The Abiotic Environment

(wind-dependent or wind-independent, feeding en route or proceeding directly). For exam- 681

ple, the swarming flights of social insects, such as ants and termites, involve only a fraction of a colony's population, may be completed in a matter of minutes, and may take the migrating individuals only a few meters from the original colony. In contrast, the migrations of locusts are undertaken by all members of a population and may cover several thousand kilometers. The migrations extend over a number of weeks and are interspersed with short periods of feeding activity.

The nature of the navigational cues used by migrating insects is not well understood. Species that are wind-dependent respond to weather disturbances in which temperature, wind speed and direction, and light intensity may change rapidly. In the "correct" combination these variables trigger a mass take off, and the direction taken is wind-determined. When the combination changes (e.g., at lower temperatures and wind speeds), the swarm will land.

Species that are wind-independent, that is, migrate within the "flight boundary layer" under their own power, require an orientation system in order to reach a predetermined goal. For some diurnal species, a light-compass reaction, similar to that used by bees, exists. Other diurnal migrants may use the earth's magnetic field or local landmarks as cues (Srygley and Oliveira, 2001). On the basis of somewhat tenuous evidence, it has been suggested that the well-known migrant, the monarch butterfly (see below), might use geomagnetic cues for orientation. However, recent experiments appear to have shown this suggestion to be unwarranted; rather, the monarchs use a light-compass system (Mouritsen and Frost, 2002). Day-flying species that rely on wind currents for their movements typically also show positive phototropism. Their upward flight takes them out of the boundary layer (the shallow layer of relatively still air adjacent to the earth's surface) and into horizontally moving airstreams. Such insects also may make use of thermals, columns of warm rising air, to gain height.

Insects from a wide range of orders, but especially Ephemeroptera, Trichoptera, Diptera, and Lepidoptera, show nocturnal flight activity that can be correlated with the phase of the moon. Such species may use polarized light reflected from the moon's surface for navigational purposes. For other nocturnal migrants that show a common orientation even under dark and completely overcast conditions, other cues have been suggested, including the earth's magnetic field, infrared energy perception, and stellar navigation (Danthanarayana, 1986).

An important component in migration, whatever cues are used for direction finding, is the ability to compensate for varying conditions, notably the changing position of the sun or moon and crosswinds that may blow the insects off course. Thus, an integral part of the light-compass response and possibly astronavigation is the ability to time-compensate. To compensate for wind-drift effect, insects appear to use a ground reference mechanism (estimation of the relative motion of landmarks beneath their flight path), which causes them to turn into the wind until the cross-track component of their upwind speed is equal and opposite to the crosswind drift (Srygley and Oliveira, 2001).

5.2.1. Categories of Migration

Johnson (1969) suggested that all forms of migration may be arranged in three major categories, though there is gradation both within and between each of them. In the first category are included species that as adults, migrate from the emergence site to a new breeding site where they oviposit, then die. Johnson includes in this group species such as the

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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