An extraordinary cricket (Brachytrupes membranaceus)
By Prof. Giovanni Costa, University of Catania
Among the research carried out within the long-term project “Behavioural adaptations of desert animals”, by collaborators and me since 1983, the giant cricket Brachytrupes membranaceus (Drury) certainly deserves special consideration. We have studied a population of this insect inhabiting the Gobabeb area within the Namib Desert, at a very short distance from the local Training and Research Centre about arid ecosystems and their diversity, located 120 km south-east of Walvis Bay.
Several individuals are also found in the dry bed of the Kuiseb River. This presence in a place that is occasionally flooded due to heavy rain in the Khomas Hochland to the west of Windhoek had surprised us at first. But we quickly discovered that, like its sister species Brachytrupes megacephalus living above the Saharan Africa and in the south-western Europe, this species also has the ability to easily float and swim. The swimming technique is exceptional. The propulsion is mainly generated by violent backward strokes of the cricket hindlegs; at the same time, the forelegs enter the water following a "keyhole" trajectory, as in our "butterfly" swimming style, and the midlegs horizontally perform an undulation similar to our breaststroke. The rate of water progression of this "terrestrial" animal is impressive and makes us understand that no sudden flood of the Kuiseb River represents an insurmountable obstacle to its survival.
But let's get to the reproductive behaviour of this cricket, which is no less sensational. A brief reconstruction of the insect cycle. Having one generation a year, the adults die between February and April, which is at the end of its reproductive season. The post-embryonic development takes place between March and November: in this period, these insects carry out a very intense food activity. Then, at the beginning of the rainy season, they hibernate individually remaining inside their burrow. This refuge is a cylindrical and curved tunnel 80-100 cm long, 2.5-3.0 cm large, and 50-70 cm deep. The burrow entrance is plugged with a sandy layer. After the hibernation period, each cricket at sunset removes the plug and re-enters in contact with the outside world. Each female waits inside its tunnel near the sand surface. Each male, using its mandibles, digs a shallow semi-oval depression, which will serve as a resonance chamber. In fact, immediately afterwards, it will position himself in its chamber with their head turned inside their tunnel and the abdominal end turned outwards, and will begin to carry out its ear-piercing stridulating. The powerful sound produced by the rubbing of the tegmina (forewing serving to cover the hindwing in grasshoppers and related insects), amplified and directed by the resonance chamber, can propagate up to over 1 km away with the aim of attracting some female.
If after 1-2 hours no females respond, the male returns into its tunnel, closing it with sand. This plug will be removed the following sunset to make another attempt. When a female arrives near the burrow of a male, the latter stops its singing and enters his tunnel followed by the female. Then mating takes place: insemination is very fast and consists in the application of a spermatophore in the female genital tract. After 15-50 minutes, the male comes out and resumes stridulating to attract other females. At the end of his lucky attempts, the male closes his tunnel definitively. A curious thing: the females entered the male tunnel (in one case we counted three in the same evening) do not come out after copulation. Each of them it is sealed by the male with sand in a secondary small branch of its tunnel. This male is it a sort of Landrù in the world of insects? Not at all. The behaviour of the male is that of an animal very interested in the protection of his partner and his offspring. In fact, if a predator were to enter the male tunnel, it would only find the owner, but not its fertilized females. The latter, on the other hand, after having laid the fertilized eggs, are free to leave the tunnel in which they were housed a few days after. And, why not accept the advances of some other male?
In conclusion, this cricket provides us with numerous evidences of how essential it is to acquire important eco-ethological adaptations to be able to survive under harsh conditions. The excellent swimming abilities, the accentuated seasonality, and above all the reproductive promiscuity provide this animal with an excellent chance to win its battle for the preservation of its species. The B. membranaceus population living in the Namib Desert should not be at particular risk unless global climate change alters environmental conditions excessively. However, the populations of this phytophagous species that have colonized agricultural areas (for example, in Angola) have been exterminated as they are considered a pest. The same fate, on the other hand, has been reserved for the sister species B. megacephalus, once very abundant in the coastal dune systems of Sicily and now virtually disappeared.