Sheet webs are a distinct and effective strategy for capturing prey, differing significantly from the more commonly recognized spiral orb webs. These intricate silk constructions offer a unique approach to insect trapping, revealing much about the spiders that build them and their survival in various environments.
Characteristics of Sheet Webs
Sheet webs are flat, dense, and often horizontal or slightly domed silk structures. Unlike orb webs, they typically do not feature sticky spirals; instead, their primary function is to act as a barrier or trap, forcing insects to fall onto the sheet.
These webs often include an upper layer of loose, crisscrossing threads, known as knockdown or tangle lines. When an insect encounters these threads, it is disoriented and falls onto the main sheet below. The main sheet itself is sometimes non-sticky, relying on the insect’s confusion and the spider’s rapid response. The spider typically positions itself underneath the sheet, waiting to detect vibrations from trapped prey. This design allows the spider to remain hidden while its web actively collects food.
Key Sheet Web Spider Families
Several spider families construct sheet webs, each with unique variations.
Linyphiidae (Sheet Weavers or Dwarf Spiders)
One of the largest spider families, Linyphiidae are generally small (1-8 mm). They build horizontal sheet-like webs often situated near the ground or in low vegetation. Linyphiidae spiders often hang upside down beneath their webs and capture prey by biting directly through the silk. Some species create distinctive web forms, such as the “bowl and doily” web (an inverted dome above a flat sheet) or the “hammock” web (a concave sheet). Many Linyphiidae species do not use sticky silk, instead relying on prey becoming tangled or the spider’s quick movements.
Agelenidae (Funnel Weavers or Grass Spiders)
Agelenidae construct notable sheet webs characterized by a flat sheet narrowing into a funnel-shaped retreat, where the spider waits. These spiders are medium to large (4-20 mm). When prey lands on the sheet, the funnel weaver quickly dashes out from its retreat to capture it. Their webs are often non-sticky, relying on exceptional speed to subdue prey.
Hahniidae (Dwarf Sheet Spiders)
Hahniidae are even smaller, usually around 2 mm. They build delicate sheet webs that often lack a distinct retreat. Their silk is so fine it can be difficult to see unless covered in dew. A distinguishing feature is the arrangement of their six spinnerets in a transverse row, with the outer spinnerets being notably long.
Habitat and Hunting Strategies
Sheet webs are commonly found in various natural settings, often close to the ground. They frequently appear in low vegetation, tall grasses, shrubs, corners of structures, or under rocks and boards. This placement maximizes the chances of intercepting ground-dwelling or low-flying insects. In certain environments, particularly during periods of heavy dew, these webs become highly visible, revealing their widespread presence.
The hunting strategy for sheet web spiders varies depending on the specific family and web design. For spiders that reside beneath their sheet, like many Linyphiidae, prey falling onto the web is detected by vibrations. The spider then bites through the web to paralyze the insect before pulling it down. Funnel weavers wait inside their funnel-shaped retreat; when vibrations signal prey on the sheet, the spider rapidly emerges to capture the insect and drag it back into the safety of the funnel for consumption. This reliance on a physical barrier and swift attack makes sheet webs effective trapping devices in diverse habitats.