Migration Isn't Just For The Birds

 

Wandering Glider dragonflies do more than wander...

Most people who are interested in nature know that each spring many species of birds migrate north, and each fall they migrate south.  But birds aren’t the only animals that participate in this amazing natural phenomenon, as even some insects (other than the well-known Monarch butterfly) participate in long-distance migrations as well.  While insects are the most species-rich and abundant group of macroscopic organisms on the planet, understanding many aspects of their annual cycles and behavior drastically trails behind the ability to identify and classify them. 

Migration is defined as the cyclical travel of an animal as it returns to its place of departure.  It can be accomplished in one or multiple generations.  Some animals don’t migrate at all, and others move only short distances south or north.  While the guiding factors for navigation are still being discovered, migration is typically triggered by local climate, food availability, seasonality, or reproduction.

Adult Common Green Darner

Interestingly, about nine dragonfly species are known to migrate, including the Common Green Darner (Anax junius), Wandering Glider (Pantala flavescens), as well as some species in the Skimmer family such as meadowhawks and saddlebags. These dragonflies are known to travel up to 1500 miles over the full cycle of their migration, taking three generations.

Adult Wandering Glider

The secrets of dragonfly migration were discovered and confirmed by biologists measuring the chemical makeup of their wings.  In their immature or nymph stage, dragonflies grow their wings while still developing in local water.  Because the stable hydrogen isotope concentration varies in concentration by latitude, biologists can estimate the geographic region where adult dragonflies developed as nymphs.  

The female Common Green Darner (bottom) is held by
the male (top) after mating, when ovipositing in water.

From February to August the first generation emerges in the south and migrates north.  From June to October the second generation emerges in the north and migrates south.  Both of these generations have the ability to undergo rapid development from egg to adult.  But from November to March the third generation emerges in the south and is non-migratory, focusing mainly on producing another first generation for the cycle to start again.  It is this generation that employs a different strategy, developing slowly in late fall, entering diapause or actively induced dormancy over the winter, and emerging in early spring.

Dragonfly migration was not well understood until very recently.  Research published in 2018 detailed the journeys of Common Green Darners by analyzing 852 wing samples from eight different countries and utilizing 21 years of citizen scientist observations to link each specimen to its place of origin and establish its migration history. However, it is important to note that the timing of migration and the development of nymphs are both highly temperature dependent, so continued climate change could lead to fundamental changes in the migration cycles for dragonflies and other migrating insect species. So as you can see, migration isn’t just for the birds!

Cooling Conduct

Eastern Ringtail dragonflies obelisking

The unrelenting heat this summer has affected all living creatures, and while humans can escape into air-conditioned spaces, wildlife needs to rely on other ways to regulate their body temperature.  Often this is accomplished through different types of behavioral strategies acquired through adaptation, and some may seem quite odd until you understand the dynamics at work.

One of the most obvious behaviors (for humans at least), is sweating.  But many creatures don’t have sweat glands, or only sweat from their feet (like some canines), so they rely on panting instead.  In panting, animals breathe rapidly and shallowly with their mouths open, thereby increasing evaporation from the surfaces of their mouths.  This evaporation removes heat and acts as a cooling mechanism.  Panting is used by most mammals (like dogs) and many species of birds. While panting is effective at removing heat, dehydration is a risk so a nearby water source is important.  Birds may also spread their wings and body feathers to allow heat to dispel, and they have an elaborate breathing system that includes air sacs (in addition to lungs) to help them get rid of excess heat through panting.

A molting Northern Cardinal, panting and spreading its wings to cool down

Some birds, such as some species of vultures and storks, will voluntarily defecate on their legs to cool down.  Since bird poop is mostly liquid, it works by having an evaporative cooling effect, much like sweating.  As the poop dries on the bird’s legs, the heat is carried away and it brings down their body temperature.  Pelicans, herons, doves, owls, quail, and nighthawks, can employ gular fluttering, vibrating the muscles and bones in the throat and exposing moist throat membranes to the air, which helps regulate temperature by increasing evaporation.

Insects, like most all creatures, can also maintain a stable body temperature in at least a portion of their bodies through some interesting behavioral means.  While many insects need some heat to warm up their flying muscles, many will adjust/reduce their activity levels or seek out shady spots to handle extreme heat.  Dragonflies, for example, will often obelisk while perched in the midday hot sun, or point the tip of their abdomen directly up at the sun.  It is believed that this behavior reduces the heating effect by reducing the amount of surface area exposed directly to the sun’s rays.

