There are over 7000 different answers to this question because there are over 7000 species of cockroaches (non-eusocial cockroaches and termites). Well…actually 5000 (just non-eusocial cockroaches). Or is it 9000 (non-eusocial cockroaches, termites and mantises)?

Ok, so clearly there might be some complications about how many cockroaches there are. This stems from complications about how to answer the question “what is a cockroach?”. What is any animal for that matter? What is a horse? Is a zebra a horse? What about a giraffe? Is a giraffe a horse? Is a termite a cockroach?

We need clarification if we are going to make this any less confusing. I am going to break down the question, “what is a cockroach?” from all angles. Below you will learn:

the bodily features that are used to define cockroaches how cockroaches are defined through their ancestry problems resulting from classifying animals the right way to define cockroaches and termites

Answer number 1: What is a cockroach? Things that look like cockroaches!

If I asked you to pick a cockroach out of a line-up you would you be able to do it? Most people have an image in their head about what they think a cockroach “should” look like. This is one of the oldest and most widely used systems for classifying organisms.

In this method we can answer the question by defining cockroaches based on a set of features which are shared by most cockroaches. This is the morphological approach (morphology = the physical features and shape of an organism).

Here are a few physical traits that DEFINE cockroaches.

Prominent cerci

Cerci are structures on the hind quarters used for sensation (among other things). Simply put, cockroaches have big cerci.

Laterally expanded and prominent pronotum

The pronotum is the big shield like structure that most people mistake for the head of the cockroach. Once you take a close look you see that the head is actually right underneath the pronotum. The fact that it is “laterally expanded” just means that it is wide, flat and doesn’t curve around the body, like in grasshoppers.

Cursorial limbs

This means they have legs modified for running. It is particularly important that the front pair of legs are used for running. Compare this to mantises, whose front legs are modified for rasping and killing prey (raptorial) or compare this to grasshoppers whose legs are modified for jumping (saltatorial).

Purse-like egg case

Cockroaches develop an egg case that is hard and leathery.

Lack of an ovipositor

Female cockroaches lack an ovipositor, which is an appendage used to lay eggs.

Presence of endocellular symbiotic Blattabacterium

Blattabacterium are common to most cockroaches and are necessary for their survival. They assist in processing the nitrogen in their diet.

There are other traits that are common to almost all cockroaches but these are some of the most important ones.

Problem with this: There are too many exceptions! Many species don’t have cursorial limbs; many species don’t have a laterally expanded pronotum; many species don’t produce a purse-like egg-case; and there are species that don’t have Blattabacterium. If we have to make exceptions then it becomes okay to start including things that aren’t obviously cockroaches.

Using this definition, mantises and termites are separate from cockroaches.

Answer 2: What is a cockroach? All of the descendants of the ancestral cockroach!

Willi Hennig. If you haven’t heard of him you’re probably not a biologist, or at least an evolutionary biologist. He’s a very important guy.

Willi Hennig set out the rules for how we define and name groups of organisms. You can read more about him here but basically what he said is that the only valid way to define groups is based on common descent i.e. their evolutionary family.

Back to cockroaches. If we are going to answer the question of what a cockroach is we need to look at their evolutionary tree.

Based on this tree, everything in the red box is a cockroach. That includes that little line next to the blue dot, the ancestor of all cockroaches that lived 175 million years ago.

Problem with this: By using this definition we begin to include things that don’t look or behave like cockroaches. Under this definition, termites are included in cockroaches. Mantises can or can not be included depending upon how early you want to draw the line (in the image above I have not included Mantises as part of cockroaches).

Answer 3 (cop-out version): What is a cockroach? It’s complicated.

One definition leaves out cockroach like things and another definition includes non-cockroach like things. Needless to say, it’s complicated.

You may have thought to yourself, “if cockroaches are defined by all the descendants of the ancestral cockroach, then how do we define the ancestral cockroach?”. That certainly is a valid point and we’ll get to that below. Also, if you did some quick googling you could have found some examples of cockroaches that lack those physical features I mentioned. I will also address this below.

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But first it is valuable to note the reason why we run into these problems. It’s that the idea of a “cockroach” (or any animal, plant etc.) is a human fabrication. It’s something we do to make nature a little more organized so it’s easier for us to wrap our heads around. That doesn’t mean it’s random or completely arbitrary though. We do try and classify based on real patterns in nature. But since nature is complicated there are always problems or exceptions.

So, we need a concept to classify (define) cockroaches that accounts for problems and exceptions that also is somewhat simple to use. Who figured this out? Can you guess?! Willi Hennig once again. In his principles he accounted for this. Willi Hennig said: 1) we classify based on ancestry (I said that above)and 2) that shared traits are the best evidence for supporting ancestry and evolutionary relationships. These two principles unite the different classification concepts discussed above. This is called Hennigian Systematics and it’s the gold standard for classifying organisms.

Now let’s unite our two classification schemes above to answer our question the way Willi would.

Answer 3 (the Hennig/Awesome version): What is a cockroach? Cockroaches are all organisms derived from the single common ancestor in whom traits arose that were then retained among the daughter lineages.

Ahh!!! Too many words! The simple version is: cockroaches are this one ancestral species and all of its descendants that share similar features.

This is the same as answer 2 but with the addition that we use shared traits. This gives objectivity to our cutoff point of what is and what is NOT a cockroach. Also, the process of using the evolutionary tree to interpret the traits allows us to consider that traits can be lost in some of the more recent species.

When we use physical characteristics in the context of when and how they evolved, we will find that one branch of the resulting evolutionary tree will be defined by: wide pronotum, running limbs, purse-like egg case, and the presence of Blattabacterium. Those are the cockroaches.

This resolves most of those problems I mentioned above. The many exceptions to the morphological traits are resolved because we have an objective way of inferring trait gains AND losses over evolutionary time.

We can now justify the “cockroach-iness” of our ancestral species. The evolutionary tree tells us what species came from which ancestor. We get to an ancestor that has most of those cockroach-y traits (the branch with the blue dot) and we then say that this species and all of its decendants are cockroaches.

Now we are still including things that don’t look like cockroaches BUT we have an explanation. Although most modern termites are lacking a large pronotum, purse-like egg case, and Blattabacterium, their ancestor probably had ALL of those things. Although, Saltoblattella has jumping hind-legs its ancestor probably had running hind-legs.

Problems with this: Other than being slightly more complicated than the others, this is by far the best answer.

A summary:

What is a cockroach? Everything circled on the tree below is a cockroach. The groups on the tree represent over 7000 species of cockroaches. The traits that define cockroaches are: large cerci, wide pronotum, running limbs, purse-like egg case, lack of an ovipositor, and the presence of Blattabacterium. However, the presence or absence of one, more, or even all of these traits does not necessarily mean that the animal in question is a cockroach. However, all animals within the branch of the tree that is defined by these traits is a cockroach. So, termites are cockroaches, because their phylogeny shows that their ancestors were cockroaches.

Dominic Evangelista is a PhD candidate studying the biodiversity and systematics of the cockroaches of the Guiana Shield. Follow him on twitter @Roach_Brain or ask him a question about cockroaches!