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Ambush Bugs: More Controversy


Last week, we talked about some of the issues with classification surrounding Phymatinae and Reduviidae. Today we'll talk about a more nuanced question: are Phymata americana americana and Phymata pennsylvanica, two well-known species in Southern Ontario and the North-Eastern USA, actually the same species? Strap yourself in for a longer post, because we need some background first.


Above is a beautiful Phymata specimen, perched on some goldenrod. As you can tell from his partially out-stretched wings, he's getting ready to take flight! Can you identify it? What features do we check to differentiate between Phymata americana americana and Phymata pennsylvanica?

W. Connor Lynch - [The Prince] - Personal Collection

There is massive confusion when it comes to identification of Phymata species, at least in North America. A review of old species records from Michigan determined that individuals from P. pennsylvanica were commonly confused with P. wolffii, P. erosa, or even P. actutangula, a species from Cuba. Individuals from P. americana were mistakenly recorded as P. erosa or P. erosa fasciata. In Southern Ontario, many people identify local ambush bugs as P. erosa despite the fact that multiple studies (including one by the National Research Council of Canada) have found that the only species found in the area are P. americana americana and P. pennsylvanica. Why are there so many issues here?

The genus Phymata is notoriously difficult to identify. Trying to compare two species is like looking at a set of twins and trying to find ways to tell them apart without marking one of them with permanent marker while they aren't looking. There is an enormous amount of intra-specific variation in the genus, which can include not only colour pattern (which aids with camouflage tactics), but also morphological features such as antennal ratios and connexival shape. On top of this, we see sexual dimorphism, which can cause even more confusion if you're trying to compare a male of one species to a female of another. A few weeks ago, we talked about how species-level identification can often be done through studies of the genitalia, but, other than an in-progress initial investigation of genitalia by Christine Weirauch, there is no current key to identification of different Phymata species based on this feature, though I'm sure some real bug fanatics out there wish there were (wink wink, nudge nudge). Checking other keys that guide identification by morphology, you'll see that differences between closely related species can be very subtle. We'll talk about P. americana americana and P. pennsylvanica to demonstrate this and take a closer look.

Intra-specific variation - Differences in individuals within the same species

Connexiva - The flattened lateral border of the abdomen in hemipterous insects. Hard to describe, but you can find a Phymata diagram in this paper, which we'll also be pulling from later on.

Sexual dimorphism - When the male and female of the same species look different (have different morphology)

Spot the differences

P. pennsylvanica and P. americana americana are two very closely related species that are exceedingly difficult to tell apart, especially to the naked, untrained eye. Daniel Swanson puts forth a key to distinguish between the two species in his paper on their identification, which calls attention to the following features:

  • Overall size

  • Male antennal segment ratios

  • Fourth connexivum dilation

  • Shape of lateral margin between the third and fourth connexiva

  • Outline of connexival segments 1-3

  • Angles at connexival junctions

You might think that size would be easy to quantify, but with P. pennsylvania ranging from 7.5-9mm in length and P. americana americana ranging from 8.5-10mm in length, there is a bit of overlap. Additionally, you need to be very careful while measuring to accurately measure these tiny guys! Of course, even if you get an intermediate or unintelligible result, you can rely on the other sections of the key. Just make sure you don't skip too many parts!

Above, you can compare the connexiva and antennal ratios of P. americana and P. pennsylvanica. On the left, you can see how P. pennsylvanica's central segment juts out a bit more prominently and P. americana's upper connexival segments have little "spiky" bits at their bottom edges. On the right, you can compare the lengths of the terminal antennal segments between both species - look at how even though both antennae are (roughly) the same length, P. pennsylvanica has a much longer terminal segment relative to the other segments.

Please note: These drawings are not meant to be completely anatomically accurate/to scale, but simply to demonstrate the differences we see between these two species. For high resolution pictures of both species, please see Daniel Swanson's paper.

