The family Thomisidae is a large family with 160 genera and about 2000 known species. In Southern Africa they are represented by 39 genera and about 140 species.
Common names: Thomisidae (crab spiders); Thomisus (flower crab spiders); Runcinia (grass crab spiders); Xysticus (ground crab spiders); Phrynarachne (bird-dropping crab spiders); Synema (African mask spiders); Heriaeus (spiky grass crab spiders)
Life style: Wanderers (plant and ground
dwellers); free-living spiders found mainly on foliage with only a few genera
living on the ground; abundance: very common.
Body size: 3-23 mm.
Common genera: Camaricus (4 spp.);
Diaea (4 spp.); Heriaeus (4 spp.); Misumenops (1
sp.); Monaeses (7 spp.); Parabomis ( 5 spp.);
Pherecydes (6 spp.); Platythomisus (4 spp.);
Runcinia (8 spp.); Simorcus (4 spp.); Synema (14
spp.); Thomisops (7 spp.); Thomisus (14 spp.); Tmarus
(7 spp.); Xysticus (10 spp.).
characters: colour: body
colour variable, from brightly coloured (pink, green, yellow) to dark brown or
grey with a mottled appearance, abdomen frequently decorated with patterns;
carapace: variable from
semicircular, ovoid to elongate, usually with simple erect setae while in some
genera strong protuberances or eye tubercle are present; eyes: 8 in 2 rows (4:4) with lateral eyes usually
on tubercles that varies from rounded to distinct;
abdomen: variable in shape from round to ovoid to
elongate, sometimes extending caudally past the spinnerets; legs: laterigrade with legs I and II
usually longer than III and IV and the anterior legs frequently with series of
strong spines on tibiae and metatarsi. Web and retreat:
Web: absent; retreat: absent, hide beneath vegetation and
ground debris when not active.
Habitat: Thomisids are very commonly on plants
and they inhabit grass, shrubs, flowers and trees and are frequently encountered
on crops. They are found in all the different biomes.
Behaviour: Thomisids have lost their agility,
and have become semi-sedentary to excel as ambushers. With their cryptic
colouration most species await their prey, usually on plants. They are mainly
active during the day and their gait is sideways or crab-like hence their common
name. They have strong bodies and robust front legs which enable them to attack
prey much larger than themselves. They are able to see motion 20 cm away. Prey
are seized, frequently from the air when 0.5-1 cm away. Although they have weak
chelicerae, they secrete extremely potent venom which enables them to attack
insects 2-3 times their size. They have no cheliceral teeth and their prey is
consequently not mashed but sucked dry. The life-like carcass is held in a
natural position while it is being fed upon and thus offers some protection to
the spider, while sitting on the plant. Thomisids display an interesting range
of adaptations to their habitat. Genera such as Tmarus and Pherecydes with their
mottled brown and grey bodies, decorated with tubercles are primarily found on
the bark of trees, while Monaeses and Runcinia with their elongated bodies are
living on grasses; Heriaeus with their spiny appearance live in inflorescences;
members of Bominae with their small, round bodies are found between seeds;
Xysticus with their dominantly brown colouration is soil-dwelllers while the
brightly coloured Thomisus species live on flowers. In Phrynarachne the spider
spins a small irregular silk disk on a leaf. Their bodies are covered with
numerous tubercles and when resting in the middle of this disk they look like
bird dropping . Their ability of some thomisid species of Thomisus to change
colour to conform with the background of their habitat has intrigued
naturalists for many years. The process usually takes a few days. The colour
ranges from white to yellow or pink.
Prey: Thomisids prey on a variety of small
invertebrates. They are commonly found on crops and play an important role in
the natural control of pests such as aphids, red spider mites and thrips.
Distribution: The thomisids have a very wide distribution
throughout the region. They are easily distributed by wind.