Soil burrowing cockroaches (Blattodea : Blaberidae : Geoscaphinae) are unique to Australia. They live in sandy soils in permanent undergound burrows and feed on dry leaf litter. Of the 19 described species, Geoscapheus dilatatus has the second widest distribution. It occurs in discontinuous populations throughout eastern South Australia, western Victoria and NSW, and the south-western corner of Queensland in localities with Callitris (native pine) being the predominent tree species.
Most if not all insects have an outer waxy layer on their cuticle much of which comprises normal, branched and unsaturated long-chain hydrocarbons, and different species frequently have their own characteristic blend of these components. GC/MS of these hydrocarbon profiles can be used as a chemotaxonomic character for the recognition of species which are otherwise hard to resolve. The technique has proved useful for termites (close relatives of cockroaches), a difficult order taxonomically; where new species have been described on the basis of cuticular hydrocarbons [1].
The hydrocarbons were obtained by an external wash of the cuticle with hexane followed by mini-column chromatography on silica gel to remove non-hydrocarbons. Saturates and unsaturates were separated by argentation chromatography in silica gel/silver nitrate. Hydrocarbon profiles were obtained by GC on a methyl-silicone capillary column with temperature programming, and component structures elucidated from EI-MS. Double bond positions were located by GC/MS of dimethyl disulphide derivatives which cleave at the position formerly occupied by the double bond.
The cuticular hydrocarbons of 23 geographic populations have been analysed and most fall into 2 distinct groups. Though these groups are somewhat variable, a clear difference is the presence of unsaturated components in one and not in the other; the latter also tends to contain a series of very long chain components (C40-C50) which is generally lacking in the other. These hydrocarbon phenotypes correlate with morphological differences; the group with unsaturates tend to be smaller and the males have tubercles (prominences on the anterior edge of the pronotum) not present in the other group, they also occur in the southern part of the known range of G. dilatatus. Both forms are found together at Moonie in southern Queensland. Three populations appear to be atypical for both morphology and hydrocarbon composition.
The presence of two hydrocarbon phenotypes in conjunction with morphological differences may indicate that these should be regarded as separate species, while the status of the other populations is unclear.