Jumping spiders (Salticidae) and crab spiders (Thomisidae)
Jumping spider eyes
They have the greatest visual acuity of all arthropods. Their eyes give them binocular vision and they can see in colour and polarized light. The arrangement of eyes is shown above right. The pair of small eyes furthest to the back may just detect light and movement.
The spider often rears up to get a better look at things, and they can spot prey 20 cm away. The pair of eyes between the 4 at the front and the two at the rear are hardly visible to the naked eye. The two large front-facing eyes are like headlamps (see left and right) and do reflect the light just like the eyes of a cat. They stalk prey then leap on it. They have a fondness for leafhoppers, and will eat 4 - 8 per day. Good vision is also needed during courtship.
Jumping spider courtship
The male waves his legs and palps semaphore-style as he dances around the female trying to gauge if she is willing to mate. If she is willing to mate his dancing will have reduced her to such a passive state of ecstasy that he can mate and make a retreat with his life; free to delight another girl with the dance of his life and hers.
Salticus sp. The most common species in the genus is Salticus senicus, commonly called the zebra spider because of its stripes. It is often found on walls, fences and window sills during warm, sunny weather. It's body length is 6 - 7 mm when fully grown. Greenfly are a favourite prey of this genus. It mates from May onwards.
The Salticidae move in a jerky fashion making a short, fast run, then stopping to have a look around, and sometimes raising the cephalothorax to get a better view, then another short, fast run.
The photograph on the left was taken on a wall in my garden. The movement of the fly attracted my attention, but I didn't see the spider at all at first as it was so well camouflaged. As you can see the fly is much bigger and heavier, but was already tiring. The spider just held on to the fly with its fangs and to the stone with its legs and drag line. It was totally oblivious to me, so I could get close enough to take this.
Curious courtship in crab spiders
In the Thomsids mating usually lasts 5 - 45 minutes, and the male is usually free to escape. However some Thomisids, in the Xysticus genus such as X.lanio and X. cristatus, have a curious courtship. The male is slightly smaller than the female. He climbs over her body tying her down to the ground with numerous lines of silk. Then he pushes up her abdomen and squeezes underneath her to insert a sperm-loaded palp in her epigyne. However after this the female just gets up and walks away. The silk does not restrain her, so we do not yet know what is the purpose of the male tying her down.
Thomisidae - crab spiders
There are 2062Thomisidae species world wide.
The spider on the
left is a male of the Thomisidae family. It is easy to see he is a
mature male because of his swollen palps. And also that he belongs to the
Thomisidae family because his front two pairs of legs are longer than the rear
Thomisids can live in a wide variety of habitats, and are often masters
of camouflage. Some (mainly those which capture their prey while they lurk in flowers) even have the ability to change colour.
They tend to sit
and wait for their prey then make a quick grab with their first 2 pairs of legs, injecting a relatively powerful
and quick-acting venom. Because of this method of prey capture none of them spin webs for
ensnaring prey. The potency of the venom allows them to capture prey larger and heavier than themselves, even bumblebees can be taken. They also have the ability of walking sideways which gives them
their common name of crab spiders.
The drawing on the left shows the typical arrangement of eyes in the Thomisidae family, and the drawing below the typical body shape.