An Up-Close Look at the Tiny Sensory Pits That Ticks Use to Smell
By Meredith Swett Walker
If you ever find a tick before it finds you—that is when it’s still hanging out on vegetation hoping you’ll brush past it—you may notice the little bloodsucker waving its “arms in the air like it just don’t care.” But ticks aren’t fans of 1980’s hip hop. They’re waving their arms because they are trying to get a whiff of you.
While insects primarily smell with their antennae, ticks are not insects; rather, they’re arachnids, and they don’t have antennae. Instead, a tick smells using a structure on its forelegs called the Haller’s organ. The Haller’s organ is described as a tiny “sensory pit” that can detect chemicals like carbon dioxide, ammonia, or pheromones. It can even sense humidity and infrared light, which includes body heat emitted by the warm, blood-filled creatures that the tick wants to find.
Despite the importance of the Haller’s organ in tick’s ability to find hosts, it hasn’t been described in detail for many important tick species. But in research published in December in the Journal of Medical Entomology, Tanya Josek, Brian Allan, Ph.D., and Marianne Alleyne, Ph.D., of the University of Illinois examine the Haller’s organ in three medically important species of tick: the blacklegged tick (Ixodes scapularis), the lone star tick (Amblyomma americanum), and the American dog tick (Dermacentor variabilis).
Each of these species is an important disease vector: I. scapularis transmits the bacterium that causes Lyme disease, A. americanum transmits the bacteria that cause ehrlichiosis, and D. variabilis transmits the bacterium that causes Rocky Mountain spotted fever. A better understanding of how these ticks find their hosts may aid in reducing disease transmission.
Josek, a graduate student in Alleyne’s lab, was interested in arthropod sensory structures. The Haller’s organ seemed a great research topic: Relatively little was known about it, and the potential findings could have important applications in real-world tick-control efforts. The only problem? Josek had a phobia of ticks stemming from some bad experiences in childhood. “This was a complete shock to me because I have loved spiders and insects my whole life,” she says.
But Josek conquered her fear with knowledge, reading up on where you’re most likely to pick up the parasites and how long it takes them to transmit disease. She trained with Allan to learn tick collection skills. Then, arming herself with a white Tyvek suit and ample duct tape, she set out into the forests and fields of Illinois in search of ticks.
Previous studies of the Haller’s organ were mostly qualitative, which, while informative, are not useful to making quantitative comparisons between species. In addition, recent advances in scanning electron microscope technology provided an opportunity to look more closely at the tiny structure. Josek and her collaborators used environmental scanning electron microscopy (ESEM) to get high resolution images of Haller’s organs.
The team focused on the overall shape of the Haller’s organ in each tick species as well as the organ’s major components: the capsule aperture, anterior pit, and the number of setae and sensilla (hair-like structures) in the pit. They analyzed their images using Geometric Morphometrics Analysis, a technique that essentially translates shape measurements into data that can be used in quantitative comparisons.
They found that the structure of the Haller’s organ was significantly different in each species of tick. In one species, D. variabilis, the morphology of the Haller’s organ was significantly different between females and males. This study did not test the consequences of these differences in shape, but the detailed quantitative measurements it provides can serve as a basis for future studies in differences in function of the Haller’s organs between species.
I. scapularis, A. americanum, and D. variabilis have different preferred hosts and different strategies for finding them. The differences in the structure of their Haller’s organs may reflect this. In order to understand how Haller’s organ morphology relates to tick life histories, “within-genus comparisons, as well as comparisons between ticks with different host-seeking behaviors or between ticks that have a generalist or specialist host-range,” are necessary, say the authors.
Meanwhile, Josek is working on determining the specific chemicals, infrared wavelengths, or humidity variables the Haller’s organ is sensing. Josek and Alleyne are also looking at the organ from a genomics perspective and will soon publish a paper that examines the chemoreceptors and binding proteins expressed in the Haller’s organ.
Alleyne’s main research interest is bioinspired design, and, while she’s not a huge fan of ticks, she finds inspiration in the Haller’s organ. “To think that arthropods have an exoskeleton that protects them from the environment … and yet are able to sense minute amounts of chemicals is amazing to me. The Haller’s organ is an example of a multi-functional sensor that is very sensitive yet rather simple in design, compared to a vertebrate’s nose, for instance,” she says.
If we can better understand the structure ticks use to find us (and their other hosts,) we might devise ways to elude them. This could reduce transmission of serious diseases, as well as make ticks less of a creepy problem for people who work or play outdoors. Josek has overcome her tick phobia and is now comfortable collecting and handling them, but “I still have the occasional nightmare about them,” she admits.
Journal of Medical Entomology
Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.