4.3.5 — Invertebrates

A wide variety of displacements among invertebrates is considered as migration by the respective research schools, harking to the broad definition given by Baker ([...] movement from one point in space to another, l.c. p. 1). For example, there are diurnal migrations of plankton, or studies of "migration" of parasites within their host. A recent book on "Insect migration" (Drake & Gatehouse 1995) treats species which emigrate as a consequence of population increase. The concept of "true migration" does not exist among entomologists, but Roer (1971) gave a clear definition of "Zuginsekten" which is close:

"Unter Zuginsekten sind Saisonwanderer zu verstehen, die ihren Bestand im Migrationsgebiet nur aufgrund jahreszeitlicher getrennter Hin- und Rückflüge zu halten vermögen." (l.c. p. 187).

"Migratory insects are seasonal migrants, which maintain their population within the migration area only by seasonally separated back-and-forth flights" (l.c. p. 187, translation by the author).

This definition contains the concept of a necessary "return ticket". In addition, one could differentiate between return flights by one individual, or performed by different generations. The latter raises the fascinating concept of "migrating genes", brought up with the study of "migrant bees".Brower (1995) postulates that both forms seem to co-exist in the monarch butterfly (Danaus plexippus). The Monarch - an "International Wanderer" (Urquhart 1960) - is the only invertebrate listed in CMS Appendix II, and probably the most popular migratory insect. The migratory behaviour of the "migratory locust" (Locusta migratoria) gave it its name. Together with other species, this forms "locust plagues", which must be considered as "mass evasions", and therefore do not fulfil the definition given by Roer. However, some grasshoppers do follow predictable paths (cf. Skaf et al. 1990), and the Rocky Mountain locust (Melanoplus spretus) once regularly crossed the Rocky Mountains in huge quantities, as shown by organic residues in glaciers (Lockwood et al. 1994). It became extinct at the beginning of the 20th century for unknown reasons.

Migrating invertebrates often occur in large quantities, and can trigger or herald migrations of other animals. Larsen (1992) showed that migrations of the African White Catopsilia florella heralded the arrival of the great herds to the Kalahari bushmen.

Further migratory insect species can be found among the ladybirds (Coccinelidae: Hippodamia convergens), the syrphids (Syrphidae) and the dragonflies (Odonata).

The scientific information on insect migrations in the broad sense is abundant, in part because of the economic aspects of mass evasions and invasions (pest control). However, there is much less information on "Zuginsekten" as defined by Roer (1971).

Crustacea and Cephalopoda
Among the other invertebrates, migrations are reported for lobsters (Crustacea) and squids (Cephalopoda). Lobsters of the genus Palinurus show a remarkable behaviour known as "row migration" (cf. Lohmann et al. 1995). Anecdotal reports of migratory squids are given by Squires (1957). Besides this short introduction, and the few species mentioned here, migratory invertebrates will not be covered by this volume of GROMS.

This document should be quoted as part of the publication "Riede, K. (2001): The Global Register of Migratory Species ­ Database, GIS Maps and Threat Analysis. Münster (Landwirtschaftsverlag), 400 pp." + CD

 by Klaus Riede