Hydrostatic Pressure

Hemolymph, especially in softer bodied forms, such as caterpillars, may work synergistically with the musculature of the body wall as a hydrostatic skeleton. Paralyzing the muscles causes the bodies of soft-bodied insects to become flaccid. For normal tonus, muscles contract against the skeleton of the body wall. As the muscles increase their tone squeezing the hemocoel and its enclosed hemolymph, hydrostatic pressure within the cavity increases by Pascal's principle, in that the increase in hydrostatic pressure transmits undiminished to every part of the volume and to the walls. The pressure within the body depends upon the volume of fluid hemolymph, muscular forces, and resistances to flows of hemolymph within the body. Some insects may even achieve a sub-atmospheric pressure within their hemocoels. Sometimes this pressure is so far below atmospheric pressure that air and fluid may enter an insect following puncture wounds in the cuticle. This poorly understood finding, even occurring when small volumes of hemolymph are present, may have something to do with how rapidly the heart beats. Increased ventilatory movements transporting air into and out of air sacs associated with the tracheae may contribute to these differences in pressure (Ref: Negative Pressures in Hemolymph).

As an insect molts or emerges from a larva or pupa, hemolymph under muscular pressure flows into spaces in the wings and appendages expanding them into their adult forms. Localized regions of increased hydrostatic pressure assist with movements of the body to expand the proboscis. Together with local mechanisms hydrostatic pressure can swiftly autolyse a damaged antenna, leg or wing. In mosquitoes, localized movements of the pharynx and hydrostatic pressure activate a hatching spine of cuticle that pokes a hole in the eggshell to release the larva.

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