biology zoology blog benno meyer petioles

Petioles aren’t Petty at all

But what really are they then?

Petioles are not petitions and botanic petioles aren’t entomological petioles. So, what are they? Their etymology gives us a clue, for petiole comes from petiolous (= little foot, stalk). In entomology, however, it does not refer to a foot, but to the narrow waist characteristic of the bodies of wasps and all other members of the insect group known as Apocrita, i.e., the hornets, ants and bees. We are keeping wasps and hornets here in our laboratory in Korea, because farming wasps has become an industry in China and wasp larvae, which incidentally (just like the larvae of the other Apocrita) do not possess the petiole, are considered not only edible, but delicious in taste when lightly fried in oil.

The bodies of adult insects always consist of three parts: the head, the thorax (with three segments, 6 legs and the wings), and the segmented abdomen. In the Apocrita something strange has happened around 250 million years ago: the first abdominal segment has shifted to the thorax, and a “stalk-like” connection between the modified thorax and the remaining abdomen (now one segment short and no longer termed ‘abdomen’ but ‘gaster’) has evolved. But what can be the advantage of this narrow “wasp waist” through which nerves, the intestine and haemolymph must pass? What is its function? There are several hypotheses to explain how and why the petiole has evolved.

One idea is that ancestral Apocrita were parasitoids, which means they laid their eggs (just like hundreds of parasitoid wasp species even now) into larval insects like caterpillars and spiders. They use their pointed and tube-like ovipositor for that, but in order to increase the precision with which they can deposit their eggs, they need a high degree of freedom and flexibility for the abdominal movements to do the job quickly and accurately. Obviously the thin and stretchable membrane around the narrow petiole rather than the hard and stiff chitin cuticle elsewhere covering the insect body, was a help and when later in evolution some ovipositors became modified and used as a stinging device, movements to direct the sting and by curling the gaster also benefitted from the presence of the petiole.

Another advantage of possessing a petiole over species that lack it has been suggested. Any thin and narrow structure, especially one that is not surrounded by hard cuticle, can warm up very quickly when exposed to the warming rays of the sun. Insects possess a dorsal heart, actually more like a dorsal heart vessel that contracts rhythmically and usually pumps the haemolymph (which is the name of the colourless blood of insects) forward towards the thorax and the head (although it is capable of reversing the direction). During its forward flow, the blood already warmed up rapidly in the narrow petiole, first reaches the flight musculature. It raises the temperature of the flight muscles and allows them to function with no delay.

The narrowness of the petiole, however, means that in some species, e.g. fire ants, solid food particles can apparently not pass through it and only liquified food can be transported to the abdomen. It seems that this has led some ant species to use their own larvae to render food edible by the adults. The larvae possess powerful enzymes in their saliva and the adults expose dehydrated food to the larvae, whose saliva changes the solid food into a slush that can then be used by the adults. As usual, there are always two “sides to a coin” and a compromise between advantage and disadvantage is often the only way.

© Dr V.B. Meyer-Rochow and, 2020.
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