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cryptic Bombus mixtus

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  • Doug Yanega
    Hi, folks. I m putting out a notice regarding the western NA species Bombus mixtus, that potentially affects hundreds of misidentified specimens scattered
    Message 1 of 1 , Oct 19, 2011
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      Hi, folks. I'm putting out a notice regarding the western NA species
      Bombus mixtus, that potentially affects hundreds of misidentified
      specimens scattered around. If you have a collection containing
      specimens of Bombus "edwardsii" (the black form of B. melanopygus),
      from counties ranging from central California northwards into Oregon,
      you need to be aware that there is a morph of Bombus mixtus in that
      area that is extremely difficult to distinguish, and every collection
      I've examined thus far (including our own) has had some of these
      mixtus specimens mixed in among the melanopygus. It is possible that
      this morph is the same as the taxon named "Bremus edwardsii russulus"
      by Frison in 1927, but I have no definitively IDed specimens to
      compare to.

      The solid diagnostic features are fairly subtle (a shorter malar
      space, and a more arcuate metabasitarsal margin, finer and sparser
      clypeal punctures), and it's not possible to just casually scan for
      these traits. Fortunately, there *is* one very quick and easy feature
      that will greatly reduce the number of specimens one needs to
      examine: if the yellow hairs on T4 have a narrow black "part" along
      the midline, then that is melanopygus (or bifarius, which sometimes
      gets mixed in, but can be easily diagnosed by the pale golden-brown
      metabasitarsi and short malar space). There are occasional specimens
      of melanopygus where T4 is entirely yellow, or where the midline is
      obscured or rubbed, and then one checks the midline of T5; if the
      middle of T5 is black including the posterior fringe, then this is
      also melanopygus. In the "edwardsii-like" morph of mixtus, T4 is
      always entirely yellow, and T5 has, at best, a few black hairs in the
      middle of the disc (rarely any black hairs at all), and never
      including the posterior fringe. Those few melanopygus with similar
      tergal coloration can be readily separated by the structural
      features. Also, the mixtus morph *almost invariably* has yellow hairs
      medio-basally on T2, very similar to the common color pattern of male
      melanopygus. Yellow hairs in this position are rarely seen in
      melanopygus females, and only a very few mixtus lack them.

      A few additional details: (1) this morph is represented almost
      exclusively by queens. Workers from these populations have enough
      black hairs on the face that they have historically almost always
      been correctly IDed as mixtus, and males are no different from normal
      mixtus males. (2) the majority of specimens of this mixtus morph I
      have found are from Plumas and Shasta county, but they also appear to
      be common in the Sequoia National Forest (Fresno & Tulare), as well
      as occasional specimens from Tuolumne, Nevada, El Dorado, Siskiyou,
      Mariposa, Yuba, Sierra, Lassen and Santa Cruz (the one outlier, of
      which I am slightly suspicious) counties. One can certainly expect
      them in other parts of the Sierras, at least, though probably not
      along the coast (that Santa Cruz specimen would not be the first
      specimen someone mislabeled, but it might be correct, who knows...)

      So, check your collections - I'm betting that lots of these will turn
      up when people start looking.

      Peace,
      --

      Doug Yanega Dept. of Entomology Entomology Research Museum
      Univ. of California, Riverside, CA 92521-0314 skype: dyanega
      phone: (951) 827-4315 (standard disclaimer: opinions are mine, not UCR's)
      http://cache.ucr.edu/~heraty/yanega.html
      "There are some enterprises in which a careful disorderliness
      is the true method" - Herman Melville, Moby Dick, Chap. 82
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