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A guided journey · 4 stops

Reading movement in what is around you

Water being pushed aside, and the ground shaking — two channels no list of the senses mentions, and the animals that live in them.

The route

  1. The lateral line
  2. Sharks
  3. Feeling the ground
  4. Spiders

The senses everyone can name detect things that travel: light, sound, chemicals in the air. This route is about two that detect the medium itself being moved — water displaced by something swimming, ground shaken by something walking or calling — and the animals for which those are the primary way of knowing what is nearby.

Neither fits the categories. The lateral line is called a kind of hearing and it is not: sound is a pressure wave that propagates, and the lateral line reads the near field, the water actually being shoved aside, which disappears within about a body length. Substrate vibration is called feeling and it is not quite that either, given that it carries signals produced deliberately by other animals for the purpose.

The order runs from water to land, and from a sense with a dedicated organ to one that is probably the most widely used and least studied channel on the planet.

  1. Stop 1 of 4. Senses and abilities

    A sense that is neither hearing nor touch

    Start with the clean case, because it has a dedicated organ and a decisive ablation experiment. Block the neuromasts and a fish keeps its hearing, keeps its vision, and loses schooling in the dark and obstacle avoidance. That dissociation is what establishes this as a separate modality rather than as a variety of something familiar.

    Read The lateral lineNot hearing. A fish’s lateral line reads the water actually being pushed aside — which is why it works within a body length and nowhere further.

  2. Stop 2 of 4. Marine life

    One animal running four of these at once

    A hunting shark uses smell to get into the area, hearing and the lateral line closer in, and electroreception at the moment of the strike — when its own body may be blocking its view. Placed here, it does two things: it puts the lateral line in a sequence rather than in isolation, and it explains why the famous claim about smelling blood from miles away is a claim about the wrong sense.

    Read SharksA shark will attack a pair of bare electrodes that smell of nothing — because a living body leaks electricity.

  3. Stop 3 of 4. Senses and abilities

    The same physics, in a solid

    Now onto land, where the medium is stiffer and the wave is faster. An elephant call low enough to couple into the ground arrives twice — once through the air, once through the earth, at different speeds — and animals respond to the seismic version with the airborne part below their threshold. This is also where the honest difficulty of the field sits: separating the two channels experimentally is the whole problem.

    Read Feeling the groundAn elephant call is low enough that much of its energy goes into the ground, where it travels faster and further than it does in air.

  4. Stop 4 of 4. Arachnids

    An animal that builds its own sensory surface

    Ending here makes the point the rest of the route builds towards. A spider in a web is not detecting vibration in its environment; it constructed the thing the vibration travels through, and it tunes what it transmits by how it tensions the threads. That is the far end of what a mechanical sense can be — not an organ that samples the world, but a structure built to make the world legible.

    Read SpidersA spider with 100,000 neurons will take a detour to prey it can no longer see.

Where this leaves you

You end with the two limits that define these senses. The lateral line is extraordinarily informative and almost absurdly short-ranged, which is why everything it is good for happens within a body length. Ground vibration is the reverse — it carries a long way and is very hard to separate experimentally from the airborne sound arriving alongside it.

What the route does not settle is how much of the world reaches animals this way. Estimates suggest most signalling insect species use substrate vibration, and it is invisible to anyone not holding a contact microphone.

Last reviewed 2026-09-02. All guided journeys