T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network


Crab Spiders Information)

Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Araneae
Superfamily: Thomisoidea
Family: Thomisidae
Genus: Sidymella
Common name: Crab spiders

Crab spiders are of the family Thomisidae A crab spider is any member of several genera of free-living spiders that resemble small crabs. Crab spiders have short, wide, flattened bodies. The first two or three pairs of legs are longer than the rest and are normally held out from the sides of the body as a crab would hold its claws. Crab spiders are easily recognizable if you tease them. They widen their legs and move sideways like a crab. The crab spiders in the photos below are of the Genus Sidymella which are blunt-ended spiders which live in leaf litter and other forest floor debris. Sidymella spiders are all cryptically coloured and have been named for their distinctive triangular-shaped abdomen and to date in New Zealand have not been fully described. Their size is approximately 4 - 10 mm. and can be found on flowers and foliage where they are well camouflaged by their colouring, ranging between brown, orange and yellow. Crab spiders do not spin webs to trap prey but hunt on the open ground or on vegetation or flowers where they wait for prey to get close, before seizing them with their long front legs. The principal adaptation of this capture system is their large, elongated front legs. They are also aided by a body that is often superbly camouflaged. Unlike other free-living spiders, however, all of a crab spider's eyes are small and serve primarily as motion detectors. Though their chelicerae or jaws are rather small and slender, many crab spiders possess potent venoms (not dangerous to humans) that quickly immobilize their prey. Their venoms enable them to successfully attack insects much larger than themselves. they do not wrap their prey in silk after biting, but instead remain with the immobilized prey until they have sucked it dry.

Some Thomsids 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 the purpose of the male tying her down is. The females usually stand guard with their egg sacs. The egg sacs are fastened to the vegetation and are usually flat. A square ended crab spider who's colour mimicking the colour of tree trunks etc.

Unidentified crab spider.


Unidentified crab spider

If they are knocked off a plant they will land on their back and remain motionless for some time before
righting themselves. Photos below show a spider turning over after playing dead lying on its back.
It stretches its legs on one side straight above its head.  Then uses the weight of the legs on the other side to roll its body over.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/