Sac spiders are medium sized (5-12 mm body length) pale spiders. Like all spiders they have two body-segments, a cephalothorax in front and an abdomen behind. They have eight legs, all attached to the cephalothorax. The legs are fairly long in this family, often twice as long as the body.
On the front of the cephalothorax they have two small "mini-legs" called pedipalps. These are used to grab prey, and they are used in mating. Pedipalps are much bigger in male spiders than in females. In this family all species have eight eyes in two rows of four. Their eyes are small and all the same size, though back row eyes are sometimes oval-shaped and longer than the front row. These spiders have strong mouthparts and their fangs are sometimes a bit longer than other spiders of the same size. All sac spiders have venom glands.
Sac spiders are often rather pale: a common house species is yellowish white, some are even lighter and others are light brown. No matter how pale they are, their mouth parts and the area near their eyes is much darker, usually dark brown.
This family of spiders is found all around the world, and about 540 species are known. Only about 30 species are found in Michigan.
These spiders climb around on vegetation looking for prey. They can be found in most habitats that have lots of plants. A few species in this family are often found in houses and other buildings.
Spiders hatch from eggs. The hatchlings look more or less like grown-up spiders, though sometimes their colors change as they age. To grow they have to shed their exoskeleton, which they do many times during their lives.
Most spiders in this family live for about a year. Females might live longer. Males often die soon after mating.
Sac spiders are mainly active at night, and they roam to new places every night. They'll only stay in a place as long as they find food there. These spiders are not social. They only come together to mate.
Since they are active in the dark, these spiders mostly rely on smell, touch, and taste for communication. They can see, but not very well.
Sac spiders are aggressive hunters. They search vegetation at night, grabbing, biting, and eating the insects and other invertebrates they find there.
Sac spiders spin a little silken room to hide in during the day. Sometimes they fold a leaf over and spin in there, sometimes they hide under a stick or a stone. They are only active at night, so they are hard to see. They will bite to defend themselves if they can.
Though not very dangerous, some species in this family can give a painful bite.
Sac spiders are important enemies of many insect pests.
No sac spider species are known to be endangered or in need of protection.
Sac spiders get their name from the little silk sacs they make to hide in during the day. They also use the sacs to protect their eggs.
George Hammond (author), Animal Diversity Web.