Do Monarch butterflies have a polarization compass?

Those insects, in which the polarization compass has been studied most thoroughly, i.e. honey bees, desert ants and crickets, use their navigational capabilities mainly for central place foraging or homing. Other insects, including a number of butterfly species, are well-known for their migratory capabilities, and most famous among them is the Monarch, Danaus plexippus (Fig. 1). The Eastern group of its North-American population makes an annual migration that takes them from Central Mexico to the Great Lakes and back. Individual butterflies may cover distances of up to several thousand kilometers. Although their migration is well-described, the sensory basis of this impressive navigational feat is only scarcely studied.

We are presently investigating the dorsal part of Monarch compound eyes for structural specializations indicating polarization vision (compare project Evolution of the insect polarization compass ). The first results of a histological study show that specialized polarization-sensitive ommatidia (POL-ommatidia) are indeed present in a narrow dorsal rim area of the eye (see Fig. 2). This strongly supports the view that Monarchs make use of a polarization compass.

For more information see references:

_{Labhart T, Meyer E P (1999). Detectors for polarized skylight in insects: a survey of ommatidial specializations in the dorsal rim area of the compound eye. Microsc Res Tech 47:368-379.}

_{Brower L (1979) Monarch butterfly orientation: missing pieces of a magnificent puzzle J Exp Biol 199:93-103.}

Recent link: www.MonarchWatch.org