(So this is a combinated of CM lored and the orgional movies. Plus some added modification to make the make it seem more like a liveing animal. Also I plan to cover other castes like the runner or queen in part 2 and 3.)
Shell- This is often called an exoskeleton, but its function is purely to provide protection making it a shell instead. The shell is made of 3 different materials. The first is its extracellular matrix that makes up most of the shell. In drones this layer is half an inch thick and provides little protection. The matrix is made up of chitin with large deposits of iron in it. However, the amount of iron is relatively low compared to larger castes. Due to these factors this layer cracks easily. The second is the epidermis. Unlike in humans, the top layer of the skin is made of living cells due to the matrix on top. This layer maintains the matrix above. The third layer is called the dermal quilt. This is a protective weave made of non-living protein filaments. It acts like an organic gambeson as it is extremely hard to cut yet flexible. Although again drones only possess an extremely thin version. On the skull are two eyes hole that are covered with a translucent eye lid. The reason said eyes cannot be seen is because the eyes are all black much like the shell.
Digestive tract- All castes possess a second mouth mounted on the tongue. It is unable to swallow food and tis exact purpose is unknown. They have been seen using it as a weapon before but only rarely. It is likely an ancestral trait from whatever creature the black goo turned into the first xenomorph. Its esophagus, stomach, and intestines are similar other animals. All are omnivorous but depending on what species the individual was incubated in their preferences may change slightly. One’s incubated in cows were seen to graze as opposed to one from deer who preferred to browse for shrubs. However regardless of host they all hunt for meat. They often seek out animals for their blood and brains, sometimes only eating those parts. The blood is to gain the large amount of iron needed to form their shells. The reasons for the brains are unknown, but it is most often done by younger members. It is suspected to help in their development in some way. This strategy is employed by chimpanzees on earth.
Body cavity- Like most large earth animals they have a fluid fill cavity in their torso that stores several large organs. Inside is two lungs, two hearts, plasma sack, one stomach and intestine, one host receptacle, and two kidneys. The kidneys are much larger and more complex than with normal lifeforms. One is attached to the intestine and the other to it arteries. There is a vestigial sack of tissue located under the host receptacle. This is the remains of the liver that was replaced by the kidneys. The smaller liver allowed for more room to fit the host receptacle. The entire cavity is covered in a weave similar to the dermal quilt. It serves the same purpose.
Skeletomuscular system- This is heavily dependent on what the incubation host was. Most captured specimens were incubated in either humans or dogs. See section on reproduction for more details. Regardless of host, drones always take a bipedal stance to free up their hands. Regardless of host, all will possess either digitigrade or hooved legs. All castes except for runners have extremely dense muscles and bones. Much like the shell the bones continue small iron deposits. This is usually around the ribs and skull, but the bones still primarily use calcium. The muscle is mostly unremarkable and is very similar to normal muscle tissue.
Circulatory system- It has two hearts in its body cavity. In its head is a pseudo heart located at the very tip. This pseudo heart is needed to help get blood all the way around the body. Since this organ is rich in left over grey plasma it is highly valuable. Much like normal animals they have veins and arteries. Their blood is like that of normal animals except it has a much lower Ph and is rich in chlorine. Many think xx-121 blood is a bright yellow, but it is in fact red like most animals. What people describe as its acid blood is not its blood at all. It is concentrated sulfuric acid that is stored in tubes that do not connect to organs. The acid is purely for defense and the tubes sit near the surface of the body to burn anything the breaks the skin. The reason for the yellow color is the real blood has a special pigment that turns yellow when exposed to acid. This is suspected to help nurse drones identify what areas of the body are most injured. The acid blood does help sterilize wounds and is used to melt objects. The acid blood itself is also colored green to help other xx-121 identify what object have been covered in acid. Strangely the red blood has no immune cells in it. XX-121 rarely get sick due to the lower Ph of their blood high chlorine content. It is suspected this was an intentional weakness by the designers. If a hive got out of control, then a bioweapon that could survive in their body could easily kill them. An example of a naturally evolved pathogen that can do this is the black mold.
Nervous system- Like normal animals they have nerves that run through their entire body that provide singles. In the body cavity is a nerve cluster that regulates the organs. This means that if a xx-121 was to take significant head trauma it would be able to breath automatically. Similar verve clusters are placed through the body that allow the body to engage in instinctual actions without input from the brain. Examples would be recoiling when hurt or for a drone building. When a drone builds a structure, it activates the instinctual pattern to do so. This leaves its brain to plan or stay on lookout as the body constructs. All castes have a large head that extends towered the backside of the body. The brain will be extremely different depending on the incubation host. The brain is split into two parts. The frontal section and the posterior section. The frontal section is what we call an entire brain. It directs most thought and processes sensory information. This section is often around the size of a chimpanzee brain. The posterior section takes up the remaining space in the elongated head. It is suspected to be responsible for the xx-121 hivemind. It is unknown how the hivemind works but the removal of this section of the brain has been shown to remove an individual’s ability to use the hivemind. If the frontal section remains intact, then the individual will keep all its memories and behaviors.
Reproduction- Drones do possess an underdeveloped reproductive system. This will fully develop if a drone grows into a queen. Genetic and anatomical testing shows that all xx-121 are female. Many vestigial parts remain and show that the original species did lay eggs. But it also suggested at some point two sexes existed. The gene that triggers male development has been made dormant. Despite the name king xenomorphs are not male. All castes possess a host receptacle. When one captures a host, it will swallow it for transport. However instead of using it esophagus it uses a secondary tube that leads to the host receptacle. This is a muscular sack that holds the victim until they are regurgitated inside the hive. To prevent the victim from moving too much the inside of the sack is covered in tentacles that hold the victim in place. Most xx-121 that have been studied are incubated in humans or dogs. This is for two reasons. The first bring that xx-121 that live near human settlements are easier to capture. The second is that any caste incubated inside a human are more intelligent. Hives seek out humans over other hosts for this exact reason. As for dogs, they are often the pets of the captured humans who got unlucky.