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I photographed these (unidentified) wasps on a sunny but cool winter day in northern Taiwan because they were conspicuously hanging out on a hand railing and had much lighter coloring than I'd ever seen before.

But after I took the time to look at a close-up I realized "Wow, they really have long, very thin waists!" Mechanically, it looks like they are two large masses connected by a very long thin beam, which must have quite a tiny canal in the center to let all the "juices" flow through.

While other insects and spiders may have segments separated by a tiny connection, this one is both small in diameter and quite long.

Why? Does this specific configuration provide some benefits to them?

wasps in December, northern Taiwan

wasps in December, northern Taiwan

wasps in December, northern Taiwan click for full size

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  • $\begingroup$ companion question: What are the three dots (eyes?) on the top of this wasp's head? What species might it be? $\endgroup$
    – uhoh
    Dec 26, 2023 at 7:05
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    $\begingroup$ Note that mechanically, the required slenderness ratio of a self-supporting beam is inversely proportional to scale. So this feature is much less costly for insects than it seems to be from macro photos. $\endgroup$
    – Therac
    Dec 27, 2023 at 23:03
  • $\begingroup$ @Therac agreed for sure - but... why? The reason I mentioned that it looks like two masses connected by a beam (and with wings attached to one of them) is that I wondered if this configuration has some mechanical advantage - perhaps during flight. As you suggest, it can be small in diameter and still plenty strong for the job, but I also wonder why it's so... long. $\endgroup$
    – uhoh
    Dec 29, 2023 at 4:45

1 Answer 1

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Why do humans have such a flexible shoulder? Our ancestors relied on throwing things so the ones who could throw things better did better. What is the wasp's equivalent weapon? The stinger. Wasps have such thin "waists" to facilitate maneuverability of the stinger. This is especially important in wasps because for many, and for the ancestors who first evolved this feature, a flexible thorax with an ovipositor capable of depositing eggs into grubs and caterpillars was fundamental to their reproduction. The path to their current form seems to have been as follows: 1) early stingless wasps develop an ovipositor and begin implanting their eggs into other insects, 2) the thorax evolves to be thin and flexible to facilitate this method of reproduction, 3) the ovipositor modifies into a stinger and, combined with that flexible thorax, the wasp becomes the Zorro of the skies.

See New Scientist's 20 November 2019 Hourglass figure: Why do wasps have such narrow waists?

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    $\begingroup$ I had also thought about that, but for the specific types I wonder whether it could also have aerodynamic reasons (in terms of efficiency and balance). $\endgroup$
    – U. Windl
    Dec 28, 2023 at 11:34
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    $\begingroup$ The ovipositor argument sounds (to me) like special pleading. While the wasp-waist may make ovipositoring somewhat more efficient it seems unlikely that this is of such major selection advantage to drive such a change in so many wasp variants. || Stinging flexibility as suggested by one respondent in your link sounds more likely. But even that seems an unlikely selection driver. $\endgroup$ Dec 29, 2023 at 1:10
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    $\begingroup$ @RussellMcMahon hmm... we have to remember that the stinger is a modified ovipositor, so only females can sting or "oviposit". In Sexing a Black & Yellow Mud Dauber Wasp "abdomen (A) to petiole (B) length ratio (A/B) - (male = 0.6 to 1.3, female = 1.5 to 1.9)" the abdomen is significantly longer for females. That's one species only, and I don't know if males have different aerodynamic requirements... $\endgroup$
    – uhoh
    Dec 29, 2023 at 2:02

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