Sense of Smell

      Have you ever wondered how insects talk to each other? If humans want to communicate with one another, we have several different ways to do it: we can visually communicate by wildly waving our hands at someone to get attention, we can yell at the person and use sound (audio) communication, or even physical communication by giving a parent a hug to show we love them. Insects use these methods of communication as well, but in different ways. For visual communication they use color and movements to get attention. Audio communication can be the buzzing of wings in a certain way, or squeaking sounds produced by squeezing air out of their spiracles (breathing holes). Physical communication could be grabbing a rival male by the horns like in Rhinocerus Beetles to let them know they better get out of the way.

Example of the male horns in the Rhinocerus Beetle on the far right. Image from Katrina Menard

    One type of communication that insects (and many other animals) use that we don't use as much anymore is chemical communication (chemosensory). To humans, this is smell, which in the past probably played a big role in communication with humans. Insects, though, use it to communicate constantly. For example, many female moths will leave a smell trail (the smell being pheromones) that the male moths detect with their fluffy antennae. The males then can follow the trail in the air to find them.

Example of a male silk moth found at a light sheet with its fluffy antennae. Image from Katrina Menard

    My main focus of research is plant bugs (Miridae), and those of us that work on them know that they also use chemical communication to find mates, plants to feed on, and other important information in their lives. For most plant bugs, the main "nose" for them to smell things are their antennae at the front of their heads. On these antennae they have lots of tiny hairs that pick up the molecules of different pheromones and smells to send the information to their brains.

Scanning Electron Micrograph of a plant bug (Spanagonicus albofasciatus) female showing all the hairs and shape of the antennae on their heads. Image from Katrina Menard

   When looking at one group of mirids in particular, in the genus Spanagonicus sp., it was pretty well known that the males and the females have differently shaped antennae. The males have a much "bigger" first and second segment of the antennae (if you count starting from the head, each "piece" of the antenna is a segment).

Male Spanagonicus specimen. Note how the first and second antennal segments are bigger than the image of the female above; they look more like footballs. Image from Katrina Menard

     So, I was wondering why the first and second antennal segment in the males is bigger than the females: what is it being used for? Taking a closer look at the underside of the male antennae, I found an opening in one of the common species around here in Oklahoma, Spanagonicus albofasciatus (albofasciatus meaning "white striped"). This opening is documented in any of the books or papers about the insect, so I wanted to take a closer look and see what was going on in there: were they there to "hear" females nearby, or "smell" females?

Underside of the second segment of the male antennae, showing the opening, which is filled with super-small hairs. Compare this to the image of the female antennae above! Image from Katrina Menard

      When we used the Scanning Electron Microscope to look as closely as we could to that opening, and we found that inside of the opening there are super-small hairs lining the inside. The hairs are so small, they can't even fit nerve cells in them! The width of each hair in there is 1/100th the width of a human hair. Because they are so small and can't fit nerves in them, we think they are being used to "smell" females and find them. However, we are still investigating exactly how the hairs actually work and test out if that really is what the males are using for finding females versus other things (food, etc.).
      So this spring as the flowers start coming out and we can literally start smelling the roses, these little bugs will also be about using this neat structure to smell the world around them too! And I'll be outside watching them to investigate how.