Tiny Ones

Sunlight streaming through the goldenrod branches glinted off a dust-like mote in the air. The speck hovered purposefully, like a living thing, but with the diaphanous, baffling aspect of a fruit fly. Finally it came to rest on a flower where inspection was possible through the magnification of the close-up lens. Indeed it was a tiny insect.

A Perilampus wasp at the tip of a goldenrod cluster

This full-bodied creature with a plan was less than a sixteenth of an inch long. It made a fascinating contrast to the 'bruisers' we looked at in the last posting. As it turns out, they are all wasps.

An enlargement of the wasp astride a 3/16" long flower

The minutely detailed anatomy of the wasp appeared as complex and capable as in larger-scaled insects. Its smooth, shiny shell, or exoskeleton distinguished it from the hairy body and legs of bees. Exoskeletons support and protect the bodies of insects, anchor their muscles, and prevent desiccation.

A 1/8" long braconid wasp

Other bits of animated dust fluttered near the goldenrod. Until it alighted on a flower this braconid wasp looked very much like the Perilampus. Close observation revealed a bit larger and more intricate structure. The long appendage at its rear is an ovipositor, the device by which females implant their eggs.

Cocoons of a braconid wasp on a Sphinx moth caterpillar

During the summer this Sphinx Moth caterpillar was victimized by a braconid wasp. When the eggs hatch the larvae slowly consume their host.

Braconid wasp

The wasp explored the flower surfaces with inquisitive antennae. The fine functioning of its tiny parts replicated the vast and varied sophistication of the whole animal kingdom.

A Couple o' Bruisers

A certain well-watered clump of goldenrod flowers at Halibut Point has provided a steady arena of insect activity through the recent arid months. Like an oasis in the savannah it has concentrated the insects for their subsistence, and consequently drawn predators on  the prowl. 

The goldenrod grows about 5 feet high. That's a convenient height for observing the bevy of nectarivores. Then a relatively enormous creature came crashing like a pinball through the flower canopy. It looked like an oversized bumblebee in black Kevlar with racing stripes.

Four-banded Stink Bug Wasp, Bicyrtes quadrifasciatus

I'd seen this fellow earlier in the month excavating a burrow on the Bay View Trail. It's a solitary sand wasp. Individual females lay their eggs in subterranean galleries provisioned with paralyzed prey to feed the growing larvae. She ends her part in the life cycle by sealing the tunnel.

The wasp almost never comes to rest. It searches high and low, inside and out of the vegetation, for food to bring back to its brood. Most desirable are nymphs (juveniles) of stink bugs and leaf-footed bugs. Adult wasps subsist primarily on flower nectar.

These behemoths of the branches frequented the goldenrod fairly briefly but another hefty hunter followed their forays in late August. The two species didn't seem to overlap, which may be the practical result of evolutionary design. It may also be due to the dominant size and aggressiveness of the newcomers.

Bald-faced Hornet, Dolichovespula maculata

The newcomers were Bald-faced Hornets, which bounced relentlessly through flowers and foliage scattering the smaller pollinators. Their fore and aft ends were boldly marked with what looked like war paint.

Bald-faced Hornet nest

The hornets‒actually large wasps closely related to Yellowjackets‒were on a similar nest-provisioning mission. They have constructed a papery, foot-ball sized, fiercely defended domicile with cooperative working assignments. They are considered Social Wasps. Each colony may number upwards of 300 individuals.

Flying through the goldenrod the Bald-faced Hornets seldom come to rest. They roam within the canopy inspecting the undersides of leaves and branches as well as flowers, evidently capturing a variety of insects and spiders to bring back to feed their larvae, along with the nectar that is also a mainstay of their diet as adults.

Black-shouldered Drone Fly, Eristalis dimidiata, yesterday

By mid-September the brawny carnivores have disappeared from the glade, leaving it a more tranquil place, though certainly not without hazards for emergent insects. For those of us who anticipate a long life it's hard to fathom that most insects spend only a small part of their lives‒days or weeks‒in a recognizable adult phase, if they happen to survive to maturity. That brief period is meant to ensure propagation and dispersal. It ends in death and the cycle of renewal through the usually much longer phases of dormancy, pupation, and juvenile growth.

Arms or Legs?

 

Common Baskettail

Like all insects, dragonflies are considered to have six legs. Though they don't walk, they use those six limbs in various ways that make me wonder about the difference between an arm and a leg.

Black-tipped Dasher

When alighting on earth many dragonflies hang rather than stand. That raises the question of whether they're employing feet, hands, paws, claws, or some special solution I don't have an adequate word for.

Eastern Forktail

They do grasp, aided by flexible joints and barbs for holding onto things. But then grasping involves the kind of dexterity I associate more with arms than legs.

Blue Dasher female

Dragonflies catch and consume their prey in midair. Their limbs are configured like basket staves to facilitate scooping and clutching, rather than a linear arrangement advantageous for walking. The limbs to the rear appear to be more weight-bearing, the ones to the front more nimble for functions close to the mouth and for settling encounters with friend or foe.

In this way a dragonfly's limbs are specialized, like with many other animals. However tradition has categorized very few of even the most supple limbs as arms, in that exalted class with primates and octopi. Not even squirrels and raccoons, with their superior dexterity, have been so designated, presumably because they move about on all fours. Nevertheless, along with dogs, cats, and horses, they have skeletal parallels with us: powerful hind limbs that extend from hip sockets and articulate at knee joints; and relatively mobile front limbs fluidly connected at the shoulder, with flexible elbows and wrists. These anterior limbs are thought of as forelegs rather than arms, terminating in something not quite as marvelous as a hand.

