nymphs; and aggressive, courtship and copulatory sounds produced by adult males. Henschoutedenia epilamproides has a stridulatory organ between its thorax and abdomen, but the purpose of the sound produced is unclear. Several Australian species practice acoustic and vibration behaviour as an aspect of courtship. They have been observed producing hisses and whistles from air forced through the spiracles. Furthermore, in the presence of a potential mate, some cockroaches tap the substrate in a rhythmic, repetitive manner. Acoustic signals may be of greater prevalence amongst perching species, particularly those that live on low vegetation in Australia's tropics. Biology Digestive tract Cockroaches are generally omnivorous; the American cockroach (Periplaneta americana), for example, feeds on a great variety of foodstuffs including bread, fruit, leather, starch in book bindings, paper, glue, skin flakes, hair, dead insects and soiled clothing. Many species of cockroach harbor in their gut symbiotic protozoans and bacteria which are able to digest cellulose. In many species, these symbionts may be essential if the insect is to utilize cellulose; however, some species secrete cellulase in their saliva, and the wood-eating cockroach, Panesthia cribrata, is able to survive indefinitely on a diet of crystallized cellulose while being free of microorganisms. The similarity of these symbionts in the genus Cryptocercus to those in termites are such that these cockroaches have been suggested to be more closely related to termites than to other cockroaches, and current research strongly supports this hypothesis about their relationships. All species studied so far carry the obligate mutualistic endosymbiont bacterium Blattabacterium, with the exception of Nocticola, an Australian cave-dwelling genus without eyes, pigment or wings, which recent genetic studies indicate is a very primitive cockroach. It had previously been thought that all five families of cockroach were descended from a common ancestor that was infected with B. cuenoti. It may be that N. australiensis subsequently lost its symbionts, or alternatively this hypothesis will need to be re-examined. Tracheae and breathing Like other insects, cockroaches breathe through a system of tubes called tracheae which are attached to openings called spiracles on all body segments. When the carbon dioxide level in the insect rises high enough, valves on the spiracles open and carbon dioxide diffuses out and oxygen diffuses in. The tracheal system branches repeatedly, the finest tracheoles bringing air directly to each cell, allowing gaseous exchange to take place. While cockroaches do not have lungs as do vertebrates, and can