An Ode to Odonata

In the spring I love to go birding in the Arnold Arboretum and elsewhere on the Emerald Necklace.   By late June, however, most of the migratory birds have either moved farther north to their breeding grounds or are nesting locally and want to be as inconspicuous as possible to avoid detection.  That’s when I look elsewhere in the natural world for some interesting and colorful sightings.

Dragonflies are an ancient invertebrate group, at least 250 million years old.  Some of the prehistoric ones had wingspans of three feet, probably due to the higher levels of oxygen in the atmosphere way back then. Today’s versions are still large for an insect, some with body lengths of more than three inches.  There are two main subgroups– damselflies and dragonflies.  Here are examples of each, showing the primary differences:

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Blue Dasher male. This is a typical dragonfly showing the two separate shaped wing sets held open at rest, and the robust body. Dragonflies are very strong and agile fliers, sometimes traveling at more than 20 mph!
Photo: Bob Mayer

 

Azure Bluet, male. This pond damselfly is fairly characteristic of the damselfly sub-group; slender, delicate and holding its wings closed when at rest. They are weaker fliers than their cousins the dragonflies.

 

All members of the order Odonata (odes for short) have peculiar mating behaviors. The male grasps the female either behind the eyes (dragonfly) or by the neck (damselfly) as seen below:

Probable Northern Spreadwing tandem pair; the male is the bluish one. Several odontata experts studied this image taken recently in Vermont and couldn’t positively identify it. Many damselflies can only be distinguished from one another by a careful study of their external genitalia!
Photo: Bob Mayer

After mating the female dragonfly stores sperm in her spermetheca. The males compete for fertilization and some will scrape out previous sperm from the female’s spermetheca.  Talk about male competitiveness!

The eggs are laid on the leaves of water plants or in water where they hatch into larvae.  The larvae can be nearly as large as the adult insect but because they live in water they are seldom seen. They also live much longer than the adults, most of whom fly around for less than a month. The larvae of some odes live and grow for several years, moulting their exoskeletons many times before finally undergoing metamorphosis into the winged adult.  Here is an image of a dragonfly larva my grandson discovered while digging in the sand in a lake in Vermont:

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This dragonfly larva may live longer than the adult form, but it sure doesn’t compete on an aesthetic scale!
Photo: Bob Mayer

All odes are carnivores, in both larval and adult form, and adult dragonflies capture and eat all sorts of flying insects, including other dragonflies, on the wing.

Over the last ten years I’ve recorded about a dozen different species of Odonata in the Arboretum. They can be found almost anywhere but are usually not far from some water source.  The easiest place to find them is near the three man-made ponds off Meadow Road.  In the remainder of this post I will discuss some of these pond species, showing images of both sexes for many of them as they are often quite dissimilar.  All the displayed dragonflies are members of the same family— the skimmers.

One of the smallest and prettiest pond species in the AA is the Eastern Amberwing:

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A well named Eastern Amberwing male resting on a Pickerelweed leaf on Rehder Pond
Photo: Bob Mayer

 

Like many dragonfly species, the female is patterned differently

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Eastern Amberwing female. Note the dark patches on her wings, not seen on the male of this small dragonfly.
Photo: Bob Mayer

 

Dragonflies are aerial acrobats, showing many different flying habits that are reflected in their common names.  There are Painted Skimmers, Blue Dashers, Wandering Gliders, Eastern Pondhawks and Stream Cruisers, each displaying favorite flying modes.

Blue Dashers are medium-sized dragonflies that are common around the ponds during much of the summer.  The male is pictured above, representing a “typical dragonfly”.  Females are seen less frequently and have different markings:

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The female Blue Dasher has no blue color on it’s abdomen, instead showing paired yellow streaks along the sides.
Photo: Bob Mayer

Another very common dragonfly near the ponds is well named:

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Common Whitetail male resting on a rock beside Faxon Pond. Note the single broad black wingband.
Photo: Bob Mayer

Here again the female of this species is quite different:

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This female Common Whitetail does not have a white abdomen and has three black patches on each wing
Photo: Bob Mayer

 

Perhaps the most striking large ode flying lustily around the ponds in the Arboretum is the Twelve-spotted Skimmer:

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Twelve-spotted Skimmer male. They are named for the three black spots on each wing but the white patches in between are what makes this ode shine.
Photo: Bob Mayer

Once again the female is less flashy, and closely resembles the female Common Whitetail:

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Twelve-spotted Skimmer female.
Photo: Bob Mayer

 

Much less common, but equally flashy, is the male Widow Skimmer:

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Widow Skimmer male. The single large black band and white patches on all four wings are diagnostic for this species.
Photo: Bob Mayer

 

The female lacks the white on the wings:

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This female Widow Skimmer could be easily confused with other skimmers without careful observation.
Photo: Bob Mayer

 

There is another large skimmer which frequents the ponds, seen here resting on a plant at pond edge:

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Male Slaty Skimmer on Swamp Mallow flower.  This species has no wing markings (except for the small stigma seen on most dragonflies) and a uniformly slate-blue thorax and abdomen. The female is a drab brown.
Photo: Bob Mayer

Also in the skimmer family is the Eastern Pondhawk:

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Eastern Pondhawk male. The entire body is blue-green except for the small white abdominal appendages at the very tip.
Photo: Bob Mayer

 

You’re probably expecting the female to be drab; not so with the Pondhawk:

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The female Eastern Pondhawk is at least as colorful as its male counterpart.
Photo:Bob Mayer

 

Finally I should mention two more members of the skimmer family that are actually in the same genus, Spot-winged and Wandering Gliders.  The former shows up earlier in the dragonfly season (which is early June through mid September) and is less colorful than the Wandering Glider, which is golden in color.  The characteristic flight behavior of these stocky odes is that they spend long periods in gliding flight, with interspersed bursts of flapping, but they seldom perch. That’s the reason I have no photos of either!  But their flight patterns give them away when they do show up at the ponds or over open fields in the Arboretum.

