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.