Excerpts from Jim Conrad's
Naturalist Newsletter

Entry from field notes dated May 7, 2023, atop hill forested with pines and oaks about 1km east of Curva de la Doctorcilla, on unnamed road connecting Hwy120 and El Doctor; limestone bedrock; elevation ±2650m (8700 ft); Eastern Sierra Madre mountains of east-central Querétaro state, MÉXICO, (N20.88°, W99.62°)
RAMBUR'S FORKTAIL FEMALE, OLIVE-FORM

Rambur's Forktail, ISCHNURA RAMBURII, female, olive-form phase

Here at ±2650m (8700 ft) elevation atop a droughty hill with no ponds, streams or lakes I knew about, it was surprising to see the above damselfly land beside me, turning her head this way and that, apparently waiting for prey to fly by. Damselfly larvae, or naiads, are nearly always aquatic, so where was the home water? Later, on Google Maps, a small, dammed lake was noticed in the valley just below the hilltop, to the east.

Within the insect order Odonata, which is home to both dragonflies and damselflies, damselflies constitute the suborder Zygoptera. Within the Zygoptera, there are various damselfly families. With such a smallish, slender damselfly with transparent wings, it's a good bet that you have a member of the Narrow-winged Damselfly Family, the Coanagrionidae. That's the largest and most commonly encountered damselfly family, with more than 1300 species found worldwide, in over 110 genera.

Rambur's Forktail, ISCHNURA RAMBURII, adult female, olive-form phase, head and abdomen

As seen above, the black, white and tan markings on the sides -- lateral stripes on the abdomen -- were good field marks. By doing an image search for Mexican Coenagrionidae displaying such lateral stripes, several species of the genus Ischnura looked like good candidates. Ischnura species are known as forktails, because males bear a forked projection at the tip of their abdomen. Another forktail field mark is that their big compound eyes display a dark upper region and a contrasting lower part, often even more conspicuous and well defined than displayed above.

Rambur's Forktail, ISCHNURA RAMBURII, adult female, olive-form phase, end of wing close-up and tail

The bodies of male forktails often are predominantly blue and black, with the females plainer colored. We first noticed that on the Pacific Forktail in Oregon. Our hilltop individual was a female. Abdomen tips of males often are brightly pale blue, perhaps encouraging predators to attack the body's brainless rear end. At the right you see that our female's abdomen tip was plain. One wonders whether, in adaptive and evolutionary terms, this is because a female damselfly's egg-laying rear end is at least as important as her brain.

On the GBIF Ischnura occurrence map, graphically displaying the genus's worldwide distribution, by zooming in on our part of upland central Mexico and clicking "explore area," it became clear that here mainly we could expect two forktail species, Ischnura denticollis and I. demorsa. Their females were similar to ours, but the lateral striping wasn't quite right. However, well down the list there was a single observation of one other species, ISCHNURA RAMBURII, and that one's females were similar to ours. Usually this species is called Rambur's Forktail.

Actually, our female only approximately matched the vast majority of Internet images of female Rambur's Forktails. That's because female in the species display different "forms." On BugGuide.Net's Ischnura ramburii page they do their usual excellent job showing the variations, of which there are four:

Our hilltop one is an Olive-form Adult Female.

Despite the meager observation records of Rambur's Forktails in our part of upland central Mexico, the species is widely distributed from about the eastern and southern half of the US, and south into Chile in South America; in fact Rambur's Forktails are the most widespread of all New World Ischnura.

The matter of the females exhibiting several distinct forms has interested evolutionary biologists. Rambur's Forktail's male-mimicking females are considered an example of sexual conflicts shaping biodiversity. The 2017 study by Eben Gering entitled "Male-mimicking females increase male-male interactions, and decrease male survival and condition in a female-polymorphic damselfly" dealt with our species.

Observations were reported supporting the hypothesis that some males were quicker than others to recognize that certain of their population's females halfway looked like males, and to mate with them. Males slower to catch on mated less, thus contributed fewer genes to future generations. Confuse the males, then mate with whomever sees through the subterfuge. In the end, maybe the species evolves a little more discrimination for dealing with life's subtler deceptions.