Slender Mallee Longhorn Beetle
Black Stag Beetle
Twig Borer Beetle (Bostrychopsis jesuita)
Retired Professor of Anatomy, Ian Gibbins, has kindly shared his thoughts with us to help people learn:
From a developmental point of view, antennae are effectively modified limbs. There is a famous mutation in Drosophila called "antennipedia" in which the fly sprouts a pair of legs instead of antennae from its head. The underlying genetics of this turned out to be a major breakthrough in understanding how bodies are put together in all organisms, but that's another story...
The antennae are primarily sensory structures. They can be moved around by muscles in and around their base, but in most insects, there are no muscles in the antennae themselves.
The sensory functions are wide and varied.
Most have mechanosensitive receptors => they are "feelers". They also have sensitive mechanceptors that indicate distortion of the antenna itself. Other receptors, especially those on antennal hairs, respond to air-borne vibrations (sound, air-currents). The various combinations of these structures can lead to a great variety in shape. The short stubby antennae of flies respond to air currents and probably act with the halteres to give the fly a sense of direction and airspeed during flight. The long feathery antennae of male mosquitoes are tuned to the vibration frequency of the females' wings => they "hear" the wing hum.
The antennae of many insects contain chemoreceptors of various kinds. Some are activated by contact with the substrate, ie, the insect "tastes" the ground or plant or prey or whatever by touching it with the antennae. Others are activated by air-borne molecules, such as pheromones, ie more like "smell", eg the feathery antennae of many male moths.
Other receptors respond to humidity, ie they can effectively measure the water content of the air, and temperature.
In every case, more complex antennal morphology leads to increased surface area which allows for more receptors => higher sensitivity or discriminatory power. Almost certainly aerodynamic considerations come into play for flying insects. This affects both mechanosensory and chemosensory functions.
The thing to notice here is that for an animal the size of an insect, the air would feel as thick as water does to us (another whole story there!), so controlling airflow around the antennae is vital for their effective function. It's interesting in this context that the overall form of many feathery antennae is not very different from that of fish gills, which need to extract oxygen from water in a way analogous to insects extracting scent from the air. When you take the scaling factors into account, this is probably not coincidental.
I have never seen, and, so far, I've not been able to find any explanation for the series of rings etc on the antennae of beetles other than that they increase the surface area.
Hanging over all of this is the very real possibility that at least some of the variation in form is more or less random, and has no specific adaptive advantage, in the sense that, within some limits, it doesn't matter exactly how you increase the surface area of your antennae. But once the pattern has been locked in developmentally, further variations will occur, built from that model.