Dragonflies are full of surprises. They have six legs, but most can’t walk. Their giant, 30,000-lens eyes can detect ultraviolet light. And though they lack the brain architecture normally required for a sense of smell, a new study finds that dragonflies may use odors to hunt prey.
Smelling, as we humans understand it, requires certain hardware. Our noses are packed with olfactory receptors, each of which is tuned to a precise scent molecule. (Indeed, a recent study suggests we can detect a trillion smells.) When one wafts into our nostrils, these receptors send nerve signals to sensory way stations called glomeruli, which pass them along to the brain for interpretation—“Oh, a rose!” Glomeruli are shared by most terrestrial mammals and insects, and until now, scientists believed they represented the only possible route to a sense of smell. Because dragonflies and their close cousins, damselflies, don’t possess glomeruli or any higher order smell centers in their brains, most scientists believed these insects were unable to smell anything at all.
Invertebrate biologist Manuela Rebora at the University of Perugia in Italy was not one of them. When her team took a closer look at dragonfly and damselfly antennae with an electron microscope, they spotted tiny bulbs in pits that resembled olfactory sensilla. Like the insect equivalent of a nose, these sensilla house olfactory neurons. When Rebora’s team exposed the suspected sensilla to scents, they emitted nerve pulses, supporting the idea that damselflies and dragonflies perceive odors.
But to count as a sense of smell, the sensilla would have to do more than chemically react to scents. Their signals would have to actually affect behavior in dragonflies and damselflies. So the researchers built a wind tunnel where fruit flies—a dragonfly delicacy—were concealed behind a cotton screen through which odors could pass. Damselflies gravitated to the spot of the screen just on the other side of the fruit fly cluster, providing the first evidence that scents guide damselfly and dragonfly behavior, the team reports this month in the Journal of Insect Physiology.
The results have triggered a debate over what a bona fide sense of smell truly looks like. “No olfactory glomeruli and no higher olfactory processing suggests [dragonflies’] ability to detect a range of odors is strongly limited,” says neuroscientist Nicholas Strausfeld of the University of Arizona in Tucson, who was not involved with the research. This could be the remnants of a regular olfactory system that devolved as dragonfly vision improved, he says.
This discovery of smell sans glomeruli echoes a recent one in carrot psyllid bugs, and together, they may open a new view into how olfactory structures are organized, says Joshua Martin, a biologist at Case Western Reserve University in Cleveland, Ohio, who was not involved with the research.
But the biggest takeaway is that scientists should be careful before declaring an animal can’t sense something, says Robert Olberg, an insect neuroscientist at Union College in Schenectady, New York, who was not involved with the research. “Animals do surprise you.”