Mysterious Migration

Wandering Glider, Pantala flavescens

Fall migration season is upon us, and that usually conjures up thoughts of songbirds and hawks using the central flyway through Texas to make their way south to the subtropics and tropics for the winter.  However, birds are not the only ones who migrate, and while much has been said about the complex, annual migration made by Monarch butterflies, the record for the longest insect migration (twice the distance of the Monarch) belongs to a dragonfly species, the Wandering Glider (Pantala flavescens).  In fact, dragonfly migration has been suspected for over 100 years, and up to 50 of the world's 5,200 dragonfly species are thought to migrate (about 16 out of 326 in North America), but not much is known about where they are coming from or where they are going.  

Green Darner, Anax junius

In Texas, there are several species of dragonflies that migrate in addition to the Wandering Glider. They include the Common Green Darner (Anax junius), Black Saddlebags (Tramea lacerata), Variegated Meadowhawk (Sympetrum corruptum), and Spot-winged Glider (Pantala hymenaea).   Cooler nights seem to trigger the dragonflies' journey south, and like birds, they build up their fat reserves before setting out.  They may use the lay of the land as a navigation guide, and some scientists speculate that they have an internal magnetic compass, as those that fly off course and out to sea appear to realize their mistake and reorient themselves.  

Black Saddlebags, Tramea lacerata

Dragonflies migrate during daylight hours, and green darners have been found to break their journeys every three days to rest and feed, using oak and juniper trees as stopover sites.  Like monarchs, the full migration circuit takes multiple generations to complete, as it is the offspring of the generation that traveled south in the fall that is migrating north again in the spring.  

Variegated Meadowhawk, Sympetrum corruptum

Swarms of dragonflies can create one of nature's most impressive spectacles, with tens to hundreds of thousands of individuals streaming southward along lakeshores, mountain ridges, and coastlines.  Even with the origins and destinations poorly known, the migration in the fall is more noticeable than that in the spring, presumably because the spring event occurs over a wider front and a longer period of time.  However, migration is the only explanation for how dragonfly adults appear in early spring in places where their nymphs or larvae from the previous year or years have not yet emerged.

Spot-winged Glider, Pantala hymenaea

The ecological role of migrating dragonflies is another facet of the mystery yet to be resolved. Since several species use the same migration strategies and timing as migratory birds, traveling at the same times and concentrating in the same places, it is likely that certain bird species are exploiting the abundance of dragonflies as a source of fuel for their own migration.  More research is being done to solve these mysteries, most notably the Xerces Society’s Migratory Dragonfly Partnership initiative, which uses “research, citizen science, education, and outreach to understand North American dragonfly migration and promote conservation.”    

Traveling Tarantulas

Tarantula

Relatively common throughout Texas, the Tarantula (Aphonopelma sp.) is Texas’ heaviest and largest spider.  Typically, the head and legs are dark brown, and the abdomen is brownish-black. Coloration varies between individuals as well as between the 14 different species found in Texas. Identification of individual species is difficult, however, and is often performed only on mature males under a microscope.

Tarantulas are typically found in grasslands and semi-open areas, and use burrows, natural cavities under stones or fallen logs, spaces under loose tree bark, and even old rodent holes as shelters.  They are also capable of digging their own burrows, and often line them with webbing, placing a few strands across the front to help detect passing prey.  Laying several hundred eggs in a hammock-like web constructed inside the burrow, females will guard them until they hatch. Females have lived in captivity for over 25 years, while males rarely live over two or three months after reaching maturity.   

Like many animals, tarantulas molt their exoskeletons several times as they grow.  The skin on the hard upper shell and abdomen splits, and the tarantula begins the process of squeezing through the opening.  Most of the time, tarantulas molt while positioned on their backs, twitching, stretching, and kicking until the entire exoskeleton has been cast off.  After they have wriggled free of their old skin, this discarded exoskeleton is a perfect replica of the spider, minus its head and fangs. 

Other insects such as crickets, beetles, grasshoppers, cicadas, and caterpillars form the basic diet of the tarantula.  They inject their prey with a poison when they bite, which liquefies the prey’s insides, making it easier to ingest.  While they can climb, they are usually restricted to the ground, where the majority of their prey is found.  The hairiness and large size of tarantulas often evokes concern, but the bites of Texas species are not serious to humans.  Tarantulas maneuver quickly to face whatever disturbs them, often raising up on their hind legs and stretching out their front legs in a threatening posture.  They have also been observed rapidly brushing the top of their abdomen with their hind legs to dislodge hairs that can be used to irritate the attacker’s eyes or skin.

For a few weeks late in the summer or early fall, one of the most spectacular spider events occurs in Texas.  Not well understood, this phenomenon is often called a migration, but it may be related more to mating rather than seasonally motivated movement.  Males actively wander to seek out females, and can travel 50 miles in search of a mate.  Populations seem to follow a boom and bust cycle, depending on weather patterns and the availability of food, but a good year can be a sight to behold if summer rains have been plentiful.  While the males are out searching, females wait in their burrows for a suitor to appear.  Larger and more robust, the female does not always accept any male that comes along, and will kill and eat males that are deemed unsuitable. 

Tarantula Hawk

As formidable as they may seem, tarantulas are not without their own enemies.  In fact, in late spring and early summer they are routinely hunted by female Tarantula Hawks as food for their larvae.  Belonging to a group of spider wasps in the genera Pepis and Hemipepsis, tarantula hawks are large, 2-inch long wasps with iridescent blue-black bodies and bright, rust-colored wings.  This vivid color combination is a form of aposematism or warning coloration, a type of advertising signal to both predator and prey that these species are potentially harmful.  These wasps have the ability to deliver a powerful sting, and their long legs have hooked claws for grappling their victims.  

Flying low over the ground, the female tarantula hawk will find a tarantula and sting it, which paralyzes the spider but does not kill it.  She then drags the inert tarantula into her burrow or transports it to a specially prepared nest, where she lays a single egg on the spider’s abdomen, then seals the opening to the burrow as she leaves.  When the wasp larva hatches, it creates a small hole and enters the spider’s abdomen, where it feeds voraciously, avoiding vital organs to keep the spider alive as long as possible.  After several weeks the spider dies, the larva pupates, and then it emerges from the spider’s abdomen to continue its lifecycle.