In a subsequent post I will discuss some other odonates that can be found on the AA grounds, often at some distance from open bodies of water.  In the meantime please check out the ponds, especially in the mid-day. You will almost certainly see some beautiful dragonflies if you look carefully!

There will be an “Insect Mob” on Monday August 12th at 4:00 PM starting at the ponds in the Arnold Arboretum. This brief field visit will be led by an expert on dragonflies from Harvard’s Organismic and Evolutionary Biology Department.  Check it out if your available then!

Good Odeing!

Return of the Cicada Killers!

Five years ago I was walking along Forest Hills Road near Lawson Pond in the Arnold Arboretum when I was confronted by several large wasps who seemed to have a great interest in me; I beat a hasty retreat- but not before I got a picture of one of them:

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Eastern Cicada Killer wasp
All photos- Bob Mayer

 

After reviewing some field guides I determined the insects were solitary wasps called Cicada Killers Sphecius speciosus.  I sent the image to an AA staffer who tends to the plant collection in that area and she put me on to a great learning experience a few days later.

Gerry Bunker is an amateur entomologist who had been asked by a fellow bug guy to help him collect specimens of the annual Dog-day Cicada Tibicen linnei.

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Dog-day Cicada Tibicen linnei. This is not the 17 year periodical cicada that recently had a massive emergence along the east coast (but not in Massachusetts).

 

He was going to use the Cicada Killers in the lek I bumped into as hunters to collect cicadas for his friend and I was invited to watch and learn.

The first thing I learned was that despite their formidable appearance and intrusive flight behavior, Cicada Killers are mostly harmless.  The males that had been buzzing me cannot sting (like all male wasps), and the larger females do so very rarely and only if handled roughly or stepped on with bare feet.  In fact, I watched Gerry remove cicadas from the female wasps bare-handed without concern.  But I get ahead of my story.

Male Cicada Killers emerge from the ground in July in areas that were used as leks in the previous year.  They establish territory and search for virgin females who emerge a week or so later.  Following a complicated ritual mating sequence the females begin to dig a hole in the dry, sandy soil using their jaws as diggers and their hind legs to shove the loose dirt up and out of nest holes that can be up to two feet long.

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The entrance to the nest hole is the small dark circle in the middle of this photo.

 

 

The female wasp then goes hunting.  Apparently using sight rather than sound (since most of the cicadas caught are females that don’t emit any noise) they search nearby deciduous trees and capture and sting a cicada, paralyzing it.  Dog-day Cicadas are more than twice the size of the hefty female Cicada Killers so it often takes some time and effort to deliver the prey to her nest hole.

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This female Cicada Killer is struggling to carry the paralyzed cicada through the grass to her nest site.

 

 

This is when the intrepid collector intervened, snatching the cicada away from the female before she could place it in the nest. In an afternoon he amassed an impressive collection of several dozen specimens he later sent to his entomologist friend.  What a clever way to collect these bugs that hang out high in the tree canopy and are therefore hard to get!

Returning to the story of this insect’s fascinating life cycle, the female carries her prey down into the nest where it is to become a living host for an egg that will hatch into a grub. This larvae will feast on the prey for the next week or so until it forms a dry cocoon and lies dormant for the winter.  The female chooses whether to lay a male or a female egg.   Female eggs require some of the stored sperm she has from the earlier mating; males eggs do not.

If she lays a female egg on the paralyzed cicada her work isn’t over.  Females, presumably because they are considerably larger than males, must have at least two cicada bodies in order to prosper.  So Mamma Cicada Killer ventures out again to find and subdue another Dog-day Cicada to provide for her later-to-be daughter.  Then she covers over the entrance and proceeds to dig another hole (sometimes she may just make a second tunnel connecting to the original opening).

The lek is a busy place for several months in late summer and early fall.  The Cicada Killer males are buzzing around defending their territory and looking for unfertilized females. The females are mating, digging nest holes and hunting cicadas to haul down to their nest hole.   The resident birds hang out to take advantage of the opportunity to capture and eat both predator and prey.

Property owners may be unhappy when a Cicada Killer lek is established on their lawns.  The temporary damage done to grass by these insects has to be weighed against their  biological control on cicadas, which damage deciduous trees by laying eggs under the soft bark of the new growth on the trees’ terminal branches.

Like a number of other insect species (dragonflies for example), Cicada Killers spend ninety percent of their lives in various stages of dormancy underground. But the sixty days or so they are out and about cause quite a stir.  A recent article in The Atlantic discusses this insect from a different vantage point.  If you want to get even more involved in Cicada Killers check out Professor Chuck Holliday’s website and read about his research on these bugs.

I am doing a “Bug Mob” at the lek this Monday August 5th at 12:30 PM. Check that out on the Arboretum website.

Bob Mayer