Sac spiders, primarily species in the genus Cheiracanthium, are frequently encountered indoors and outdoors. Accurate identification is useful for homeowners, as these spiders are often confused with other household species. Learning their specific visual cues, such as their pale coloration and unique silken retreat, helps distinguish them from other arachnids. They get their common name from the small, silken structures they build for resting and reproduction, not for catching prey.
Overall Size and Body Shape
Sac spiders are small, pale arachnids, typically measuring between one-quarter and one-half inch (5 to 10 millimeters) in body length. The body is divided into two main sections: the cephalothorax (head and thorax combined) and an oval-shaped abdomen. Their overall color is usually pale yellow, cream, or light tan, sometimes exhibiting a slightly greenish tinge on the abdomen.
The cephalothorax is often a slightly darker shade of yellow or tan than the abdomen and has a smooth, glossy appearance. The abdomen is large and oval, occasionally featuring a faint, narrow stripe running lengthwise down the top surface. Sac spiders are not heavily covered in hair, giving them a sleek, almost translucent look compared to species like the wolf spider.
Distinctive Head and Leg Features
The legs provide a clear visual cue for identification. The first pair of legs is noticeably longer than the other three pairs, resulting in a distinctive, forward-reaching posture when the spider moves. These legs are slender and delicate.
The tips of the legs, particularly the tarsi and chelicerae (jaws), often display dark brown or black coloration that stands out against the pale body. Sac spiders possess eight eyes, arranged in two horizontal rows of four across the front of the cephalothorax. This eight-eye arrangement is a key feature distinguishing them from some common look-alikes.
The Identifying Sac Structure
Sac spiders are active, nocturnal hunters that do not rely on sticky webs to capture prey. The “sac” refers to the small, silken retreat they construct for daytime shelter, molting, or egg laying. This protective enclosure typically appears as a small, flattened tube or a loose, hammock-like cocoon.
These silken retreats are commonly found in sheltered locations both indoors and out. Indoors, they are often tucked into corners where walls and ceilings meet, behind framed pictures, or in the folds of curtains. The sac is made of thin, white silk and is generally about the size of a thumbprint or a small cotton ball.
Comparing Sac Spiders to Common House Spiders
Sac spiders are frequently mistaken for other species, particularly the Brown Recluse spider (Loxosceles reclusa). Sac spiders lack the distinctive “violin” marking found on the Brown Recluse’s cephalothorax. The most reliable distinction is the eye arrangement: sac spiders have eight eyes in two rows, while the Brown Recluse has only six eyes, arranged in three pairs.
Compared to common Cobweb Spiders (Theridiidae family), sac spiders are more robust. They also lack the extremely long, spindly legs characteristic of species like the Cellar Spider. Cobweb spiders are identified by their messy, tangled webs used for prey capture, whereas the sac spider is a free-roaming hunter.