily, and then it is eaten.
The most common spider web type is the ground built sheet web - prey gets tangled when walking on the sheet - the spider comes out and bites or wraps the prey insilk, then drags the prey back into the burrow, where it gets eaten. Such a web can be improved by moving it up above the ground using some vertical elements as a foundation. After all, there are lots of flying insects as potential prey.
The only known aquatic spider occurs in freshwater environments of Europe and Asia - it captures air bubbles on its abdomen and takes the air bubbles to an underwater web and fills the web with the air bubbles, forming a diving bell; the spider captures aquatic prey. It doesn’t rely on the oxygen in the air bubbles; the bubbles bring in oxygen from the water it turns out.
Beyond a tangle of silk thread, can have webs with a few radials and a zigzag concentric - only a partial circle.
The next step puts a catching spiral all the way around the spider waiting station.
How webs are constructed - saw video. The catching spiral is made from the outside in; there are actually 2 spiral in most forms - there is a scaffolding spiral made after the radials, from the inside out, and then the catchment spiral is made from the outside inward.
The smallest spiral webs are ~15 mm across; the largest are ~2m in diameter.
One form makes a 4-sided puffy silk web and uses it to grab out at and capture pedestrian prey; this form has reduced the amount of silk used and has used muscular movement to help capture prey; muscular movement is less metabolically expensive than silk production.
Another form makes a ball of silk, partly camouflaged by leaves and twigs and an opening - it is dark inside; insects are attracted to the inside because of the darkness, where they get caught on radials and eaten by the spider.
Another form makes a web of only radii - it dashes down to capture passing pedestrian prey.
Moths’ wing scales attach to sticky silk and are easily shed, allowing a moth to readily escape from spider webs. Spiders have responded by making finer mesh webs and then attacking quickly when a moth comes into contact. Another anti-moth strategy is to build a vertical extension of the web; the moth loses all its scales while falling downward and eventually gets stuck with no more scales to shed.
Spider superglue in some webs is also used to better capture moths.
Another anti-moth strategy is building 3 radials with cross-strands loosely attached to the outer radials and firmly attached to the inner radial. When a moth gets stuck in the superglue silk, the loosely attached ends come loose, and the moth is tethered by a single strand - it eventually exhausts itself flying around in a circle, and the spider reels it in without risk of injury from an energetic moth.
Another strategy is a spider waiting at the end of a simple web for prey to fly up to the spider; this particular form preys on male moths only by secreting a female moth sex pheromone chemical.
Another pinnacle in web building is a single line with globs of superglue, and the spider swings it around to grab out at passing moths, but they are not trying to hit prey, rather they are simulating an orb web - increasing the chances that the prey will fly into it - an increased area of capture with very little silk production.
Orb webs are not the apex of web building behaviour, but it is a point from which other strategies were developed. One orb weaver spider has a bird dropping camouflage - it sits & waits for insects to come and feed on the fresh dropping. Spiders are known to be active scavengers as well as capturing live prey. Silk is ultimately derived from chemical trails back to a lair/home, probably, derived from modified coxal glands.
There are Devonian spiders. Most Carboniferous spiders aren’t really spiders. There are no Permian spiders known. There are Triassic and Jurassic spiders - the Mesozoic is the first time when we get fossil spiders belonging to living families; the Triassic & Jurassic fossils are morphologically orb weavers.
Silk in amber doesn’t help understand web building behaviour, because they are almost always draglines, not webs.