Hydrostatic skeleton

Definition

A structural support system in soft-bodied invertebrates in which body shape and rigidity are maintained by the turgor pressure of internal fluid against a flexible body wall, rather than by rigid skeletal elements. Muscle contractions generate and modulate this pressure, allowing extension, flexion, and locomotion. In arthropods, hydrostatic skeletons occur primarily in soft-bodied life stages and serve as an adjunct to the later-developing sclerotized exoskeleton.

Etymology

From Greek hydro- (water) + static (standing still, in equilibrium), referring to the pressurized fluid that provides structural support.

Example

Spider legs operate partly as hydrostatic skeletons: the hemolymph pressure within each leg segment resists collapse and enables extension, which is why a dead spider's legs curl inward when muscular contraction and fluid pressure cease. Many lepidopteran and dipteran larvae also rely on hydrostatic skeletons to generate the body rigidity needed for crawling before their cuticle hardens.

Synonyms

• hydroskeleton

Related Terms

• Exoskeleton
• Endoskeleton
• turgor pressure
• Hemolymph
• arthropod locomotion
• soft-bodied larva
• molting

Usage Notes

Distinguished from rigid exoskeletons by its reliance on fluid pressure rather than sclerotized cuticle; the two systems often function sequentially in arthropod development. Not all fluid-filled body cavities constitute hydrostatic skeletons—the term implies active muscular generation of pressure for mechanical work. Some authors reserve 'hydroskeleton' for structures like the penis or proboscis where fluid pressure drives specific movements, using 'hydrostatic skeleton' for whole-body support.