While some animals are content to eat the fruits and nectar of plants, others want to eat the important, often vital structures of the plant's body. This can be potentially life-threatening to plants, which is why they've come up with a number of measures to defend against herbivores. Now, some physical external measures include thorns, which you can see here and is a type of modified stem, spines, which you can see here in this cactus's characteristic needles, and spines; recall, are a modified type of leaf. And, also, they'll use trichomes, which you can see here covering the leaves of this plant. They're kind of scattered all over the leaves here. And these contain some nasty chemical surprises for the herbivore. Now, plants will actually use what are called secondary metabolites. These are like chemical weapons against herbivores, and they will have a number of effects. Sometimes they just smell bad, other times they'll actually poison the animal, and sometimes they can actually alter the animal's nervous system. And, some of you out there might recognize what this plant is behind my head, that is cannabis. People often like to use it recreationally, but the chemicals in the plant were actually intended to ward off herbivores. They're going to mess with the nervous systems of herbivores, and for example, with insects can be super super toxic. So, you know, that is really the intention behind those chemicals in cannabis. It's to kill or just make sick or ward off herbivores.

Now, if herbivores can get past those outer defenses, plants have some other surprises for them. Plants will have, some plants will use what are called protease inhibitors. These block digestive enzymes found in animal saliva and stomachs. So if an animal eats a bunch of a plant with protease inhibitors, it's gonna get sick because it's not going to be able to digest the material it ate from the plant. Believe it or not, if you don't have the proper enzymes in your gut, to digest the food you've put in your body, you'll get very sick. Your body will reject that. And herbivores have actually learned to recognize the taste of protease inhibitors. Now, in, you know, because producing these chemicals can be costly for the plant, they don't wanna do it willy-nilly. So there's actually a hormone signal that plants use to produce or to signal to produce protease inhibitors, and, this is, called systemin. This is a hormone a hormone that, wounds will signal, and it, again, leads to the production of protease inhibitors. So, basically, if an herbivore takes a bite of your body and you have a wound, it's going to, trigger the release of systemins, and that's going to produce protease inhibitors. So, hopefully, the herbivore, if it continues to eat the plant, will taste those protease inhibitors and be like, no way. I'm not getting sick. I'm out of here. I'm done with this plant.

Now, sometimes plants can go a step further and get downright sadistic with their defenses. And probably the creepiest, let's say, example of this is with, caterpillars and wasps. It's believed that when some plants, like, for example, this here, are being eaten by caterpillars, these caterpillars are having a field day just munching on that plant leaf, that the plant will actually give off a chemical attractant that will draw wasps to them. This is because wasps are actually, or some wasps, are parasitoids. That means that they live freely as an adult, but they are parasitic as larvae. And, in fact, wasps lay their, or some wasps lay their eggs inside caterpillars, and when the larvae hatch, they eat the caterpillar from the inside out. And that is what this disgusting image is behind my head. You can still kind of see the shape of the caterpillar, but all that's left is these gross little wasp larvae that have just compl