A planthopper with sex-limited mimicry

November 11, 2012 • 6:49 am

Oddly, the Guardian has published an account of a new insect species by Quentin Wheeler, a systematist at Arizona State University and director of the International Institute for Species Exploration. (He has also proposed a new species concept, but the less said about that the better.)

His article, however, is intriguing, for it describes a planthopper (an insect in the order Hemiptera, superfamily Fulgoroidea; also called “leafhoppers”) that apparently mimics an ant.  As Wheeler says of the Fulgoroidea:

 . . . [it is] a worldwide group of true bugs [JAC: remember, only the Hemiptera are “true bugs” to an entomologist] with about 12,000 species. That is, by the way, more than twice as many species as all mammals combined. To say that planthoppers are diverse is putting it mildly. They range in size from less than 2mm to over 100mm, populations exist with both flighted and flightless morphs, many are camouflaged green while others are brilliantly coloured including reds, blues, and hot pinks. And some, like F. indicus, are ridiculously shaped.

The new species is Formiscurra indicus, found (as the name indicates) in India; this specimen was collected near Bangalore and described by Vladimir Gnezdilov of the Russian Academy of Sciences and Chandrashekharaswamy A. Viraktamath of the University of Agricultural Sciences in Bangalore.  The intriguing thing is the sexual dimorphism, with the males looking antlike (I don’t have a picture of the female):

Formiscurra indicus

 

As Wheeler notes:

Sexual dimorphism is nothing new in the tribe Caliscelini, to which the new species belongs. In fact, males and females of two species of the genus Gelastissus look so different that they were initially described in separate genera! Females of the new species have reduced wings, a very compact body form, and the front of the head elongated into a cylindrical process. But the male is something else entirely.

Even as an entomologist I had to do a double take when I first looked at the photograph of F. indicus to be sure I knew which end was which. Males of the species apparently mimic ants, although it can be debated whether the ants should be flattered. The abdomen is exceedingly shortened and bulbous and the main part of the cranium almost looks as if it is part of the thorax, which has impressively long legs, but these modifications pale in comparison to an elongate protrusion on the front of the head that is approximately the size of the abdomen and similarly bulbous. Weird.

There are two questions that Wheeler raises implicitly but doesn’t address:

1. Why mimic ants?  Ant mimicry is common in many diverse groups; in fact, Wikipedia has an article on it.  There could be several explanations for why the planthopper is such a mimic.  The mimicry could be aposematic, that is, the ants that are being mimicked are poisonous and distasteful, and predators have learned to avoid them.  By mistaking the leafhopper for an ant, the hoppers gain respite from being eaten, an obvious selective advantage.  Alternatively, the leafhopper could live in an ant colony and gain advantages that way, including protection by being in a group or getting access to the ants’ food. I find this less plausible since ants are good at sniffing out intruders.  And there are undoubtedly other possible reasons for mimicry.

2. Why are only the males and not the females mimetic?  This is tougher; one would think at first blush that males and females would be subject to similar selective pressures. But that’s not necessarily true.  In swallowtail butterflies of the species Papilio dardanus, for example, males retain the ancestral wing pattern, which is similar among populations, but females diverge, with females from different places mimicking different unpalatable models.  P. dardanus is palatable—to birds. In the picture below, we see different “races” of P. dandanus from different places in Africa mimicking different local model species, all of the models unpalatable (the P. dardanus female mimics are in columns at left and right, while the local models are in the center).

The males in all populations look pretty much the same: like the male at upper left. (A female at upper right is the presumed “ancestral” form, which occurs—and looks like the males— in areas where there are no unpalatable models.) But note that the females are free to vary among populations—a remarkable case of inter-population differences within a species.  Why do the females but not the males vary, responding to local selective pressures?

One explanation involves sexual selection, and I’ll leave you to ponder that and make suggestions below?

By the way, which end of the hopper above is the head?

h/t: Matthew Cobb

13 thoughts on “A planthopper with sex-limited mimicry

  1. “(A female at upper right is the presumed “ancestral” form, which occurs—and looks like the males— in areas where there are no mimics.)”

    Should the last word be models, not mimics?

  2. How does one tell that one should stop writing R code on a Sunday afternoon?
    When one looks at an African butterflies distribution map, and one reads the caption, repeatedly, as “Bayesian Mimicry”.

  3. I’m guessing the hopper in the picture is facing to the left, but as the text indicates, the main part of the head is somewhere in the middle — presumably the part with the dark circular patches on top. (Are those eyes?) The saddle-shaped plate to the right of the patches would then be the thorax.

  4. Yep, facing left and the head is in the middle. The false ‘head’ is the bulb on the far left. This reminds me of another WELL KNOWN INSECT IN THE SAME FAMILY, the ‘crocodile head bug’ . These HUGE insects have a big head bulb which is thought to mimic a lizard or alligator head. See lots of pretty pictures of them at http://www.webandtime.com/flying-crocodile
    Those are amazing insects. Insects are amazing in general.
    As for the sexual dimorphism, I do not know of cases in the fulgoridae, but perhaps it is similar to the ‘scale insects’ in the same order. There, females stay on their food plant and do not move around and the males do move (and fly) to find females.

  5. Don’t know about the sexual selection, but could the participating genes be correlated (say, co-located) with sexual chromosomes? Half a population mimicry may be better than none.

    Less likely for wing patterns perhaps.

    Bug facing left, which also should be the mimiced ants orientation. By the way, are there examples of species mimics facing the “wrong” way?

  6. “(He has also proposed a new species concept, but the less said about that the better.)”

    I disagree on that. I think Wheeler’s phylogenetic species concept, although not perfect, is one of the most reasonable approaches out there to the question of what exactly a species is in empirical terms (i.e., a species criterion rather than a species concept, although that distinction doesn’t get used enough). I know you’re a biological species concept guy, but in very few cases is the biological species concept empirically applicable in any kind of direct fashion. In a few model systems, yes, we have data on reproductive isolation that can be used to examine species boundaries… in everything else, meaning perhaps 99.9999% of the species out there, the best we can do is look for morphologically or genetically discrete clusters of critters. Under the biological species concept, you assume that these discrete clusters are also reproductively isolated and therefore species. The phylogenetic species concept just takes out that step where we have to make assumptions about stuff that isn’t known and in practical terms is rarely empirically knowable. That makes sense, because most of the primary taxonomic literature doesn’t say anything about reproductive isolation (because the authors don’t have data on this point) or, when reproductive isolation is brought up it tends to be through sheer speculation (again, because the authors don’t have data). Either we assume that species that occur together and don’t form intermediates are reproductively isolated (which is a reasonable assumption, but speculative and rarely with any direct empirical foundation), or we guess about whether species with disjunct distributions might interbreed if they did somehow find themselves in the same place (this is where the really egregious arm-waving starts). In practice, the biological species concept and Wheeler’s phylogenetic species concept are identical in almost all cases, -except- that the biological species concept requires some additional arm-waving, and thus another step in the process where authors can get it wrong and end up basing their conclusions on guesses rather than data.

  7. My guess for the sexual dimorphism is that there is actually some psuedo-social / parental care behavior in some treehoppers. Females are known to stay on the same branch they oviposit onto and live alongside their offspring. The males are more mobile, I believe. Perhaps avoiding predators requires better aposematism for these males.

  8. The mimicry could be aposematic, that is, the ants that are being mimicked are poisonous and distasteful, and predators have learned to avoid them.

    I thought that was simply Batesian mimicry, and that aposematicism referred specifically to bright warning coloration…

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