Brittany Smale - Diagrams of Phymata Morphology - Original Work

Next up are male antennal segment ratios, which are more reliable. You'll need a magnifying glass or dissection microscope to be able to properly view the antennal segments, but as long as you have that (and know what antennal segments are), then you're generally good to go. Antennal segment ratios between the terminal segment and the previous two segments are very obviously higher in P. pennsylvanica, making this a great way to tell the difference between the two species. While ratios are always larger for P. pennsylvanica when compared to P. americana americana, note that antennal ratios are also generally larger for males in each species. For this reason, it's best to identify by antennal segment ratio if you know the sex of your specimen and have something to compare to.

The final four differences between the two species all have to do with the connexiva. These parts of the key are somewhat more difficult to navigate both due to the differences being more subtle and the measures being less objective. For example, what constitutes "more or less straight?" The figures presented in Swanson's paper are certainly helpful and can provide something to compare to, but the intra-specific variation within both species can still cause issues.

In general, P. pennsylvanica will have their fourth connexiva more abruptly dilated, resulting in more obvious points where the brown stripe is on their abdomens. Up until that point, the outline of the abdomen is smooth. In P. americana americana, however, the outline of the abdomen is not smooth - there are usually "small salient angles posteriolaterally," also referred to as "spiky bits on the bottom of each connexiva." All scientific language, of course. It's best to explain this with a picture, so see below for a better look.


So who the heck is this? The fourth (terminal) antennal segment looks roughly the same size as each of the previous two antennal segments, not as large as both of them combined - point to P. americana americana. Additionally, if we look at the left side of this individual's connexiva, we can see that the outline of it isn't smooth - it's jagged, and kind of looks like each connexivum overlaps over the one beneath it.

Brittany Smale - Phymata americana americana - Personal collection

I get it. It's Hard.

As you may now know, it's difficult to tell these two species apart. But then again, calculus is hard, too - why should we care? My first answer would be, "It's cool, duh," but there are more legitimate reasons as well. Morphological differences between different populations are often used to help us with classification - are two groups different species or the same species? To get further into this, we need to define what a species is. There are about 30 different definitions of species because nature doesn't like to follow our labelling rules, but we'll focus on the following three:

Morphological Species - The smallest groups that are consistently distinct and distinguishable by morphology

Ecological Species - A population that fills an ecological niche that differs from the ecological niche of other organisms in its geographical range

Biological Species - A natural population that produces fertile offspring and is reproductively isolated from other similar populations

Morphological species. While P. americana americana and P. pennsylvanica do look frustratingly similar, their differences are significant enough that we could consider them to be different species if we were basing our classification on the morphological species definition. This might seem counter-intuitive based how subtle the differences we just discussed are, but consider crocodiles and alligators - these two groups are completely different families, but only someone who has researched into what the difference between them is would be able to tell that they are different upon first glance.

Ecological Species. Both species are generalist foragers, grabbing onto just about any tasty insect that comes their way. They also live and forage in the same places, not showing any preference for the plants they hide on to forage. Taking this into account along with their overlapping ranges, we would probably consider them to be the same species!

Now we're tied - one point for P. americana americana and P. pennsylvanica being different species, and one point for them being the same. Who will win the title of Ambush Bug Champion?! Err, wrong blog.

Biological Species. Groups that can breed and still produce viable offspring are generally considered one species – by utilizing laboratory breeding and rearing techniques, David Punzalan found that P. americana and P. pennsylvanica fulfill this requirement. Their relation is even further evidences by the fact that the two groups have overlapping geographical regions and seem to not care who they breed with (i.e. P. pennsylvanica males will hop on not only females from their own group, but also females from P. americana americana).

So we have two points towards these two groups being the same species and only one against. That means that they're the same, right? We should be calling the papers!! Not quite - scientists like to be really sure of things like this before names are changed. We might want to do some more research before declaring a change as big as this. More morphological differences might be discovered upon investigation of genitalia between populations, we might change how related we think these two groups are based on DNA evidence as additional molecular studies are completed. Changing naming and taxonomy is bad enough, but to change it and then have to change things back as we uncover more of the puzzle? Absolutely atrocious.

We don't have a final answer right now, but as more information comes to light, we'll get a final answer. What do you think? It might be kind of unsatisfying to not know for sure right now, but hopefully this was a nice example of how nuanced taxonomy can be! Next time, we'll move away from identification and taxonomy and get into some fun stuff!

#identification #classification #controversy #hemiptera

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