Going back to my normal format of listing everything important first.
3. Vascular bundles of woody xylem found within the the appendages of the mother longlegs suggests that trace elements of actual bamboo are within its physiology, as though the flora of its natural habitat has become infused within its DNA.
4. Evolved razor sharp spines at the end of its legs
5. Poisonous spurs on spikes immobilize prey
6. Drink up liquids from victims through feet like a straw
7. Underbelly releases muscular tendrils that drag prey up into its open stomach
Okay this one is gonna be hard.
Usually I save taxonomy for last, but that’s honestly the easiest thing to do this time. Mother Longlegs are simply a giant parthenogenetic member of the Opiliones, and despite their appearance Opiliones are actually more closely related to scorpions than they are to spiders. And honestly it’s nice to see them taking the spotlight instead of the very overused Araneae. Also, considering that their cephalothorax and abdomen haven’t fused suggests that they diverged from other harvestmen before that happened… and I guess became harvestwomen.
They’re parthenogenetic because they’re basically stationary in their adult phase so they can’t really go and find mates. Although if my florafauna theory holds water then this has to be something recently evolved or else they wouldn’t be so seamlessly merged with their plant parts. Parthenogenesis basically results in clones and doesn’t make a lot of genetic variety, and thus slowing down the process of sorting out the new plant genes generation after generation.
Instead of retaining the normal foot of an arthropod, Mother Longlegs have legs that end in a single sharp point. While this is useful for what they use their legs for, those points are going to dull down very, very quickly. They’re constantly impaling their legs deep into the ground to get a strong footing so they don’t fall over via wind or the clumsiness of a large animal, but because of this and how thin their legs are they’re most likely going to snap their legs if they attempt to walk anywhere. And harvestmen legs don’t grow back, and they’re going to need all the legs they can get to support their body, which looks to be pushing half a ton in weight, without an endoskeleton. Their legs probably support them through both a complete muscular system and hydrologic system (somewhat like spiders). They also apparently have venomous spurs on the end of their foot that’s used in immobilizing prey. While this is useful, they still run the risk of snapping their legs, but I suppose the reward in nutrients is well worth it to them.
On a related topic, Mother Longlegs can seemingly suck nutrients and fluids from prey impaled on their feet. Arachnids already partially have some of their digestive tract in the base of their legs, and sea spiders (which might not even be chelicerates at all) have most of their digestive system in their legs and even use the rhythmic pulsing of it to move around. So it’s plausible to evolve leg straws. Speaking of their leg straws, Mother Longlegs have xylem in their legs. Xylem is a vascular tissue in plants that conducts water and nutrients from the root and up the plant using suction and pressure. Xylem also helps form woody elements in the stem of a plant. So this is likely what Mother Longlegs are using to suck fluids up from impaled prey, and their feet mouths might actually be an extension of the xylem that connects to their digestive track. Xylem is also responsible for why their legs look and feel like bamboo.
Mother Longlegs also can release muscular tendrils that pull prey up into their exposed stomach. This simply isn’t going to happen since the stomachs of arachnids are on top of their nervous system and sometimes even their brain. The movie seems to notice this since we never see their underside open, and the tendrils are instead dragging a soldier up towards its mouth. Prey dragged towards the mouth are crushed by the pedipalps of the Mother Longlegs, and are devoured (harvestmen don’t have a filter system or a sucking stomach so they just eat bits of food). Also their chelate are reduced to large fangs. I don’t know what purpose they serve. They have plants on their abdomen that photosynthesize and provide extra energy.
Harvestmen don’t have very well developed eyes and they’re pretty small compared to their body, so that begs to question why Mother Longlegs have such large and well developed eyes. My guess is that they’re hold overs from when they were cave dwelling creatures, and that they eventually came to the surface of skull island through its many cave systems.
On the bottom of their abdomen are two sphincter-like organs. They could be derived from book lungs or the same chemical spraying organs that other harvestmen use for defense.
Speaking of lungs, at this size ordinary book lungs aren’t going to cut it. The Mother Longlegs have to have a well developed respiratory system.
As far as their life cycle goes, I assume that they start off small, and are omnivorous scavengers like normal harvestmen, and as they molt and grow bigger and slower they migrate back to the bamboo forest. Once there they molt one last time and become fully sedentary.
Let me know if there’s something I didn’t cover here.