I am growing Rubiaceous ant-plants (Myrmecodias and Hydnophytums) in a greenhouse in Michigan. The ants in the 3 photos attached here have moved into some of the Myremcodia plants.
The ants are 2 mm long. The are a reddish-tan color except for a black abdomen, black eyes and black clubs on the end of the antenna.
The photos are not too good. The lines in the photos are mm lines on a ruler
Thanks for your help.
Thanks for the question! Luckily, this genus of ants, Cardiocondyla, have some very distinctive characteristics, especially when top and side views are available. The post-petiole (the second segment of the "waist" of an ant) is always much wider and at least a little shorter and more shallow than the petiole (first segment of the "waist"). This character alone is usually enough to identify them. Another feature every member of this genus that I am aware of shares is the matte texture of the head and much of the thorax (for myrmecologists using a microscope: the dorsal surface of the head, petiole, post-petiole, and often much of the dorsal and lateral alitrunk is pocked with evenly spaced, shallow fovea, yielding what appears to be a fine punctate sculpturing under low magnification). As a general rule, one should not use texture as a character to identify ant genera, but I know of no Cardiocondyla that are smooth-headed, or have groves or wavy lines like some ants (compare an average Cardiocondyla with a Tetramorium or Diacamma).
With regards to the species, one of the most common and widespread species is Cardiocondyla emeryi. This species has been reported from many latitudes on every continent and many tropical islands around the world. A very nice representation of some of these localities is given on its species page on AntWeb (linked above). Another common, widespread species is Cardiocondyla obscurior.
More information on Cardiocondyla and its species can be found in Seifert's very thorough revision from 2003 ( linked here). As Seifert notes, the genus has many species that cannot be easily distinguished with a microscope, and C. emeryi itself has such a wide range and is so variable that many would not be surprised if it turned out to actually be multiple species. Because there seems to be almost as much variation within some of these species as there is between them (I think no one could have done a better job with this group than Seifert, but it does make me a little bit suspicious that he had to use mathematical formulae to make sense of his morphological measurements...), the last word may not have been written about the species limits even of the most familiar members of this genus. I encourage you to read more about the fascinating behavior and reproductive strategies of these ants (an excellent overview in the introduction of the Seifert 2003 article linked above). Perhaps we'll have to do another blog post some day on the fascinating biology of Cardiocondyla!
I hope this helps!
Jesse Czekanski-Moir & the AntAsk Team