Neurology of Insect Migration and Navigation
Their Behaviors and Neurologies are More Complex Than Realized
Oct 24, 2009
Insects have complex neural mechanisms that guide them from location to location. These include centers that stimulate migratory behavior, provide a sun-compass to tell direction, store a map, and neural pathways that help correlate the compass with map location.
Stimulating Seasonal Behaviors
Seasonal photoperiod affects centers in the optic lobes of the brain that control annual rhythmic cycles, and elicit migratory or sedentary behavior. Similar processes occur in the neural visual pathways in the brains of most other animals, but the centers have different locations. In vertebrates, this center is the pineal body of the brain. In monarch butterflies (Danaus plexippus), long days foster both mating and sedentary behaviors, and shortening days suppress mating while triggering migratory behavior.
The Time Sense of Insects
Insects able to view the sun or polarized light can utilize a time-corrected compass providing direction. Honeybees use this sense to return to the hive and locate food sources from information provided by returning foragers. In the monarch butterfly, this compass is located in the antennae, with neurons connecting the antennae to the optic lobes of the brain. If the antennae are painted black or removed, these butterflies cannot orient their flight direction toward the winter roosts.
Wind Propulsion
Dragonflies migrating across the Indian Ocean use the monsoon winds to propel them up to 900 miles (1500 km) across the ocean. These insects also must have a map and use their sun-compass to inform them of direction. Dragonfly antennae are much smaller than butterfly antennae, but it is still possible the compass is located there.
Detecting Where to Land
Although insects have poor vision, they can see large land masses and detect odors at large distances. Silkworm males are able to detect mating pheromones some 5 to 7 kilometers downwind from an advertising female, and both honey and bumble bees inform nest mates the identity of food flowers by the scent that lingers on returning foragers.
Using Wind Currents to Aid Migration
When making local movements over land, insects can fly directly into quite strong winds to move to feeding or roosting areas. Insects weighing less than three grams are capable of crossing 500 to 900 miles (800 to 1500 km) of open ocean without stopping. Although they store fat, they cannot store enough calories to make powered flights of this length. Instead, they fly to altitudes where they find winds moving in the preferred direction and soar on these winds to their next stopping point.
Warming Up to Fly
Insect muscles must be warmed to about 95º F (34ºC) to allow them to fly. Yet, dragonflies, bees, and butterflies may be seen flying when temperatures are below 50ºF (10ºC). They raise their muscle temperatures by thermal regulation and shivering.
Using the Same Trees to Roost
Monarchs spend the winter in the same eucalyptus (California) or oyamel (Mexico) trees year after year. Even more dramatic is that these butterflies will roost on the same trees in specific groves during their migratory journey. The location of these trees is thought to be transferred from generation to generation, but which organ houses this map or what information it contains has not been determined.
The information might be as simple as “rest in a tree producing such and such a chemical near water, and winter in a tree producing other chemicals at a specific elevation.” Or it could be that they roost in whatever trees are available near high quality food sources. But this does not account for the butterflies' habit of bypassing hundreds of groves of trees with similar species composition, food species availability, location, and elevation. Nor does it account for monarchs using one suite of quite dissimilar tree species for roosting in northern latitudes, and a completely different suite of species in southern latitudes.
Although much has recently been found concerning how insects find their way, there are many questions still unanswered. Among these are:
- Why do monarchs in the Northeast often congregate in the tens of thousands move to coastal dunes and feed on seaside goldenrod (Solidago sempervirens) while other tens of thousands stop in salt marshes and feed on Eastern groundsel bush (Baccharis halimifolia) - even though seaside goldenrod may be available at the same site? It may be possible that the butterflies obtain some essential nutrients feeding in groundsel then continue to the dunes to obtain other nutrients from the goldenrod before continuing their migration, but this type of local movement has not been reported.
- Why do monarchs in the Midwest bypass large colonies of flowering asters (a primary food source for this region) to descend en masse on nearby colonies of the same species?
Answering questions like these will take some time, but they are important factors to take into consideration as our tendency to develop “wastelands” continues. If these factors are not heeded, insect migration paths will continue to be disturbed – possibly to the point where these important members of our environment are no longer able to make their grand scale migrations.
Other information on insect migration is on page 1.
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