There are two kinds of tarantulas; those that have evolved to live off the ground and those that have not.
Tarantulas that live off the ground are often known as arboreal species. They will climb up trees, living in holes or behind loose bark. They often spin silken retreats in which they hide. It is therefore simply a part of their normal behavior for such a tarantula to climb.
A greater concern is why ground-dwelling tarantulas climb. This is far more unusual and suggests that there could be something wrong with the conditions in their cage.
A terrestrial tarantula that keeps climbing is most likely reacting to incorrect conditions within it’s cage. When you correct the issues within the cage your tarantula will normally stop constantly climbing the glass.
Here are some elements to consider to stop your tarantula climbing up the glass of their cage…
Some tarantulas take a dislike to specific substrates. For example, while I use coconut fibre as a substrate with most of my tarantulas, a few seem to dislike the sensation of walking on it. Changing the substrate for something firmer or less “fluffy” can encourage your tarantula to spend more time sitting on it.
Also, be aware that some tarantulas are sensitive to the moisture-level of their substrate. A substrate that is overly moist will be avoided by some tarantulas. A dry substrate is generally far more readily-accepted by pet tarantulas.
Does your tarantula have at least one hide in which it can fully fit? A tarantula without somewhere to call “home” may spend days exploring the cage, climbing up the glass, looking for somewhere cosy where it can hide away during daylight hours.
If your tarantula already has a hide, but this is not being used, consider adding additional hides, or swapping the original for something new.
It is popular to claim that tarantulas don’t need heating. Indeed, many species will survive just fine at room temperature. Others, however, seem to actively seek out warmer temperatures. It may be that increasing the heat in your tarantula cage encourages him or her to settle down.
Of course the reverse may also be true; your tarantula may be overheating and looking for somewhere cooler to escape to.
Either way, if your tarantula won’t stop climbing it is wise to monitor the temperature inside the cage carefully, and to consider gently increasing or decreasing the temperature to see if it reduces climbing.
Tarantulas that are overly hungry or thirsty may climb up the glass looking for food or water. Make certain that your tarantula has access to a shallow open water dish, and consider offering food more frequently to see this if it reduces unnecessary climbing.
One final reason why your tarantula may climb all the time is due to parasites within the cage. Mould, fungus or even mites can make a tarantula cage an unpleasant place to be. Your tarantula may be climbing the sides to avoid this.
Under these circumstances a full clean-out might be advisable, to be sure everything is clean and hygienic within the cage.