Cases of convergent evolution
What qualifies as a sting and what had better be called a bite is not always clear and most people say they’ve been bitten by a mosquito while some refer to the “bite” as a “sting”. If the sharply pointed end of a structure is pushed into the tissue of another individual, I think we should call it a sting – and stinging animals abound. The stinging devices in animals are amazingly diverse and although we tend to think immediately of the defensive stings of bees and wasps, stinging is also used to deposit, for example in female parasitic wasps, their eggs into caterpillar or spider hosts on whose tissue the larval baby wasps can feed. Injecting venom can be the fastest way to immobilise prey and some stinging animals like, for example the fish-eating slowly moving cone shells or some spiders and snakes that inject venom into their victims, make use of their stinging devices in this way. To obtain oxygen under water without having to come up to the surface, the larvae of the mosquito Mansonia spp. impale the air-containing cavities in the stems of underwater plants with their syringe-like “stinging” syphons.
Venomous snakes, depending on the species, have two ways of delivering the venom to their victims. In those with venom glands connected with teeth at the back of the mouth, the teeth are not hollow, but possess a narrow groove on their inner side, in which the venom runs along. More dangerous are the hollow front teeth of some snakes. These teeth are normally folded up and tucked away under the roof of the mouth when not used, but when needed they point forward to strike the prey and to deliver the venom through tiny pores at their tips. The bite is carried out with a considerable force and the venom is forced from its storage glands into the wound of the victim. A rather similar method is employed by spiders, which also inject venom through a canal inside their two oral appendages, known as chelicerae. The chelicerae have very pointy tips with a hole at the end and the venom stems from the glands at the base of the chelicerae or from deeper in the head. How a droplet of the liquid venom can be pressed through the very narrow tube in the hollow chelicerae and then leave through the tiny pore at the end is something for scientists interested in fluid mechanics and involves some knowledge of the viscosity of the venomous liquid. In most web-building spiders the two chelicerae work like pincers, but in the often much bigger mygalomorph spiders the chelicerae work in parallel striking prey like 2 axes from above.
Similar fluid mechanical problems would apply to the venom delivery of wasp and bee stingers, for they resemble hypodermic needles. The honey bee stinger is barbed (a bit like that totally unrelated stinger of the stingray) and stays in the flesh of a stung human, leading to the death of the bee, but continues to pump venom by itself for some minutes. Wasp stingers and those of some powerfully venomous ants like the Australian bull ant are smooth and can be used repeatedly. Giant centipedes are feared because of their strong mandibles, sometimes referred to as ‘fangs’, which possess a canal through which a venom can be injected into a bitten individual. However, stingers delivering a potent venom that can kill are known not from centipedes, but scorpions. The most dangerous stinging animals in the sea are probably the already mentioned cone shells with their single hollow chitin tooth connected to a venom bulb, further the so-called stone fishes and some others with venomous fin rays as well as stingrays (the death of Steve Irwin comes to mind). The only “stinging mammal” is the male platypus with its venomous and hollow hind leg spurs. But who’d want to pet or even have a chance to pet a platypus?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.