Observing Orbweavers

Drops of morning dew sparkle on a spider's web.

The family of spiders known for building flat, spiraled, wheel-shaped webs are called Araneidae, or the orb-weavers.  They are the most common group of spiders often founds in yards, gardens, fields, and forests.  In our area, those include the Black & Yellow Garden Spider (Argiope aurantia), Lichenmarked Orbweaver (Araneus bicentenarius), and Spinybacked Orbweaver (Gasteracantha cancriformis).  While all of these spiders produce venom, that venom is harmless to humans but helps them to immobilize their insect prey.

Also called the Yellow Garden Spider, the Black & Yellow Garden Spider is a large spider that has an abdomen with a series of pale, yellow spots on a black background, and legs that are orange at the base, black near the tips, and often held in pairs. Females are much larger than males, with up to an inch in body length. While both appear in late summer, females can be seen through December in our area.

A Female Black & Yellow Garden Spider.

While not quite as common, the Lichenmarked Orbweaver has an abdomen with a pattern of light green and reddish-brown markings and two small humps on front.  Females can grow to one inch in body length. Its legs have alternating bands of orange and brownish-black, and the coloration on its abdomen acts as a natural camouflage against the lichen-covered branches of trees where it often rests.

Lichenmarked Orbweaver, showing its characteristic green abdomen with two humps.

Often called crab spiders, one of our most distinctive spiders is the Spinybacked Orbweaver, a small, half-inch wide spider with a hard exoskeleton that has six spines around the edge.  There are many color forms, but most are yellow or white with black or dark red spines. Males are tiny and seldom seen, but females can be seen year-round. They build their webs in the morning in shrubs or low tree branches.

Spinybacked Orbweaver, yellow form.

Spinybacked Orbweaver, white form, showing relative size.

Orb-weaving spiders are one of nature’s engineers, and their process to build a web is intriguing.  They start by floating a line of silk on the wind to another surface. Once that line is secured, they drop another line from the center, forming a Y.  The rest of the web is a series of non-sticky spirals ending with a final spiral made of sticky capture silk.  The non-sticky webbing is nearly invisible, but the more visible sticky webbing is what normally traps the spider’s prey.  As such, there is always a tradeoff between the visibility and stickiness of the web.

The webs built by some orb-weavers include a vertical zigzag band of highly visible silk through the center of the web.  This is called the stabilimentum, and is most often found in the web of a Black & Yellow Garden Spider. While scientists are unsure of the exact purpose of this structure, theories include it posing as a lure to bring prey to the web, a warning for birds not to fly through the web, or as a way to camouflage the spider as it sits on the web.

A male Black & Yellow Garden Spider
rests on the web's stabilimentum.

Many orb-weavers build a new web on a daily basis.  They often hide during the day and become more active in the evening, when they consume the old web, rest for a bit, then begin spinning a new web in the same general location. Consider all that hard work the next time you think about destroying a web!

 

Fall Fliers

 

Fall-blooming Blue Mistflower and White Boneset attract late season butterflies.

Butterflies are frequently thought of as insects that fly primarily in the spring and summer seasons.  This is because the emergence of many butterfly species is unimodal, which means that their numbers increase as environmental resources increase.  These resources are defined as their host plants and nectar plants, which also become available during these warmer seasons.  

However, there are some species that have bimodal emergences, or increases in numbers in both the spring/summer and fall seasons.  It is thought that the division of offspring between two different emergence times may have evolved to avoid producing all offspring at one time.  This approach would reduce the risk of species mortality in case of potential fluctuations in habitat quality. 

Whether they be unimodal or bimodal, butterfly species typically exhibit a tightly synchronized adult emergence in order to help them locate mates. Further still, some species are present in low numbers during most of the year, but their numbers increase during the fall.  In central Texas these species include the Tailed Orange (Pyrisitia proterpia), Julia (Dryas iulia), Common Mestra (Mestra amymone), Queen (Danaus gilippus), and White-striped Longtail (Chioides albofasciatus).

In the fall, the Tailed Orange is in its winter form, yellow with brown lines and blotches below and a noticeably pointed hindwing edge.  Its summer form is unmarked yellow below and the hindwing edge is less pointed. It flies late summer through fall, and uses senna species as its host plant.

