Phragmosis

Introduction
Brandao et al. (2001) provide this excellent introduction to this topic: The term phragmosis was coined by W.M. Wheeler (1927), to describe a cryptic defensive technique employed by insects that use specially modified body structures to block nest entrances. Phragmotic-headed ants, for instance, prevent intruders from entering nests by blocking the entrances or by pushing them out of entrance galleries. Phragmosis has been described in several ants: Camponotus (subgenus Colobopsis and Hypercolobopsis), Colobostruma, Crematogaster (= Colobocrema), Pheidole (P. lamia), and Oligomyrmex (= Crateropsis) elmenteitae (Hölldobler and Wilson, 1990; and below).

Shield-like-headed soldiers and queens of the Neotropical myrmicine Cephalotes species of the pallens group appear to secrete a fibrous material from thousands of pores that resemble glandular openings, coating and eventually accumulating in a dense tangle of material over the head disk. This material is superficially similar to a mass of fungal filaments (mycelia), but does not have the branching pattern or internal structure indicative of a fungal origin (de Andrade and Baroni Urbani, 1999). In other ant genera, phragmotic structures develop through the accretion of environmental debris held by specialised hairs. These and other structural modifications related to phragmosis in ants have been discussed in detail by Wheeler and Hölldobler (1985).

Reversed phragmosis was described for the queen of Pheidole embolopyx, which has a peculiar posteriorly truncated gaster with a rear surface covered with unusual hookshaped hairs (Brown, 1967). Furthermore, the queen’s antennal scapes, anterior clypeal border, and frontal carinae are covered by gelatinous sheaths, varying in size and shape among individuals and on the same queen over time, and often absent altogether. Contrary to expectation, Wilson and Hölldobler (1985) show that P. embolopyx queens do not use the gaster to block nest entrances or otherwise to repel intruders directly, nor do the gelatinous structures seem to be especially attractive or repellent to the workers. Poldi (1963, apud Hölldobler and Wilson, 1990) described how the queen of the ponerine Proceratium melinum Roger plugs the nest entrance with her rounded first gastral segments.

Chapter 8 of The Ants also provides an overview of this phenomenon and some context regarding nest defense. Specialization by majors: defense

Examples
Wheeler and Holldobler (1985) investigated how Cepholates workers might collect or accumulate debri on their head disks. Such debri can be used as camouflage for individuals that are using their head to block the nest entrance, much like Basicerotini and Stegomyrmicini ant species affix dirt their bodies. They discovered Cephalotes pallens do in fact have encrusted material on their head disks. This varied from slight to extremely built-up material. Unexpectedly these covering materials appeared to be produced by the ants, rather than an accumulation of material that was affixed and somehow binded to the head disk. The presence of many pores, which are presumably attached to glands, are the likely source of secreted fibrous and filmy substances that were detected.

They also discovered: "Z. pallens (Figs. 4-6). To the eye, the head of the soldier shown in Fig. 4a appeared to be covered with a thin layer of dirt. Closer examination revealed that the surface was covered with irregular ribbons of material (Figs. 4b, 5a). Pores were visible along part of the central ridge (see Fig. 5b). Material seemed to be extruded in lumpy strands from a number of the pores (Figs. 5b, 6), supporting the suggestion that it is secretory in origin."