What do antennae do?

2019 December 28

We all know that insects have antennae, but the question is, what are they for, exactly? They are clearly sense organs, but what do they sense? And why are they so wildly variable, ranging from tiny little threads, to great plumose things?

















Well, first and most obvious, they provide a sense of touch. Those long thready antennae, some of which are longer than the insect’s body, allow them to poke things from a distance. This is good for finding food (you can poke something without walking over to it, to decide if it is food), or for avoiding harm (if it is dangerous, you just lose an antenna tip instead of your head). This is particularly good for poking around under the leaf litter, and for general feeling around in the dark.

Next, a bit less obviously, they provide a sense of smell. The way I like to think about it, is that they are equivalent to our nasal passages if they were turned inside out. For example, look at this deer skull. See all the intricate passages around the nose? Those were covered with a mucous membrane that carried the smell sensors.



Now, imagine that this was turned inside out [1] to project out in front of the head. Now, it looks rather a lot like a plumose moth antenna, doesn’t it? They are still providing a lot of surface area for scent molecules to be picked up by, but now the area is outside of the head instead of inside of it.


This is arguably superior to the way our noses work, because we have to expend effort to draw air through our nasal passages, while insects just need to wave their antennae around. This also combines with using their antennae for touch, because when the touch something with their antennae, they are also doing the equivalent of licking it.

In addition, most insects can use their antennae to detect sound. The little hairs on the antennae can vibrate as sound waves pass over them, and depending on the length of the hairs they can respond to different sound frequencies. This is kind of like how our ears work, except our ears have the hairs inside our heads in the cochlea, while insects have them outside of their heads.

Insects can also use their antennae to detect moisture, vibrations, and in some cases direction and elapsed time. Some work has shown that they can provide balance for flight stabilization. They may also, along with body hairs, provide sensitivity to electric fields. Overall, the antennae are very much general-purpose sensory organs, and an insect that has lost them basically has had its senses reduced to sight and maybe some vibration sensitivity due to body hairs.

Of course, that means that insects that have emphasized sight heavily, like dragonflies, can get along with very tiny antennae:


[1] It has been a loooooong time since arthropods like insects had a common ancestor with vertebrates like us. In fact, the common ancestor is so far back, that it was before the complete sequence of embryonic development had been laid out, let alone the development of things like eyes and legs. The arthropods are Protostomes, which start as a sphere of cells that develops a dimple on one side that eventually turns into a mouth, with the dimple tunneling through to form a digestive tract and emerge at the anus. Vertebrates, on the other hand, are Deuterostomes, where the dimple starts as the anus, and then tunnels through the opposite direction to form the mouth. Then, later on, the vertebrates develop with their main nerve running along the back, while arthropods develop their main set of nerves along their bellies. And then, finally, vertebrates grow their skeletons on the inside while arthropods grow their skeletons on the outside. In a very real sense, relative to vertebrates, insects are backwards, upside down, and inside out.

2 Responses
  1. January 2, 2020

    One of your best posts ever.

    Antennae have an advantage over licking in that the insect doesn’t ingest the item. If it’s a poison, it only damages the antennae can doesn’t make its way into the internal organs.

  2. January 3, 2020

    Thanks, KT. It was kind of fun to browse through all my pictures looking for good antenna shots.

    Overall, antennae are really a good idea, and it some ways I am sorry that we don’t have them. But then I think about what little kids would do, grabbing each other by their antennae, and sticking them into all sorts of foul-smelling substances, and I think, “Nah, we’re better off without them.”

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