When a tick bites, it does more than just stick you with the pointy end. Here’s what happens, in far more detail than you ever cared to know. You can thank Dania Richter from the Charité University Hospital in Berlin, who carefully analysed the mouths of ticks, and filmed them as they fed on the ears of recently euthanized mice.
The mouthparts of the castor bean tick (Ixodes ricinus) consist of three main sections. The centrepiece is a long, sword-like structure called the hypostome. It has rows of backwards-curving spines along its edges, as well as its entire bottom face. It also has a groove running down its centre, which channels the tick’s saliva into its host, and channels the host’s blood into the tick.
On top of the hypostome is a pair of chelicerae—long rods that end in hooked teeth. The chelicerae are telescopic, so they can extend to just beyond the hypostome’s tip, or pull back to around half its length. They’re also mobile—their toothed tips can swing out to a 45 degree angle.
When a tick wants to bite, it starts by gently coursing its chelicerae over the skin of its host (at about 0:25 in the video). Each one ends in a tooth that’s tapered to an especially sharp point, which scrapes and punctures the skin with very little force. This also releases smells and other chemical information that the tick can weigh up.
Now, it’s time to break in (1:35 in the video). The chelicerae start to flex their tips. At first, they take turns; eventually they move together. With each sweep, the teeth snag on the host’s flesh and bury the chelicerae even deeper into the skin. After 30 of these movements, the entire tips are firmly embedded.
The tick then contracts both of its chelicerae while flexing both tips, like it’s doing a nightmarish version of a breaststoke (2:20 in the video). The motion drives the rigid, sword-like hypostome into the skin, and the backwards-facing spines keep locked in place. The chelicerae relax and extend, before flexing and retracting again. Each stroke ratchets the hypostome deeper and deeper. After six of them, it’s completely buried and the tick starts to feed.
This is a very different sequence of events to what happens when a mosquito bites you (and you can watch a video of that too). That’s because ticks, unlike many other blood-suckers, are adapted for the long haul. A mosquito bites, sucks and quickly leaves. A tick bites… and stays there for days. It needs to attach itself very firmly so that it can’t be easily dislodged. It does so with the curved teeth and spines on its mouthparts, and by burying them very deeply.
The same sticking power is also vital for the parasites that ride inside ticks, like the spirochete bacteria that cause Lyme disease. They only spring into action when the tick starts feeding, and they need to time make their way from the creature’s guts into its bloodstream, into its salivary glands, and finally into its host. The whole process can take a couple of days. If the tick wasn’t so good at anchoring its face into a host, Lyme disease wouldn’t exist.
Reference: Richter, Matuschka, Spielman & Mahadevan. 2013. How ticks get under your skin: insertion mechanics of the feeding apparatus of Ixodes ricinus ticks. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2013.1758