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Something that doesn't quite make sense to me is why lots of insects like ants, bees and wasps have a such a small petiole when it connects many major organs to the rest of their body from a suddenly much wider abdomen.

Normally from an engineering standpoint, you don't increase the width of a cantilever from a joint because the torque applied to a joint already increases with the length of an overhanging segment, so increasing the width would only multiply the torque that joint or opposing segment needs to balance.

It could have something to do with ants being so small that large torque isn't an issue in proportion to their body size, but if it wasn't, then why do their legs also get thinner as they measure further in length from their body?

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  • $\begingroup$ Human limbs also get thinner the further they go from the body, and it's not because they'd buckle under their own weight otherwise. $\endgroup$
    – John Dvorak
    Jan 22 '19 at 18:30
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    $\begingroup$ But it would be even harder for those limbs to move due to the extra torque that needs to be counteracted to move a specific direction. Joints are especially thicker than the shaft of a bone because they face more strain during locomotion and impact, and it makes sense bones would get thinner because if they were thicker, they would be that much harder to move at the same distance. $\endgroup$
    – Vane Voe
    Jan 23 '19 at 3:24
  • $\begingroup$ The normal engineering of cantilevers at the human scale can be relaxed at insect scales. At the small sizes of ants and wasps, gravity is a very minor factor because of the gravitational force affecting small masses as compared to the electromagnetic forces within and between materials. Not only does this mean insects can easily walk up walls and across ceilings, but also the extra weight of a wider, heavier cantilever (the abdomen) can be supported and moved by the material of a narrow supporting joint.(the petiole). Similar consideration occurs for the inertial force due to mass. $\endgroup$
    – mgkrebbs
    Jan 26 '19 at 3:58
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This thin waist is called petiole (aka. wasp‐waist).

How many times did the petiole evolve?

Why did a lot of common insects evolve such a thin waist?

Reading this question make it feel that you are assuming that several lineages evolved this petiole independently. It would be wrong. The petiole evolved once only and all the species that are having a petiole today (ants, bees, wasps, ...) are descendent of this lineage that evolved this petiole.

The taxon of all these species having a petiole is called Apocrita, which is a suborder of hymenoptera. You can learn much more about the phylogeny of these taxons in Rasnttsyn and Haichun (2010).

Why did it evolve?

The early lineage who evolved this trait where parasites (or even parasitoids at that time, I am not sure). This lineage was laying their eggs in tree trunk (or maybe already in other insects larvea if they were already parasitoids). The constriction that is the petiole allowed these individuals to have greater maneuverability for depositing their eggs accurately (Peters et al., 2017; Wilhelmsen et al., 2010).

The evolution of this petiole greatly affected the future evolution and diversification of Apocrita as things like stings and even maybe eusociality. From Peters et al., (2017)

The evolution of the wasp waist, a constriction between the first and the second abdominal segment greatly improving the maneuverability of the abdomen’s rear section, including the ovipositor, was a major innovation in the evolution of Hymenoptera that undoubtedly contributed to the rapid diversification of Apocrita

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  • $\begingroup$ So if it was thicker, they wouldn't be able to maneuver their abdomen as well to use as a stinger or to lay eggs? But if their petiole was thicker, wouldn't there also be more muscle tissue that could move in a desired direction? $\endgroup$
    – Vane Voe
    Jan 22 '19 at 23:29
  • $\begingroup$ @VaneVoe Well, more muscles does not necessarily give you more flexibility. You need some room to make the movement. Just like if you take a cardboard cylinder and creat a "petiole" in the middle, the movement will be greatly increased. I am no good physiologist but it feels pretty intuitive to me that this thinner waist increase movement amplitude. Watch a video of a wasp using its sting and you'll see how much flexibility they have at their petiole compared to other, thicker, body parts. $\endgroup$
    – Remi.b
    Jan 22 '19 at 23:40
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Apocrita insects (i.e. the clade containing bee, wasps and ants) are supposed to have evolved a thin waist because it gives them more flexibility to manoeuvre their abdomen. Most Apocrita are parasitoids and they need this flexibility to parasitize theirs hosts. The others have evolved a stinger: the petiole can serve them as a lever to better defend themselves against their natural enemies.

So while it's true that this thin waist brings physiological constraits (e.g. for food transit or the torque needed to move the abdomen), these disavantages are very likely over-rode by the adaptive advantage brought by the increased maneveurability of the metasoma (which carries the ovipositor/the stinger).

You can find more information about the petiole's evolution here and here.

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Specialized mechanical anchors join up arthropod exoskeletons to muscles and tendon cells. There's a complex network of interconnected matrix constituents, junctions and associated cytoskeletal elements. F-actin based fibres used as tendons fuse onto to myosin based structures which are used for muscle contraction.

The tendons can go through relatively thin areas of the insect with massive leverage. We think about robots and humans as using bones and metal joints with pivots and motors, when arthropods muscles move the exoskeleton rather than interior bones.

The exoskeleton constriction at the abdomen-thorax joint of arachnids and ants demonstrates the relative strength of insect exoskeletons, the exoskeleton material is so light and powerful relative to the size to permit the small form while conserving enough resistance.

The abdomen-thorax joints, which can be multiple segments, are bit it stronger than the legs. They are less likely to get damage there than on their legs. The abdomen and thorax are containers for all the internal organs, which is different than a mechanical limb, so they are bigger.

The slender waist allows for a lot of arthropod behaviour like weaving, stinging, burrowing, preening, symbioses, it's a complex field.

If humans can change the magnet based rotary muscles and use tendon based mechanics they can probably develop very cool machines for the future. here is the human designed comparison for arthropod locomtion: https://www.youtube.com/watch?v=Vi1hwdWUHvU https://www.youtube.com/watch?v=eYFYSYmv4v4

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2555930/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3335404/

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