Schistocerca Information Site


BIOGEOGRAPHY

Cyrtacanthacridinae, the subfamily to which Schistocerca belongs, is distributed throughout the Old World with major diversity in Africa. The subfamily contains about 35 genera and is well-characterized by mesosternal lobes that are vertically longer than their width, and by robust flexure connecting basal and apical valves of aedeagus. There are only two genera occurring in the New World , Schistocerca and the Galápagos endemic, brachypterous Halmenus (Dirsh 1969).
Schistocerca displays an unusual transatlantic distribution. Of about fifty species in the genus, only one species, the desert locust, S. gregaria, occurs in the Old World and the rest of the species of the genus occur in the New World. These two opposite patterns of transatlantic distributions have generated considerable controversies among scientists since they were first identified in the late 19 th century (Scudder 1899). Below I summarize three competing biogeographic hypotheses concerning distribution of Schistocerca .


In the map, green represents the distribution of the New World Schistocerca and red represents that of S. gregaria. Green terminals in the phylogeny represent the New World Schistocerca species while the red terminal with a square represents S. gregaria (below).


*New World Origin Hypothesis* Earlier taxonomists considered that Schistocerca was essentially a New World genus (Scudder 1899, Uvarov 1923a, b). Although clearly a cyrtacanthacridine, Schistocerca is morphologically divergent from other members of the subfamily in the Old World (Uvarov 1923a, Dirsh 1974).
Because S. gregaria has a very similar morphology to its New World relatives, it was considered a migrant species originated from the New World (Dirsh 1974). The exact path of migration and colonization of the Old World , whether by land or over open ocean, is debatable, but this view of the ancestral desert locust originating from the New World and subsequently colonizing the Old World (to give rise to the present-day desert locust) is referred to as the New World Origin hypothesis. According to this hypothesis, the migration event by the ancestral desert locust must have happened after the genus had already diversified in the New World . This can be translated into a phylogeny in which S. gregaria is nested within the New World species (Fig. A).

*Old World Origin Hypothesis* In October 1988, there was a dramatic incident that radically changed the view of the origin of the desert locust. A large swarm of the desert locust originating from West Africa successfully crossed the Atlantic Ocean to reach the West Indies (Kevan 1989, Ritchie and Pedgley 1989).
This seemingly impossible flight was later postulated to have lasted only a few days, considering the energy required to achieve the continuous flight of 5000 km (Kevan 1989). There had been records of locusts taken at sea (Howard 1917, Waloff 1946), but this was the first publicized incident of a successful flight by a swarm. This incident was an effective demonstration of the locust transatlantic flight, and it serves as a basis for an alternative hypothesis of the origin of the desert locust. Ritchie and Pedgley (1989) and Kevan (1989) proposed an alternative hypothesis on the origin of the desert locust. They argued that the New World Schistocerca species are descendants of a “gregaria-like” ancestor from the Old World that crossed the Atlantic Ocean by flight. The diversity in the New World was therefore a result of single colonization event followed by an explosive radiation. This idea of the desert locust being ancestral to the New World species is referred to as the Old World Origin hypothesis. According to this hypothesis, the desert locust would be the remnant or descendant of the ancestor that gave rise to the New World Schistocerca. The hypothesis can be translated into a phylogeny in which S. gregaria is basal to the New World species, reflecting its being the ancestor (Fig. B).

*Multiple Crossings Hypothesis* The third hypothesis is a derivative of the Old World origin hypothesis, first suggested by Dirsh (1974), and later more explicitly developed by Kevan (1989). The Multiple Crossings hypothesis suggests that the diversity of Schistocerca in the New World could be a result of not a single colonization from the Old World to the New World , but rather of multiple crossings by the “gregaria-like ancestor.”
The 1988 flight is probably not the first time that the swarm of the desert locust crossed the Atlantic Ocean, and it is reasonable to think that it happened several times in the past. Amédégnato (1993) suggested that there were several species groups of Schistocerca in the New World, main ones being americana and nitens groups. Because they are sufficiently different, she believed that the nitens group descended from the earlier colonization event, and the americana group from the most recent colonization event. This hypothesis inherently assumes that the desert locust is the most recent ancestral stock that gave rise to the swarming species in the New World and that the New World species are polyphelytic (Fig. C).

So, which hypothesis is right? Currently, there are two independent studies available telling two very different stories. A phylogenetic analysis based on morphology (Song 2004) found the desert locust to be deeply nested in the New World species, thus supporting the New World Origin hypothesis. Another analysis based on DNA (Lovejoy et al. 2006), however, found the desert locust to be basal to the rest of the genus, thus supporting the Old World Origin hypothesis. It is difficult to say which is correct at this time, but I hope to resolve this conflict in near future.


References
Amédégnato, C. 1993. African-American relationships in the Acridians (Insecta, Orthoptera), pp. 59-75. In W. George and R. Lavocat [eds.], The Africa-South America Connection. Clarendon Press, Oxford.
Dirsh, V. M. 1969. Acridoidea of the Galapagos Islands (Orthoptera). Bulletin of the British Museum (Natural History) Entomology 23: 1-51.
Dirsh, V. M. 1974. Genus Schistocerca (Acridomorpha, Insecta). Dr. W. Junk B.V. Publishers, The Hague.
Howard, L. O. 1917. Schistocerca tartarica taken at sea. Proceedings of the Entomological Society of Washington 19: 77.
Kevan, D. K. M. 1989. Transatlantic travelers. Antenna 13: 12-15.
Lovejoy, N. R., S. P. Mullen, G. A. Sword, R. F. Chapman, and R. G. Harrison. 2006. Ancient trans-Atlantic flight explains locust biogeography: molecular phylogenetics of Schistocerca. Proceedings of the Royal Society of London B 273: 767-774.
Ritchie, M., and D. E. Pedgley. 1989. Desert locusts cross the Atlantic. Antenna 13: 10-12.
Scudder, S. H. 1899. The Orthopteran genus Schistocerca. Proceedings of the American Academy of Arts and Sciences 34: 439-476.
Song, H. 2004. On the origin of the desert locust Schistocerca gregaria (Forskål) (Orthoptera: Acrididae: Cyrtacanthacridinae). Proceedings of the Royal Society of London B. 271: 1641-1648.
Uvarov, B. P. 1923a. A revision of the Old World Cyrtacanthacrini (Orthoptera, Acrididae) I. Introduction and key to genera. The Annals and Magazine of Natural History (9) 11: 130-145.
Uvarov, B. P. 1923b. Notes on locusts of economic importance, with some new data on the periodicity of locust invasion. Bulletin of Entomological Research 14: 31-39.
Waloff, Z. 1946. A long-range migration of the desert locust from southern Morocco to Portugal, with an analysis of concurrent weather conditions. Proceedings of the Royal Society of London A. 21: 81-84.