Blue-rump Cockroach - Melanozosteria sp.
The Melanozosteria genus is according to Dr D Rentz (Australian Blattodea Authority) a large difficult group to identify; containing many undescribed species in the Blattidae family, which are often darkly coloured with limited markings. You would think the blue rump would be a distinguishing feature, but this is not necessarily the case, as the color is formed by an opaque mucous-like adhesive substance, overlying the dark blue/black exoskeleton and changing it to a lighter blue, so other species may be similarly coloured. I have called this roach by this common name purely to more clearly separate it from the many other unidentified species shown in this blog.
The opaque mucous-like adhesive substance (not present in all species) was brought to my attention by David Rentz, who remarked that he had seen a spider attack a cockroach with this rear-end coating, only to have it quickly retreat with gummed up mouth parts, thereby indicating an effective defence mechanism. In the paper by Oliver Betz and Gregor Ko¨lsch called ‘The role of adhesion in prey capture and predator defence in arthropods’ they state “Among the diverse epidermal glands of cockroaches, there are at least two sets that produce a defensive secretion……. The gland tissues are located dorsally in abdominal segments…. and on the cerci. Predators are mechanically impeded by the viscous secretion, which forms long threads that usually break on the side of the cockroach and stick to the predator…….. The secretion consists of 90% proteins (dry mass) and is not toxic…..” The on-line paper available here.
The mature cockroach above with the blue rump was around 2 cm (3/4”) in head/body length, whilst the juvenile brown ones were about 25% smaller. I encountered these insects in mallee heath, 140 km NW of Esperance in a shallow sandy loam over gravel. Similar looking juveniles were seen in broken granite at the base of a large rocky outcrop 140 km NE of Esperance, which is 230 km (140 miles) due east of the above sighting. So possibly the Blue-rump Cockroach is a common inland species of gravel/rocky habitats.