M. fluvitilis produces M. fluvitilis and M. foedus produces M. foedus. I have not tried to hybridize them though, and I have not tried to produce one from the other (though if reared in the same way, and if they are merely environmental expressions of the same thing, the resulting insects should look the same, and they don't). Since I have never even seen one mating with the other in habitat, and since they seem to breed true under identical rearing conditions, and since they can be found in nature sympatrically over a large area (though favoring different environments) yet remain true to type, I seriously doubt they are the same species; closely related yes, but the same thing I don't think so.

Also, generally they do indeed keep to themselves, but they favor different environments, so this is not particularly surprising nor even exeptional, and to me it simply reinforces the theory that they are different species. However, I have indeed found them co-occuring in the same locatiions, though perhaps the exception to the rule, it is not a rare occurence. It is (or - different topic - was) not unusual to find them together in open broad sand environments of the bottoms of the South Platte, and especially of the Arkansas Rivers, where there are hammocks of woody vegetation intermixed with sandhills type habitat. One tends to keep close to the more wooded bits, while the other favors the more open sections, but the two can be found together, sitting on the same plants. They remain distinct, I have never seen the two types mating with one-another, and have not seen any individuals that might be considered intermediate. The coloring is different enough that it is hard to confuse them, even though the details of morphology are basically the same. However, M. fluviatilis often has shorter wings when all else is equal, particularly southward as it turns into "isleyi". Just as an aside, I rarely find M. packardi with M. fluviatilis either, the habitat preference shows even less overlap, but all three can be found together in the same places, sometimes (along with also closely related, but somewhat more different and smaller M. angustipennis).

While mine is all anecdotal and subjective evidence, I don't see anything yet to convince me that M. fluviatilis and M. foedus might be the same species, but lots of evidence to the contrary.

It seems to me that an argument that cites the two as never being found in the same place, would be evidence favoring different species. If they are mere forms of one another, one should produce the other and vise-versa, or there should at least be intermediates. I personally don't think the color pattern differences are of a sort likely to be produced by environmental triggers either. The patterns are fundamentally different, beyond what is seen in other species that exibit color morphs or ecotypes. In those other species it is always the same pattern, but with differences in expression of pattern elements or hues. In the case of M. fluviatilis the pattern is actually different. Also, I don't believe any environmental differences over such a huge distribution would account for seeing only one form or the other in a given location, and never a mix of the two together. The evidence, to me, suggests a total (or at least nearly total) population separation, and likely the presence of reproductive barriers between the two.

I expect that the situation is rather more similar to that seen in Schistocerca shoshone vs. Schistocerca lineata [venusta], where two very closely related insects have relatively recently occupied different, though adjacent environments, and have become reproductively isolated, but still are genetically very closely similar. I do not consider such to be ecotypes, as they don't interact as parts of a single population, but rather maintain themselves as different populations over a large overlapping region; behaving as different species.

I believe that M. bowditchi may be a similar situation, up to a point, but I don't think any populations of M. bowditchi have reached the point of being totally separate populations, and I don't think there is any reproductive isolation between them except for the choice of different hose plants.

I think more work needs to be done with the living insects to conclusively prove one way or the other. In the mean time, I think it would cause confusion to simply lump them together as if M. fluviatilis doesn't exist, and it is a very distinctive insect.

I agree, it is a mess. I was once convinced they were separate species, but now, not so much. Still, as with anything in science, nothing can be truly proven. As many studies in other taxa have shown that gene expression associated with diet is related to what the mother fed on, rearing to f2 or even f3 generation would be a necessity to determine. At the same time, my failure to find any consistent differences in the internal male genitalia suggest one species. I agree, much more work needs to be done. I appreciate the discussion, experience, and insights, thanks.

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