A Stripe-legged Robberfly eating a captured Rust Fly

Just how adroitly dragonflies use their limbs on the hunt is hard to say, since they don't bring their prey back to an observable perch. They most likely operate like this robber fly, which launched from a stationery platform, landed with its victim on a similarly hexagonal group of grasping‒legs? ‒and fed itself handily.

Painted Skimmer

An intriguing diversity of solutions goes to satisfy the common needs of creatures top to bottom in the animal kingdom. Arms are certainly a marvelous development from legs, but wings! there's an aspiration.

Pterostigma

 

The dragonfly's wing spots

One of every dragonfly's characteristics is a pigmented spot on the leading edge of each wing. It is not simply a decorative feature.

A pterostigma

The biologist's name for it is a pterostigma, meaning 'wing spot.' It is proportionally heavier than other areas of the wing and acts as a counterweight in flight. 

Northern Spreadwing

If not for this, the outer leading edge of the dragonfly's wings would be prone to flapping and feathering vibrations known as flutter, which would rapidly stall its flight when gliding at high speed. Such resonance is known to endanger far larger structures, such as aircraft, buildings, and bridges. 

Fragile Forktail

In the case of dragonflies a small spot that accounts for less than 0.1 per cent of its total body weight allows it to safely reach a flight speed 10 to 25 % higher than it would otherwise.

Wing corrugations

A dragonfly's wings are not perfectly flat plates. Their veins form three-dimensional corrugations that make the wing's performance more efficient and prevent them from warping and deforming as a result of resonance.

Pterostigma on a parasitoid wasp

The pterostigma is common among the insect orders as well, although not usually so easily seen. By passive, inertial, pitch angle control, the pterostigma probably makes the wing beat more efficient in slow and hovering flight of small insects, while its raising of the critical flight speeds probably is of more importance to larger insects.

Calico Pennant

The evolved functional details of insect anatomy are obviously of great interest to aeronautical engineers. 

 

Source: R. Ake Norberg, "The Pterostigma of Insect Wings, An Inertial Regulator of Wing Pitch," Journal of Comparative Physiology, vol. 81, March 1972.

Insects are photographed at Halibut Point.

Dragonfly's Eyes

 

Blue Dasher

Dragonflies have two huge compound eyes and three small simple eyes on their forehead in between. They are the most effective predator alive today, hunting and eating in mid flight. In the last posting we looked at their remarkable aerial ability. It's how well they see the world that makes all that agility so effective.

One of the thousands of cells in a compound eye

Dragonflies have incredible vision. Their characteristic compound eyes are the largest in the insect kingdom. Each is made of over 30,000 individual facets or ommatidia, which are like mini-telescopes but detect light from the direction in which they point it. 

Spotted Spreadwing

As their eyes wrap around their head almost entirely, their field of vision is almost 360 degrees, perfect for surveying their surroundings for potential targets, without having to reorient themselves on their perch. 

Calico Pennant

With 30,000 ommatidia spanning a full 360 degrees, dragonflies do not have the same visual acuity as a human or a bird, but are much more sensitive to movement than we are. A common prey target in the distance will rarely span more than 1 to 3 across their field of vision before they decide to pursue it, which equates to movement across 2 to 3 ommatidia at most, meaning their brain is capable of detecting and processing movement incredibly quickly, so they can immediately perceive prey or avoid a threat.

Ruby or Cherry-faced Meadowhawk

Their eyes are also specialized based on the region. The upper or dorsal region of their eyes is most sensitive to shorter wavelengths of light, that is, ultraviolet and blue. Many dragonflies fly low above the surface of the water when searching for prey, as this allows potential targets to contrast as much as possible with the blue sky. On the other hand the middle and lower, or ventral parts of their eyes are sensitive to a wider range of wavelengths. 

Northern Spreadwing

It's not just the wings or the eyes that makes the dragonfly so successful, but it's the brain The dragonfly's brain uses a highly optimized hunting strategy that allows it to predict where the prey is going and to quickly maneuver toward where they predict the prey will be. This is more difficult than simple tracking, as the locations and speeds of the prey and predator must be processed together to make such a prediction. 

Twelve-spotted Skimmer

Even though dragonflies have the speed and agility to engage in tracking if they so desire, they primarily use interception as a form of capture. In humans interception is a learned behavior but in dragonflies it's hypothesized that such behavior must be hard-wired into their nervous system, linking their visual system and motor system directly.

Violet Dancer

However, an interception model of pursuit means that the prey being pursued is less likely to remain in the same spot within the dragonfly's field of view, which can make the prey more difficult to track. Studies have shown that dragonflies seemingly adjust their head in near synchrony with their prey, fixing their target within their areas of higher visual acuity for tracking. In fact this synchronization is so perfect that even the head movements appear not to be reactionary but based on prediction.

Wandering Glider

All of this mind and body integration have given the dragonfly the best hunting record that researchers have ever found, catching around 95% of the prey they chased. This compares very favorably with the success rate of peregrine falcons, at 23%, and lions, at 30%.

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* These text excerpts  are transcribed from the YouTube video "The Insane Biology of: The Dragonfly."