Tailed Orange, winter form

The fast-flying Julia is mostly orange above and orange to brown below, with the female being a duller orange than the male and having a dark forewing band.  Its longwing shape is quite distinctive and it prefers woodland edges and gardens where it uses passionvine species as its host plant.

Julia, male

A slower, flat-winged flyer, the Common Mestra is pearly white above with a pale orange border on its hindwing, and mostly pale orange below with a thin, white spotband.  It is most often seen from June to November, and it uses noseburn species as its host plant.

Common Mestra

The Queen butterfly is often confused with monarchs, as it also uses milkweed species as its host plant.  Rich dark brown to deep orange above with white spots in the black wing margins, it lacks the strong black veining on the wings like monarchs, and can be found in any open habitat usually visiting flowers.

Queen

Straying into our area from south Texas, the White-striped Longtail is a dark brown butterfly with very long tails, and a prominent white stripe on the underside of its hindwing.  It usually perches with its wings closed, and uses various legume species as host plants.

White-striped Longtail

One way to increase your chances of seeing these fall fliers in your yard is to provide native plant species that bloom in late summer and well into fall. These plants include Frostweed, Gregg’s Mistflower, Blue Mistflower, White Boneset, Lindheimer’s Senna, Plateau Goldeneye, and Texas Lantana.  And remember, fall is the perfect time to plant!

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!

Songs of Stridulation

 

Immature katydids, called nymphs, are initially wingless. 

As the summer temperatures heat up, songs of singing insects fill the air with a variety of sounds.  Members of the order Orthoptera, including katydids, crickets, and grasshoppers, produce a variety of sounds through stridulation, or the rubbing of one body part against another.  

Differential Grasshopper, Melanoplus differentialis

While grasshoppers typically stridulate by rubbing their hind legs against their closed wings, katydids and crickets have modified bases of their wings in order to produce sound.  Specifically, they have a sharp edge or ‘scraper’ on the upper surface of the hindwing which they rub against a row of bumps or ‘file’ on the underside of the forewing. During sound production, katydids and crickets elevate their wings and move them rapidly back and forth, and the wings vibrate as a result of the scraper rubbing against the file.

Central Texas Leaf-Katydid, Paracyrtophyllus robustus

Unlike Orthoptera, male Cicadas in the order Hemiptera produce sounds through a pair of special ribbed organs or ‘tymbals’ located on sides of their abdomens, just behind their wings. When they contract their muscles, it causes the ribs to bend suddenly, producing a sound that resonates within a large air sac in their abdomen. The distinctive sound that is produced is one of the loudest made by any insect.

Resh Cicada, Megatibicen resh

The use of sound is crucial in courtship, with each species having its own distinct song.  Males attract mates through stridulation, producing a vibration frequency that is species-specific.  Songs are distinguished both by their dominant frequency and the details of their timing patterns. Crickets generally produce musical trills of continuous notes often too fast to count, or short bursts of chirps followed by silence.  

Field Cricket, a species in the Gryllinae family

Katydids and grasshoppers have high-pitched songs, composed of atonal shuffles, rattles, scrapes, buzzes, or ticks.  Some sing more or less continuously while other species have long silences in between periods of singing.  Cicada songs are the most penetrating, as rattling buzzes or harsh trills, often with a pulsating or grinding quality.

Fork-tailed Bush Katydid, Scudderia furcata

While most insect songs are the calling songs of males intended to attract females, these songs are thought to have other functions as well.  Some songs are for courtship once a mate is found, and some serve to attract males to a group chorus or to keep males optimally dispersed within a singing colony.  Aggressive songs can also be heard, when two males encounter each other, or even disturbance calls, when an insect is touched or handled.  Whatever the reason, now is the season to hear the songs of stridulation!

Furtive Fledglings

 

Mixed woodland of oak and juniper in Central Texas is the only breeding habitat
for the Golden-cheeked Warbler.