Checkered Setwing exhibiting obelisking as a cooling behavior

When temperatures soar, four-legged animals such as squirrels, marmots, rabbits, bears, and even dogs and cats will lie flat on their stomachs on a cooler surface with their arms and legs stretched out from their bodies.  This behavior is called splooting, and it is thought that since their bellies have less fur and lots of blood vessels, they can dissipate the heat and absorb some of the coolness from the surface.  Also known as frogging or pancaking, splooting can take on various forms: the full sploot (all four legs out), the classic sploot (one leg tucked underneath the body while the other is kicked back), the side sploot (one leg tucked under the body and the other kicked out to the side), and the reverse sploot or toolps (when the animal lays on its back with its legs in the air)!

Fox Squirrel in a full sploot on our back porch

Regardless of the method, we all have to find some form of cooling conduct that allows us to survive until temperatures fall and (hopefully) the rains begin once again.

 

Late Summer Skimmers

The quiet waters of a pond like this make great dragonfly habitat

Dragonflies are conspicuous visitors to various bodies of water, especially in the warmer months of the year.    These visitors include members of the largest family of dragonflies, the Libellulidae, otherwise known as the skimmers. Skimmers can be large and colorful, some with distinctive wing patterns, and are often seen perching on twigs and branches.  In late summer, especially after summer rains, some locally common but not often observed species can be found around newly refilled ponds, including the Gray-waisted Skimmer (Cannaphila insularis), Needham’s Skimmer (Libellula needhami), and Slaty Skimmer (Libellula incesta).  

Found in shady or marshy ponds, typically with tall reeds or cattails, the male Gray-waisted Skimmer has greenish-blue eyes, a white face in front and metallic blue on top, and a dark brown or black thorax divided by several pale stripes.  Its abdomen is pruinose gray on the front half and black on the back half, and its wings are clear with extreme dark only at the tips.  Females and juveniles have a yellow-orange abdomen marked with dark brown or black.  Gray-waisted Skimmers are often found perching in the shade at the tips of vegetation, with their abdomen held nearly parallel to the perch.

Gray-waisted Skimmer, male

The male Needham’s Skimmer has reddish-orange eyes and face, and a thorax than is orange in front and paler or more tannish on the sides. Its abdomen is reddish-orange with a dark dorsal stripe down the length, and its orange wings are somewhat darker along the leading edge.  Females and juveniles have brown eyes and faces, a yellowish-brown thorax, and their abdomens are yellow with a dark stripe running down the middle.  Needham’s Skimmers are typically found perching low on vegetation or overhanging the water’s edge.

Needham's Skimmer, male

Most often seen around marshy forest ponds, the male Slaty Skimmer has dark eyes and a metallic blue or violet face, and both the thorax and abdomen have an overall slaty-blue pruinescence.  The wings are typically clear, but can have a pale bluish stripe along the leading edge.  Females and juveniles have red-over-gray eyes and a pale face, a cream-colored thorax with broad dark shoulder stripes, and a black abdomen with yellowish-orange markings. Females also have more prominent dark wing tips and develop a pale pruinosity at maturity.  Males perch on top of tall grasses and sticks most often in sunlit areas.

Slaty Skimmer, male

Check out your local pond or body of water before the end of September, and you just might be rewarded with a sighting of these less common, late summer skimmers!

Some Like It Hot

Eastern Ringtails, a type of Clubtail, obelisking.

The dog days of summer are upon us, and a long stretch of sultry weather lies ahead.  It can be a challenging time for people and for wildlife, but for some, it is their chance to put on a show. 

Bouquets of Mountain Pinks

Mountain Pink close up

Mountain Pink (Zeltnera beyrichii), also called Meadow Pink, Catchfly, or Quinineweed, is an annual herb less than a foot tall and best described as a neat bouquet of small, pink flowers.  Blooming May through July, Mountain Pink sprouts up like an inverted cone 8 to 12 inches high, on hot, rocky hillsides, limestone outcrops, and along gravelly roadways.  Its leaves are threadlike and are held below the multiple 0.5 to 1.0 inch wide showy pink five-petaled blooms that provide nectar for moths, butterflies, bees, and other insects.

Woolly Ironweed

Woolly Ironweed (Vernonia lindheimeri) is a 10 to 30 inch high clumping perennial, with woolly gray stems and long, narrow leaves.  Its bright purple flowers lack true petals, but the disk flowers are arranged in showy, terminal clusters.  A well-behaved species that should be used more often in gardens and landscapes, Woolly Ironweed blooms from June to September, and prefers open hillsides, roadsides, and fields offering full sun.  It is a good nectar source for many species of butterflies during the heat of summer, and is highly deer-resistant.

