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Phragmosis is any method by which an animal defends itself in its burrow, by using its own body as a barrier. This term was originally coined by W. M. Wheeler (1927), while describing the defensive technique exhibited by . Wheeler observed the positioning of specially modified body structures to block nest entrances, as exhibited in various insect species. The term phragmosis has since been further extended beyond just insects.
Examples of phragmosis are found in the order Frog (frogs and toads). Some species, such as Pternohyla fodiens and Corythomantis greeningi, have evolved a peculiarly casqued head adapted to protect the animal as it backs down a hole. Another example is the head-plug defense used by the aphid Astegopteryx sp., in which a banana-bunch shaped gall consisting of several subgalls is used as a barrier. Arguably, the most commonly observed phragmotic behaviour is within the ant family. The behaviour is displayed in numerous taxa such as Camponotus, Colobostruma, Crematogaster, Pheidole, Blepharidatta, Cephalotes pusillus, Carebara, Stenamma expolitum, in which the soldiers have unusually large, disc-shaped heads, which are used to block nest entrances against intruders.
Recent studies of Corythomantis greeningi, a casque-headed tree frog from semi-arid areas, have provided substantial information regarding the water economy associated with co-ossification of the head. Due to the arid environments of most casque-headed anurans, it has been proposed that head co-ossification, together with phragmotic behaviour confer protection against Dehydration. Upon further investigation, it has been found that cranial co-ossification contributes little to conservation of water, but instead has a primary role of defence. This type of Skull primarily acts to protect the animal against Predation, and in doing so, leads to an indirect enhancement of water balance within the body.
In the study conducted by Jared et al. (1999) and Navas, Jared & Antoniazzi (2002), C. greeningi demonstrated the ability to enter backwards and close the entrance with their heads, a behaviour termed 'experimental phragmosis'. The study found that while phragmotic behaviour does not provide a significant reduction in water evaporation, it is important for preventing desiccation. It was concluded that in C. greeningi, the co-ossified head likely evolved originally as a protective lid for phragmotic individuals, but does aid in reducing water permeability through the head.
In a study by Kurosu et al. (2005), of 173 examined, 90.8% were plugged, with no space among the guarding soldiers. Of the 90.8% plugged , several male intruders were found outside the phragmotic plug, attempting to enter. All intruders were blocked by the guarding soldiers, and it was nearly impossible to enter the subgall.
Astegopteryx soldiers effectively defend their subgall by plugging the ostiole nearly completely with their sclerotic, spiny heads, which are very likely to have evolved for that purpose.
Phragmosis in ants has evolved independently in the diverse ant genera Carpenter ant Mayr ( Hypercolobopsis), Colobopsis Mayr, Cephalotes Latreille, Colobostruma Wheeler ( C. leae), Crematogaster Lund ( Colobocrema), Pheidole Westwood ( P. colobopsis, P. lamia), but also in other genera, such as Blepharidatta Smith, ( B. conops), Tetraponera Smith ( T. phrag- motica) and Carebara Westwood. The behaviour is most developed in the genus Cephalotes, where all castes (both Queen ant and Worker ant), have highly adapted head morphologies. The shield-like armor which characterizes this behaviour enables plugging of nest entrances, without exposing eyes, antenna or mandibles to any potential intruders.
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