As the temperature rises and late spring turns to early summer, many bird species are in the throes of caring for newly hatched nestlings (still in the nest) or fledglings (just out of the nest).  As you hike through our oak-juniper forests in the western part of Austin, you just may run across young families of our endangered bird, the Golden-cheeked Warbler (Setophaga chrysoparia).

Adult Golden-cheeked Warblers arrive at their breeding grounds by mid-March, returning largely to the same areas each year and nesting from April to May. They are socially monogamous, with the males arriving before the females to establish their territory. Courtship behavior, rarely observed, involves the female collecting nesting material as the male sings a soft, twittering version of his song, flicking and spreading his wings and tail and sometimes bringing nesting material to the female. 

A male Golden-cheeked Warbler sings from a juniper to establish his territory.

The female chooses the nest site, often in a branched fork of an Ashe Juniper or Live Oak tree, and the nest is built in 4 days, camouflaged by bark strips from mature juniper trees and secured by spider silk. Females lay 3-4 eggs and for the first 3 days she broods or sits on the eggs continuously, being attended to and fed by the male. The eggs hatch in approximately 12 days, and the nestlings are altricial or born helpless and requiring significant parental care.  

Female Golden-cheeked Warblers typically don't have black throats,
but the ones that do are called 'bearded females'.

However, they leave the nest only 8 or 9 days after hatching, staying in the vicinity of their attendant parents, but usually huddled together and partially hidden in the trees. They continue to be cared for by both parents, who actively search for caterpillars and other insects in the foliage to bring directly to the fledglings.  

A fledgling Golden-cheeked Warbler.

The most obvious way to spot these furtive fledglings is by listening for the family group.  As a parent nears with food in its beak, the fledglings chip rapidly and flutter their wings, begging and hoping to be the one who gets the morsel of food.  They grow quiet once the parent takes off to forage again.  Once they get a bit older, they start to follow their foraging parents begging for food, eventually becoming more confident in their ability to fly and learning to forage for themselves. As they become even more independent, the young join the adults in mixed-species flocks in the woodlands before migration begins in July and early August.

This Golden-cheeked Warbler fledgling caught its own food!

If you hear or see a Golden-cheeked Warbler family foraging and feeding in our mixed woodlands, consider yourself lucky.  Of the nearly 360 bird species that breed in Texas, the Golden-cheeked Warbler is the only one that nests exclusively in Texas, so each one is a native Texan! 

Umbraphilia!

 

Would the clouds cooperate during the total solar eclipse?

A total solar eclipse occurs when the moon, the sun, and the earth align such that the moon appears to completely cover the face of the sun from the earth’s perspective.  To see this astronomical phenomenon, you must be somewhere within what is called the path of totality.  About every 18 months or so, a total solar eclipse happens somewhere in the world, and on April 8, 2024 Central Texas was lucky enough to be in the path of totality.

Typically, the path of totality across the globe is around 9000 miles long, but only about 90 miles wide, and being in the very center of this path allows you to maximize the amount of time that totality lasts.  A total solar eclipse can last for several hours, but totality can only range from a few seconds to seven and a half minutes. Observers outside the path of totality may only see a partial solar eclipse. 

The first stage or partial eclipse.

There are five different stages that make up a total solar eclipse. The first stage is when the partial eclipse begins, or when the moon starts to become visible over the sun, looking like it has taken a bite out of it.  The second stage is when the total eclipse begins, when the moon covers the entire face of the sun, and you are now in the umbra, or the darkest part of the moon’s shadow. The third stage is totality, which is when the moon completely covers the sun and leaves only the sun’s corona visible. The midpoint of this stage is called the maximum eclipse. 

Totality or maximum eclipse!

During totality, the sky goes dark, the temperature falls, a light breeze picks up, and the birds and other animals go quiet. The fourth stage is when the total eclipse ends, and the moon starts moving away as the sun reappears.  The fifth and final stage is when the partial eclipse ends, as the moon is no longer visible over the sun.

Baily's Beads appearing in Stage 2, just before totality.

During the second and fourth stages, when the moon is just about to cover the sun or just starts to move away, those in the path of totality can have the chance to see two special effects, Baily’s beads and the diamond ring. About five seconds before and after totality, Baily’s Beads appear as little bead-like blobs of light at the edge of the moon, created by sunlight passing through the gaps in the mountains and valleys on the moon’s cratered surface. They are named after Francis Baily, an English astronomer who observed and described this effect in 1836. 