Blue Dasher, a type of Skimmer, obelisking

To prevent overheating on hot, summer days, some dragonflies and damselflies assume a handstand-like position called ‘obelisking.’  They raise their abdomens until the tip points up toward the sun, which helps to minimize the surface area of their body that is exposed to solar radiation.  Both males and females of these species will raise their abdomens when the temperature is high, and lower them again if shaded. Laboratory experiments have shown that this behavior is effective in stopping or slowing the rise in their body temperature.  This method of thermoregulation is practiced by about 30 different species in the Skimmer, Clubtail, and Broadwing Damsel families.  All are considered ‘perchers’ or sit-and-wait predators that spend a considerable amount of time stationary.

A sure sign that we are in the midst of a hot summer is the sound of cicadas buzzing in the air.  For their size, cicadas make as much noise as a large animal, and can be heard up to a quarter of a mile away.  In fact, the word ‘cicada’ is a direct derivation from the Latin meaning ‘buzzer.’  Many common species of cicadas in North America are in the genus Megatibicen  and are generally called the annual or ‘dog-day’ cicadas because they emerge every year in July and August, the dog days of summer.

Resh Cicada, Megatibicen resh

Male cicadas have structures called ‘timbals’ on the sides of their abdomens, and it is with these structures that they create their buzzy songs.  Unlike grasshoppers or crickets that rub their wings or legs together to produce sound, cicadas vibrate these timbals against their hollow abdomens, which amplifies the resonance of the sounds. They can even modulate the sounds by wiggling their abdomens toward or away from the tree trunk on which they are perched.  Each species has its own distinctive sound, and they use different mating songs to attract the appropriate mate.

Even in the heat of a long Texas summer, nature is busy going about its mysterious ways, offering sights and sounds that can recalibrate our senses, and allow us to continue to appreciate all that it has to offer.

A Horse of a Different Color

Horsetail or Scouring Rush

Few plant species that grow naturally today have been around for over 100 million years, but one of the best known are plants in the genus Equisetum, which is the only living genus of the entire family of Equisetopsida, most commonly known as horsetails.  They are recognized as close relatives of ferns, typically growing in wetter areas with whorls of needle-like branches radiating at regular intervals from each single vertical stem.

The common name of horsetail is used for the entire group of plants, since the branched species resemble a horse’s tail.  In fact, the genus Equisetum comes from the Latin equus or ‘horse’ and seta or ‘bristle.’  Another common name is scouring rush, referring to the upright rush-like appearance of the plants, and the fact that the longitudinal ridges of the stems are coated with abrasive silicates, making them useful for scouring or cleaning metal items.  It is still used today as a traditional polishing material in Japan.

The primary species of horsetail that occurs natively in wet or moist areas of Texas, most commonly on the Edwards Plateau and in Blackland Prairie, as well as most of the non-tropical northern hemisphere, is Equisetum hyemale.  A spreading, reed-like perennial growing to 3 feet tall, each stem is evergreen, cylindrical, jointed, hollow, and about 1/4 of an inch in diameter.  In this species, the needle-like branches appear non-existent, but are actually small and fused around the stem at each joint or node, forming a blackish-green band or sheath.  Interestingly, the pattern of spacing of the nodes in these plants, which grow increasingly close together toward the apex, is precisely what inspired Scottish mathematician John Napier to discover logarithms in the late 16th century.

Dragonflies, like this Neon Skimmer, love to perch on the cones of
the horsetail’s upright stems.

Horsetail prefers open or wooded areas along streams, moist flats, and wet ledges. Like ferns and other related species, horsetails reproduce by spores rather than through seed-producing flowers. These spores are borne in cone-like structures at the tips of some stems, and are mostly homosporus, meaning of the same size and type.  The tiny spores have four elaters or structures that alter shape in response to changes in moisture, effectively acting as moisture-sensitive springs that assist spore dispersal through crawling and hopping motions once released from the cone.

Horsetails reproduce by spores borne in the cone-
like structures at the tips of some stems.

The upright, evergreen, segmented foliage of horsetail is an appropriate plant for a rain garden, pond edge, water feature, or area with moist soil, and is an excellent perching plant for dragonflies. While it can spread quickly by underground or underwater runners, it is easily kept in check by periodic pulling or by planting it in a container.  Few plants add as much interest or vertical structure to a wildscape as this living fossil, which is clearly a  ‘horse of a different color’!

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.”