What is "phragmosis"

n. Positioning behind a barrier (or down a hole, etc.) as a means of defense

Phragmosis is any method by which an animal defends itself in its burrow by using its own body as a barrier. An example can be seen in the mygalomorph spider Idiosoma nigrum (Black Rugose Trapdoor Spider), which protects itself in its burrow by positioning itself so as to block the burrow with its abdomen, which is unusually hard. Another example is the shield of Cyclocosmia species ( Ctenizidae). It is also commonly seen in some ant genera, in which the soldiers have unusually large, disc-shaped heads, which are used to block nest entrances against intruders.