The diamond ring in Stage 4, just after totality.

About 10 to 15 seconds before and after totality, the solar corona or outer atmosphere of the sun becomes visible, and combined with a small remaining or emerging part of the sun’s disk dazzles like a diamond set in a ring.

Prominences seen during totality.

Sometimes, during totality, observers can see small, fiery structures around the obscured sun.  Typically, these are not solar flares, which are explosions on the sun’s surface that can launch massive clouds of plasma, but rather they are called prominences or longer-lived plasma structures that are smaller and not as explosive as flares.

One who is addicted to the glory and majesty of total solar eclipses is called an umbraphile, or ‘shadow lover.’  If you have personally experienced this amazing phenomenon of basking in the moon’s darkest shadow, you too may have contracted umbraphilia!

Requisite Night

 

Light pollution is nearly non-existent in Big Bend,
allowing for spectacular star-filled night skies.

Most environmentally-minded individuals recognize the more talked about threats to our native wildlife, such as habitat fragmentation/loss, invasive species, and climate change, but not as many are aware of the dangers posed by light pollution. Up until the mid-1800s, humans and animals lived under night skies solely lit by the moon. Electric outdoor lighting became common in the early 20th century, but its use spread quickly, and the global extent of modern light pollution became clear.  

By 2016, it was possible to measure nocturnal artificial light with the advent of a comprehensive global satellite measurement system. Researchers found that more than 80% of the world’s population lived under light-polluted night skies, or skies where the glow of artificial light is significant enough that the stars disappear from view.  In the US and Europe, it was found that 99% of residents live under light-polluted skies.

Light pollution exposes animals to many dangers, including predators, starvation, exhaustion, and disorientation. Artificial light, like roads and fences, can create barriers that fragment habitat.  Slow-flying bats avoid feeding in or passing through illuminated areas for fear of predators such as owls and other birds of prey. Artificial light near their roosts can also delay their emergence at dusk when their insect prey is most abundant.  If they never leave their roost since it always appears to be light, they can even starve to death.  

Artificial nocturnal light can also lure animals in and lead to their destruction.  Many species of migratory songbirds are attracted to brightly lit structures at night, circling them, sometimes colliding into them, or becoming disoriented enough to lead to a depletion their energy stores which ends in exhaustion and the inability to complete their journey.  In some bird species, artificial light at night interferes with their ability to use natural polarized light from the sky to calibrate their internal compass.

Artificial light at night attracts insects, like this Luna Moth, 
and can disrupt normal behavior patterns.

Light pollution is also one of the many factors contributing to the rapid decline of insect populations.  Moths and other nocturnal insects orient themselves by moonlight, and this instinctual tendency is interrupted by artificial night light, luring them in to fly incessantly around a bright light, causing exhaustion, exposure to predators, and the potential to miss courtship cues from mates.  This is especially true for fireflies, as artificial night light can cause them to alter or cease their mating flashes.  Studies have also shown that light pollution can harm diurnal insects like monarchs, who flit and flutter all night when exposed to excessive light when they should be resting, and causing them to be disoriented from their migration route.

Light pollution facts and some easy solutions.

Unlike other environmental threats to wildlife, simple solutions to artificial nocturnal light exist.  The best solution is to have no nocturnal lighting other than natural conditions. If a light at night is truly needed, the amount that spills into wildlife habitat can be reduced through dimming, downward shielding, or switching to motion-activated lights. Studies are also showing that lights in the warmer color tones are less disruptive than bright white lights.

Travis Audubon promotes the Lights Out Initiative for Austin.

Austin is one of several cities across the US that participates in the migratory bird friendly Lights Out Initiative, which asks residents to turn out all non-essential lights from 11pm to 6am every night during spring migration (March 1 – June 15) and fall migration (August 15 – November 30).  This is one of many ways we can prevent light pollution from overpowering our native wildlife. Turn out your lights when they are not needed, and welcome the requisite night!