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

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

 Idiosoma nigrum

Idiosoma nigrum

Idiosoma nigrum, also called Black rugose trapdoor spider, occurs only in south-western Australia, in dry woodlands east of the Darling Scarp and north to Moore River.Females can reach a length of about 30mm, males about 18mm....

(Black Rugose Trapdoor Spider), which protects itself in its burrow by positioning itself so as to block the burrow with its abdomen

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

, which is unusually hard. Even more perfect is the shield of Cyclocosmia

Cyclocosmia

Cyclocosmia or "trapdoor spider" is a genus of spiders in the Ctenizidae family.The abdomen of spiders in this genus is abruptly truncated and ends in a hardened disc which is strengthened by a system of ribs and grooves. They use this to clog the entrance of their 7 to 15 cm deep vertical...

species (Ctenizidae). It is also commonly seen in some ant

Ant

Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

genera, in which the soldiers have unusually large, disc-shaped heads, which are used to block nest entrances against intruders.

Further reading

– ^{Scholar search} |archiveurl = http://web.archive.org/web/20070927020454/http://psyche2.entclub.org/articles/92/92-337.pdf |archivedate = 2007-09-27 | issue = 4}}