Why do ants and other social insects have elbowed antennae?

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Odontomachus clarus nurse worker tenderly caring for a larva (Arizona).

A defining feature of many social insects is an unusual antennal form.  The base segment is elongated such that the antennae take on a shape rather similar to a human arm. What’s up with this “elbowed” appearance?

Quite simply, the kink allows the insect to feel with her antennae what she is holding in her mouth. A useful trick for social insects that care for their young!

(In case you’re wondering, I am currently assembling a lecture on ants for the IB109 class…)

19 thoughts on “Why do ants and other social insects have elbowed antennae?”

    1. I’ve been told that this in combination with an elongated rostrum is an adaptation for nectar feeding. The geniculate antennae fold out of the way which means they do not get damaged when the weevil shoves its face in a flower 😛
      I think Rolf Oberprieler did some work on this?

  1. Why do so many social insects NOT have elbowed antennae or not as elbowed as some ants have ?

    Do more primitive ants have the first segment small, like many social wasp species ?

    Can we graph 1st antennal segment length vs ‘degree’ of social development and see a good correlation ?

  2. Yeah, I agree with Bob that many other social insects (termites, thrips, aphids) do not have this character. It seems like it should be “why do so many social aculeates have elbowed antennae”…and weevils…which some are eusocial

    1. Intriguingly, Matt, all those other eusocial examples are hemimetabolous with relatively autonomous nymphs. Social insects with relatively sessile larvae are those with geniculate antennae. Coincidence?

      1. I can’t see much difference in the elbowed antennae of social vs solitary Vespidae or solitary vs social Apinae and looking at pictures such as this of a velvet ant:
        I would guess that elbowed antennae per se are useful even without hungry mouths to feed. So maybe it is carrying one’s young around that contributes to ants’ long pedicles. Do any other social insects regularly transport their grubs?

  3. Most non-social aculeates have elbowed antennae, too, even if the scape is realtively a bit shorter than in many social ones. Of course, they too handle (well, they mandibulate [verb]) stuff and sniff/fell (antennate) simultaneously. So, I think social aculeates have elbowed antennae (with elongate scape, or first segment) because it was adaptive for their non-social ancestors to evolve them, and it continued to adaptive to maintain them for the social lineages. Males of the social ones don’t handle prey or manipulate paper or wax or other nest materrials, so their scapes have less resources allocated toward building them, i.e., are shorter.

    Richard Brown at Mississippi State has some ideas on this topic and I have alerted him to this post.

    1. That was my thought as well, although you elaborated on it more and put it down better then I would 😉

      However, I would add a question to why Sphecid wasps have a very small pedicel, while some Crabronid wasps have a well developed pedicell, while the life-style in many cases seems comparable?

      Actually, both cases appear within Crabronidae, for example Cerceris have a short, high-inset pedicell, while others like Ectemnius (all of Crabronini?) have a low inset, long pedicell. Could it be linked to the way the wasps excavate nests? Cerceris in sand and soil, like Sphecids, and Ectemnius excavate tunnels in wood.

      Another thought: Why do insects with pedicells have the antennal socket inset so low in the face, often at the clypeal suture?

  4. Richard L. Brown

    Robert Hickling and I co-authored “Analysis of acoustic communication by ants (2000, J. Acoust. Soc. Am. 108: 1920-1929). Given that stridulating ants have resonating chambers (expanded tracheal sacs) adjacent to the stridulating structure and that ants make different signals in different situations, we proposed that stridulatory signals were carried through air, based on near-field theory, and that sensilla on the antenna responded to the sound. Because sensilla are concentrated on the distal segments and scape, we further suggested that an elbowed antenna maximizes sensitivity to the sound-velocity gradient to determine direction and location of a sound source because the signals would stimulate apical sensilla a millisecond before those on pedicel. Honey bees, which also have elbowed antennae, also respond to near field sound. The question is whether parasitic wasps with elbowed antenna also respond to near field sound. You can get a pdf of this article by googling the title or contacting me.

    1. curious and curiouser !! Pretty amazing what all a teeny-tiny-ant-brain can do. I feel sorry for those AI devs, can’t even get close to what a bug can do, ROFL.

      Thanks Dr. Brown

  5. Marc "Teleutotje" Van der Stappen

    Long ago, there was a discussion about the elbowed antenna between Wilson and Dlussky when talking about fossil ants. (I can only find the Russian version of Dlussky’s article online, not the English translation.). Most important result: Elbowed antenna are needed to bring the tip of the antenna to the mouth, so the ants can feel and taste the food between their mandibles.

    Dlussky: http://gap.entclub.org/taxonomists/Dlussky/1983.pdf
    Wilson: http://gap.entclub.org/taxonomists/Wilson/The%20earliest%20known%20ants.pdf

    1. Richard L. Brown

      There is no correlation between elbowed antennae and the need to bring the tip of the antenna to the prey or mouth….all other insects have solved this problem without evolutionary selection for an elbowed antenna. Ants and other insects do not choose to have have an elbowed antenna, there has to be selection for this morphological adaptation….the question is what selection gradient has resulted in elbowed antennae.

      1. It would seem to be relatively simpler to evolve a patch of integument chemo-receptors or other sensing structures wherever needed versus elaborating a specialized movable-jointed structure. There are certainly no shortage of available base structures in mouth parts to work with. It is also pretty clear that selection is always at work on mouthpart structures since there is always so much variability present.

        All those deaf nasty bugs are always coming up with clever ways to hear – it spoils my appetite…..

        I kind of doubt any single ‘selection gradient’ is/was involved in elbowed antennae as opposed to more than one. There is also the possible ‘we now have lemons so we make lemonade’ factor involved. Antennae are clearly multifunctional and evolution grinds so finely because it has so large a time-grindstone and just so many bugs to work with. I suspect there are many stories to be exposed and I do so love a good story !

  6. I’ve only seen a handful of male ants, but their antennae seemed more filamentous and less elbowed. Just thought I’d offer that observation into the mix!

      1. Ya, that’s the storyline anyway, however many other species with short-lived, apparently one-use adult male/females, DO show some morphological selection of phenotype traits.

        For example, eye enlargement in male mayflies (and bee drones for that matter):

        It is possible that the results of male ant selection are chemo-sensory and not visible rather than non-existent at all since ants are often skewed toward the “chemical”. I could easily be wrong, tho.

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