麻豆影院

A mating wheel allows a precise comparison of the colors and patterns of males and females

The little marsh swarmed with insects. Tiny moths popped up from the grass, darted a short distance and then disappeared back into the grass. Small wood nymph butterflies kept searching for the perfect spot to perch. Band-winged meadowhawk dragonflies perched serenely on reeds. But it was the white-faced meadowhawks that caught my attention.听

An immature male white-faced meadowhawk, Sympetrum obtrusum, clung to a blade of grass, and a short distance away a mature male balanced deftly on a dry twig over shallow, slow-moving water. As I approached the adult male, it lifted up, moved about two feet, then settled down again. I kept getting closer for better photographs and my movement made the male move again, but it returned to its original perch on the twig. Reluctance to leave that spot at the edge of the water suggested territorial behavior--adult males defend a mating territory from other males, inviting females to mate.听

To test the hypothesis that males were defending territories I followed the water and found another male about 20 feet away and then another beyond that. But this third territory had a male and a female in a mating wheel. My approach disturbed them a bit, so they fluttered up, retaining their holds on each other, and moved less than five feet. The mating wheel in flight is a memorable event of athletic flexibility in flight. The male has claspers at the end of his abdomen that clip neatly and securely to a plate immediately behind the female's eyes. The female brings her genital opening, at the tip of her abdomen, in contact with the male's copulatory organ, on the ventral side of segments two and three at the base of his abdomen.听

A successful mating requires that the male and female sustain the mating wheel for a while, for first the male will repeatedly scoop the female's sperm receptacle, discarding the previous mate's sperm. Then he pumps his sperm into her. The time needed to accomplish this task varies from a few seconds in the larger dragonflies to many minutes or hours for the small dragonflies and damselflies.

A mating wheel allows a precise comparison of the colors and patterns of males and females. An adult male has a white face, brown thorax, and brilliant red on the dorsal side of the abdomen with black triangles on the lower sides. An adult female has a light-yellow face, a brown thorax with paler sides and an abdomen that is tan on the top and sides, but with blue and black lines running the length of the ventral side.

White-faced meadowhawks have a large geographic range with substantial environmental diversity. They are distributed from the Atlantic to the Pacific in Canada, and south to Virginia, Colorado and central California. Given this wide range, it is not surprising that dragonflies would vary from place to place in important ways. While the description of the female color pattern above is appropriate for the Colorado Front Range, it would not be a complete description of the females in Wisconsin.听

Species of dragonflies and damselflies frequently exhibit multiple female color morphs. For example, blue-tailed damselflies in the British Isles have been studied extensively and five color morphs have been described. In species with multiple female color morphs, one is usually named "andromorph" to note its similarity to the male color morph.听

In Wisconsin, some females have the tan morph described above, but others are andromorphs, with bright red on the top of the abdomen and at the bases of the wings on the thorax. Because andromorphs occur in many species, evolutionary biologists have looked for a common mechanism to understand their repeated, independent evolutions. Some observations of andromorphs have noted that they appear to be harried less by males than other morphs. For example, while a female with the normal color pattern is moving about to lay eggs, males will attempt to mate with her. Even with the most recent mate protecting her, suitors will try to intervene, triggering fights. Furthermore, andromorphs have been noted to display some male aspects of behavior, such as responses to other males. Andromorphs may benefit by flying under the radar and being allowed to lay their eggs peacefully.

While the speculative hypothesis above would explain the repeated evolution of female andromorph color patterns, it does not explain why the andromorph color pattern is seen in Wisconsin, but not in Colorado. I wonder how widely distributed the white-faced andromorph is and whether wider sampling would reveal even more female morphs. We have much to learn about white-faced meadowhawks.