Gnaphosidae: ground spiders

This is a family with 33 British species, mainly comprised of agile nocturnal hunters who spend the day in a silken retreat. They are plain brown, black or grey and have short hairs that give them a shiny appearance. Some are powerfully built, large spiders. They have long, very mobile spinnerets, but unlike the Clubionidae they are divergent and the anterior pair is longer than the rest. They use them to weave their fine silk cells. One species, Scotophaeus blackwalli (above) is commonly found in the inside of houses, and can be surprised at night, when she prowls on walls in search of prey. Several genera have adopted a diurnal existence and evolved a remarkable ant-like appearance and behaviour. These are strikingly marked with spots and covered with iridescent scales.

Mate guarding and egg guarding
They mature in May of June, although the home-living Scotophaeus can mature all year round. Males often join immature females in their silk retreats until they reach maturity. The females will also make their white, often papery egg sacs inside the retreat.

Drassodes sp. pair. Males are often found together with penultimate instar females inside silken retreats. 

Scotophaeus occupying a cell in the bee hotel, guarding her egg sac.

Panthers of the invertebrate world

That's how William Bristowe described these spiders. They are indeed powerful, sleek, and nocturnal fighters. Their ability to produce sheaths of silk with their divergent spinnerets, which can move like fingers, gives them an advantage when facing other spiders. They can use these silk sheaths, coupled with their agility, to immobilise spiders larger than themselves. 

Scotophaeus blackwalli is able to prey on Pholcus, the other formidable spider predator in the house.

Male Scotophaeus blackwalli. It's spinnerets protruding at the end of his abdomen

Male Drassodes cupreus

Zelotes sp. found under log.

Ant mimics
The genus Micaria includes several species of ant mimics. Ant mimicry is actually very common in spiders and has evolved in several groups of spiders. The resemblance is often striking and specific to particular species of ants: the species on which these spiders prey (see this fantastic album of world ant mimic spiders by Alex Wild). In other cases the mimicry seems to have evolve in order to avoid predation. Micaria pulicaria can be taken for a Lasius niger garden ant. In sunny summer days when ants run on the ground, this small spider can be seen not only looking, but also behaving in the same way as ants, with front legs held up and quivering line the antennae of an ant. Micaria does not prey on ants, so from its resemblance to ants might give her a benefit of lower predation, allowing her to exploit the diurnal niche of small predators running in the open.

Micaria pulicaria, an ant mimic, which runs zigzagging across the ground rapidly, giving a remarkable ant impersonation.