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> The latest research has uncovered more of the mechanisms used in travelling so far to one specific spot in Mexico and back to the north:
> 
> https://www.annualreviews.org/doi/full/10.1146/annurev-ento-010814-020855
> 
> "Abstract
> Studies of the migration of the eastern North American monarch butterfly (Danaus plexippus) have revealed mechanisms behind its navigation. The main orientation mechanism uses a time-compensated sun compass during both the migration south and the remigration north. Daylight cues, such as the sun itself and polarized light, are processed through both eyes and integrated through intricate circuitry in the brain's central complex, the presumed site of the sun compass. Monarch circadian clocks have a distinct molecular mechanism, and those that reside in the antennae provide time compensation. Recent evidence shows that migrants can also use a light-dependent inclination magnetic compass for orientation in the absence of directional daylight cues. The monarch genome has been sequenced, and genetic strategies using nuclease-based technologies have been developed to edit specific genes. The monarch butterfly has emerged as a model system to study the neural, molecular, and genetic basis of long-distance animal migration.
> 
> "The current review emphasizes the rapid pace of advances over the last five years. The following were key discoveries: The monarch time-compensated sun compass is bidirectional, because it is used during both the migration south and the remigration north. Anatomical and electrophysiological approaches have been used to define the central complex as the site of the sun compass.... Monarchs can use a light-dependent, inclination magnetic compass as an additional orientation system during migration. The evolutionary history of the migration has been illuminated.
> 
> ***
> 
> "Migrants may rely on various compass senses, alone or together with a map sense, for navigation during migration. A considerable body of work demonstrates that migrant monarchs possess two compass senses: a time-compensated sun compass  and an inclination-based magnetic compass . These compasses allow migrant monarchs to use reliable environmental cues, such as the sun's position in the sky and the inclination angle of the Earth's magnetic field, to fly in the appropriate migratory direction. Migrant monarchs may use these multiple modalities for directionality, such that the compasses complement each other. One compass sense might fine-tune the other, or one compass system might be a backup for a dominant compass sense.
> 
> "Monarch butterflies are diurnal and predominantly use daylight cues for orientation during migration using a time-compensated sun compass. Specifically, the sun's azimuthal (horizontal) position in the sky is the dominant source of directional information during both the southward fall migration and the northward spring remigration . When the sun is obscured but some blue sky is visible, skylight polarization patterns resulting from scattered sunlight may be a cue providing directional information. Because the sun's position in the sky constantly shifts throughout the day, migrant monarchs use a circadian clock mechanism to adjust their flight relative to shifting skylight cues, such that their sun compass is time compensated.
> 
> ***
> 
> "A startling discovery of the time-compensated sun compass system was the finding that skylight directional cues sensed by the eyes and relayed to the central complex region are time compensated via circadian clocks located in the antennae, and not in the brain.
> 
> ***
> 
> "The monarch butterfly appears to have originated in North America and had a migratory ancestor, with the migration dating back at least one million years. Approximately 20,000 years ago, after the last glacial maximum, the North American population expanded and the migration extended further northward from Mexico to ultimately fill out its current northern range."
> 
> Comment: highly complex article, hard to condense. Epigenetic possibilities are discussed in the development of the controlling genetics. And we should remember, they metamorphasize through four generations and still carry the same genetic information.