When observing spiders, a common question is why they sometimes lift the rear section of their body, an action that appears deliberate. This posterior body part, correctly termed the opisthosoma, is joined to the main body (cephalothorax) by a slender, flexible stalk known as the pedicel. This anatomical feature allows the opisthosoma a significant degree of independent movement. The act of raising the opisthosoma is a distinct behavior that serves several biological functions, ranging from self-defense and communication to simple mechanics.
A Posture of Warning and Defense
When a spider raises its abdomen, it is often a defensive or threat display, also known as deimatic behavior. This posture signals that the spider feels threatened and is preparing to defend itself. By lifting the opisthosoma, the spider attempts to appear larger and more intimidating, discouraging an attack.
This defensive posturing is frequently paired with the lifting of the front legs and the exposure of the chelicerae, which contain the fangs. For example, in tarantulas, the raised abdomen can precede the flicking of irritating urticating hairs at a threat. This combination of raised body and exposed fangs serves as a final warning before the spider resorts to biting.
For some species, the underside of the raised legs or the opisthosoma may exhibit bright, contrasting colors that reinforce the message of danger. This sudden flash of color is designed to startle a potential predator and buy the spider time to escape. The goal of the entire display is to resolve the conflict without physical engagement.
Signaling During Courtship
The raised abdomen is also a specific form of communication used by male spiders during courtship rituals. In this reproductive context, the movement signals species identity, fitness, and non-aggression to a potential mate. The accompanying movements and overall context distinguish this behavior from a threat display.
Male jumping spiders, such as peacock spiders, perform elaborate dances where they rhythmically move their brightly colored opisthosomas to impress a female. The female often judges the visual display of the abdomen’s colors and the quality of the dance before accepting the male for mating. These movements may also involve vibrations and bobbing that send signals through the substrate or the web.
For other spiders, abdominal movements are part of a complex pattern of vibrations transmitted through the female’s silk web. This signaling prevents the female from mistaking the male for prey and allows him to approach safely. In some jumping spider species, this bobbing is also used in encounters with other spiders for intraspecific communication.
Facilitating Silk Production
A functional reason for lifting the opisthosoma relates directly to the spinnerets, the silk-extruding organs located at the posterior end of the abdomen. The flexible pedicel allows the spider to precisely position the spinnerets for various tasks. Raising the abdomen is often necessary for the accurate placement of silk threads.
Spiders use this mobility when constructing a web, laying down an anchor point, or creating a dragline for safety. This elevation allows the spider to apply the liquid silk to a surface at an optimal angle, ensuring proper adhesion and tension.
The raised opisthosoma is also a component of “ballooning,” a method of dispersal used by small spiders and spiderlings. To take flight, the spider climbs to a high point, raises its abdomen, and releases strands of silk into the air. The fine silk is then caught by air currents, carrying the spider away to a new location.