Sessile

Definition

Permanently attached to a substrate and lacking the ability to move independently; describes organisms, life stages, or anatomical structures that are fixed in place rather than free-living or motile. In arthropods, often contrasts with vagile or mobile stages, and may refer to pupae, galls, scale insects, or other sedentary forms.

Etymology

From Latin sessilis 'fit for sitting', from sedere 'to sit'

Example

Scale insects in the family Coccidae are sessile as adults, with females permanently attached to host plant stems and protected by a waxy covering, while only the first-instar crawlers are motile.

Synonyms

• attached
• fixed
• stationary

Related Terms

• vagile
• sedentary
• planktonic
• benthic
• Crawler
• pupa
• gall

Usage Notes

Distinguish from the botanical sense (sessile leaves lacking petioles) and medical sense (polyps without stalks). In animal ecology, sessile contrasts with vagile (capable of movement); some authors reserve 'sessile' for permanently attached organisms and 'sedentary' for organisms that move little but are not fixed. Many insects have sessile life stages—pupae of some Lepidoptera, gall-forming larvae, and adult female scale insects—while retaining motile dispersal stages.