Hair as an olfactory organ: Feasible or dumb?

Question

Within my setting, insects, due to a rather long (millions of years) and convoluted (gods suffering from empty nest syndrome) series of events, have evolved their exoskeletons into an endoskeleton of sorts. During this transformation, which is explained in this question, many organs and sensory organs changed and were modified. One such organ was the antennae, which moved to the back of the head and multiplied until, to a human, they look like hair.

The question is simple, does this make any sense?

How well would it function as a smelling organ?

Is there another structure that the antenna can modify into that could smell better?

Answer 1

It works well

Smell is more dependent on the number of olfactory receptors in the nose than on its size or shape. For instance, dogs have nostrils about the same size as humans', but they have far better senses of smell because they have more cells dedicated to smelling.

If your insects line these "hairs" with olfactory receptor "branches", they can maximize the provided surface area and thus get a lot of smell out of a tiny space. Plus, since these hairs are exposed to the open air, they'll definitely sense a lot.

One potential challenge may be susceptibility to damage; if their "olfactory hairs" get rained on or torn by a predator, they may be damaged, incapacitated or lost altogether.

This structure makes sense; your biggest challenge is making smell so advantageous toward finding food (blind cave flies maybe?) that such a system would need to evolve.

Answer 2

How well would it function as a smelling organ?

Antennae can be used for a variety of sensory purposes, and scent-detecting antennae are already common in the world today. Solitary insects often find mates through the use of pheromones, among other methods, and eusocial insects rely on the use of dozens of different pheromones in order to organize and communicate within large groups. As @Zxyrra pointed out, increasing the amount of antennae and olfactory receptors on an insect would likely improve its ability to identify weak scents, increasing its chances to reproduce in situations where mates are hard to find.

Is there another structure that the antenna can modify into that could smell better?

From an anatomical perspective, the small muscles around the base of each antenna will not allow them to crowd together and grow close to as thick as animal hair can. If you want your insects to remain anatomically correct, you may be able to replicate the appearance of hair by instead growing a few antennae with many hairlike protrusions and sensilla. Pectinate, lamellate and plumose antennae can create the appearance of feathery hair on an insect, while short aristae and sensilla hairs can cover an insect's head and body more densely at the expense of size and possibly sensory function.

Anatomy of insect antennae

Answer 3

The increase in nerve tissue would imply a substantial increase in brain size. Their brains could support two antennae. Now, they have millions, perhaps, or hundreds of thousands, maybe. Their brains would need to increase proportionally to process the new information.

The antennae and nerve tissue would need the ability to regrow if part of it was damaged. With millions of fine antennae, a few are bound to get snagged. And, haircuts would be excruciating if the antennae would really like hair. All those nerves getting cut.

Answer 4

The primary issue I can see is that insect antennae aren't a body covering in the same way that mammalian hair, reptile/fish scales, or bird feathers are. They're a distinct pair of legs that have been twisted and modified into a pair of sensory organs, in the same way that four pairs of legs have been crammed into the head to form the maxillae, mandibles, labrum, and labium.

The way insects and other arthropods work is their body is divided into a number of segments, each of which has a pair of legs. You can fuse and compress segments together (making the thorax and the head in insects, or the "two sets of legs per segment" seen in millipedes), have multiple legs doing the same job (crustaceans have two pairs of antennae), but insects never really get around the "one segment per set of legs thing". To make an insect with antennae "hair" you would have to duplicate the segments many thousands of times so you have 60,000+ segments in the head alone.

It might be better to just have chemosensory setae (olfactory "hairs" that cover the entire back of the head) that evolved as a secondary sensory organ to help the antennae. Almost all insects have setae and in many species it can make them quite fuzzy (bees, moths, velvet ants). Though I don't know if those kinds of setae are restricted to the legs or can grow on the main parts of the body.

But insect antennae are already used for smell (that's why moths have such complex looking antennae), so olfaction shouldn't be a problem.

Answer 5

The only problems I can foresee with this are the duplication, location, and structure.

The duplication part is kind of questionable since limbs or body parts do not simply duplicate. This is especially true of complex sensory organs like an antenna.

The location is at the BACK of the head. Imagine trying to smell your lunch if your nose was there!

The final problem is with the fragility of antennae. To us, insect antenna looks thin and fragile as heck, but in proportion to the insect, they are actually quite sturdy. Now, compare the insect to antenna ratio to the human to hair ratio. Human hair is quite tough, but that is without any sensory organs built into it not to mention it isn't exactly very good at holding itself up.