17 August 2010

The mystery of meadowhawks

A putative Ruby Meadowhawk.
In our post Identifying Odonata from photographs, we talked about one of the more vexing field IDs around: the red meadowhawks in the genus Sympetrum. Since meadowhawks are a signature species of late summer, I thought they would be a good start to what we hope is a series on difficult genera.

There are 62 species of Sympetrum worldwide, and 14 in North America. More or less. Species limits are not well defined for some, hence the difficulty in telling apart some species in the field, or even in the hand. Many species vary geographically. Generally, field marks to look for (especially in confusing species) include the pattern of black on the abdomen, color of the face, and color of the legs. When immature, meadowhawks are largely yellowish-orange, with facial colors in particular not fully developed or helpful.

Here are the species that most commonly cause confusion in North America:
  • Sympetrum rubicundulum (Ruby, hereafter RUME)
  • S. internum (Cherry-faced, CFME)
  • S. obtrusum (White-faced, WFME)
  • S. janae (Jane's Meadowhawk, JAME)
In the east, the face of CFME is brownish and this species cannot be separated from RUME (face yellowish to brownish) in the field. In the hand, examining the hamules under good magnification may help. However, there are intermediate forms (with intermediate-looking hamules) that cannot be identified to species. Some consider them hybrids, a subspecies of CFME, or (at least along the Atlantic seaboard) a separate species, JAME. Genetic work on these three species is inconclusive.

Mature WFME have white faces, which should be helpful. Until maturity, though, eastern forms cannot be distinguished from CFME in the field. Even in the hand, immature female CFME and WFME cannot be separated in the east, as their subgenital plates are essentially identical (in the west, the subgenital plate of CFME is quite different from eastern forms!). WFME also hybridize, or have intermediate forms, with both RUME and CFME.

A putative White-faced Meadowhawk.
There is a more distinctive meadowhawk, Sympetrum semicinctum, the Band-winged Meadowhawk (BWME), found across most of the central and northern U.S. It varies geographically, causing problems in some areas. In the east, identification is usually pretty straightforward as the wings are amber-colored to the nodus. But beware some RUME west of Ohio may have amber wing bases, sometimes as extensive as BWME.

An eastern Band-winged Meadowhawk.
In the Great Plains and to the west, wings may be colored, clear, or diffusely colored. In the center of the country, clear-winged forms are probably not separable from CFME and RUME. The western forms are often considered their own species, S. occidentatale, but genetic work indicates that S. occidentatale is not a valid species.

Likewise, in Europe, there are no good morphological characteristics to distinguish Common Darter (S. striolatum) and Highland Darter (S. nigrescens), and genetic work was also unhelpful.

The bottom line: beware of the little red odes of summer.

Additional reading:

Caitling, P. M. 2007. Variation of hind-wing colour and length in Sympetrum internum (Odonata: Libellulidae) from the Canadian prairie provinces Canadian Entomologist 139(6):872-880. (Abstract)

Canning, R. A., and R. W. Garrison. 1991. Sympetrum signiferum, a new species of dragonfly (Odonata: Libellulidae) from western Mexico and Arizona. Annals Ent. Soc. America. 84:474-479.

Carle, F.L. 1993. Sympetrum janae spec. nov. from eastern North America, with a key to nearctic Sympeturm (Anisoptera: Libellulidae). Odonatalogica 22(1):1-16.

Donnelly, T.W., 1991, Current problems in Sympetrum and Libellula (a.k.a. Plathemis). Argia 3(4): 8-12.

Parkes, K. A., W. Amos, N. W. Moore, J. I. Hoffman, and J. Moore. 2009. Population structure and speciation in the dragonfly Sympetrum striolatum/nigrescens (Odonata: Libellulidae): An analysis using AFLP markers. European Journal of Entomology 106:179-184. (Abstract)

Pilgrim, E. M., and C. D. Von Dohlen. 2007. Molecular and morphological study of species-level questions within the dragonfly genus Sympetrum (Odonata: Libellulidae). Annals of the Entomological Society of America 100:688-702. (Abstract)

This has nothing to do with taxonomy, hybridization, or identification, but I came across it in my research and had to throw it in because I liked the title:

Horvath, G., P. Malik, G. Kriska, and H. Wildermuth. 2007. Ecological traps for dragonflies in a cemetery: the attraction of Sympetrum species (Odonata: Libellulidae) by horizontally polarizing black gravestones. Freshwater Biology 52: 1700-1709.

1 comment:

Cathy Carroll said...

Nannothemis and Stylurus,

I had a Sympetrum sp.land on my very urban bird bath this evening. It stayed only a minute and then flew off high into my Noway maple. It had a golden head and a red body. Though I'm sure I'll continue to have plenty of questions, these posts are very helpful.