|Whip scorpions also spray acid|
...but out of their other end
First EditionThe ankheg shown here is highly variable in size, ranging between ten and twenty feet in length. The head is generally insect like, with large serrated mandibles and what appear to be maxillary or labial palps beneath, compound eyes, and antennae. The body, however, consists of a series of nine similarly shaped segments, without the thorax and abdomen found in insects. Six of these segments bear a pair of legs ending in hooked claws; the first, last, and penultimate body segments do not. The creature is covered in hard brownish chitin, which is softer and pinkish underneath, but has a similar solidity to plate metal armour over most of the body. The text describes the mandibles as suitable only for crushing, although they certainly don't look that way in the illustration. Like many other giant insects, along with most fish, and, oddly, dinosaurs, the ankheg is given an intelligence rating of zero. In comparison, wild mammals are generally given a rating of 1 if herbivorous, and around 3 if predatory, although there is considerable variation.
Second EditionAs is typical for 2E, many of the basic details of the creature remain unchanged, although there is plenty of new information on the ankheg life cycle. Physically, however, there are some significant differences. The mandibles are no longer serrated, and are apparently accompanied by a row of chitinous teeth, something never found in real insects. The structure of the body is also different; we are now told that ankhegs have just three pairs of limbs, and although we can only see the first two, it appears that each pair is separated by a limbless segment. Thus, although the hind part of the body is not visible, the implication is that the torso is, as before, not divided into thorax and abdomen, fitting the text description of the animal as "worm-like". The limbs no longer end in claws.
Third EditionThe ankheg of 3E is only ten feet long, and other details also seem to match those for the smaller end of the size range given in previous editions. The intelligence is, for the first time, given as 1, even though most other giant arthropods do remain at zero. The physical changes are even more dramatic than in 2E, and this is the most insect-like version of the creature to date. The head has a strange, flattened shape with tiny eyes that might not even be compound (although it does seem to have reasonable eyesight, and, with acute sensitivity to vibrations, even a plausible justification for its ability to "see" in complete darkness). The mandibles are serrated again, but large enough that they look to be useful for grasping prey, rather than crushing - something supported in the text. So far as one can tell, the mouthparts are fully insectile. Further back, there is a six-limbed thorax, with the legs once again ending in claws, and a multi-segmented abdomen ending in a pair of short cerci. Unlike true insects, there appear to be two or three limbless segments between the head and the thorax, forming a flattened shield-shape.
Fifth EditionThe size of the ankheg is unclear here, although it's notable that the chitinous exoskeleton is far less effective as armour than in previous versions. It is also not as strong or resilient as the 3E version, which might imply that it's smaller, although that's probably not the intent. There is another significant makeover in its appearance, following a 4E version that looks very similar to the 3E one. The head has heavy ridges on it, and is much narrower than before; the text says it has long antennae, but there is no sign of them in the illustration. The mandibles now resemble those of stag beetles, but with a sharp inner edge, making them suitable for both grasping and cutting, but not crushing, as they were in the first two editions. The other mouthparts are rather strange, apparently consisting of four pairs of elongated palps - quite how they would be used is unclear, although the longest pair might be sensory (thus replacing the missing antennae). Like some spiders, it has four eyes, which don't seem to be compound. We are told that the creature has "many" legs, and three pairs are visible, so there's at least that many. The front pair end in massive digging claws, which actually makes sense given the creature's lifestyle. The abdomen, if there is one, is below ground, so we don't know the overall shape of the creature, although the artist has since stated that he intended an insectoid look, with six legs and a limbless abdomen.
We can perhaps start by asking "what the heck is this thing?" In 3E and 4E it seems to bear a fair resemblance to a giant insect, although, even there, the fit isn't perfect. The other versions, however, which presumably include the original conception (the creature was apparently thought up by the artist who did the 1E illustration), are more clearly intended to be no more specific than "fantasy arthropod". The 1E version, and to a lesser extent, the 2E one, does have some features in common with centipedes, such as the antennae, mouthparts, and general body form, although there are certainly differences.
So, the 3E version notwithstanding, I'll assume, where relevant, that the creature is somewhere between an insect and a centipede biologically. Always remembering, of course, that, for the most part, it isn't really meant to be either.
Insects and centipedes belong to a larger group of animals called 'arthropods', which also includes arachnids and crustaceans, among others. Most of the land-dwelling forms of these creatures are noted for being small, and there are thought to be two main anatomical reasons for this. First is their method of respiration. Both insects and centipedes breathe through a 'tracheal' system, with multiple openings along the sides of their bodies that lead to a network of air-filled vessels that transport oxygen into the body. This is fine for very small creatures, but just doesn't work once they get larger. The very largest insects weigh no more than 100 grams (4 oz.) or so, and, while the narrow body of centipedes means that they can be very long (up to 30 cm, or 1 foot), they're still very light.
It is possible for arthropods to get round this. Tarantulas and scorpions can get somewhat larger than insects, but the largest of all land-dwelling arthropods is the coconut crab, which can weigh up to 4 kg (9 lbs). Like the largest arachnids, they achieve this by using lungs, albeit ones that are very different structurally to those of vertebrates. There is no obvious reason why such lungs could not be adapted to allow animals of almost arbitrarily large size, but the fact is that 4 kg still isn't all that large compared with a human.
This brings us to the other key problem for arthropods, which is the way their skeleton works. In vertebrates, the bones run down the centre of the limbs, with muscles arranged around them to act as levers and move the animal about. But, in arthropods, the skeleton is on the outside, forming a sort of hollow tube, with the muscles on the inside. As the animal gets larger, and heavier, its muscles need to be larger, but there is only so much space to put them on the inside of a hollow tube. Yes, you can make the tube larger, but that makes it heavier, so you need even more muscles, which carry weight of their own. The resulting arithmetic means that you reach a limit much more quickly than you do if you can arrange the muscles around the outside of the bone.
The world's largest crabs are, in fact, bigger than a human, but that's only if you include their unusually long legs; the body itself is much smaller. Moreover, these are deep-sea crabs, where the buoyancy of the water helps them avoid the weight problems of an animal on land.
The laws of real-world physics means that we probably can't get round this to construct an animal the size of an ankheg without invoking magic. One thing that would help is to make the exoskeleton extra-strong somehow, and it's notable that, in many versions, the carapace of an ankheg appears to be as strong as steel. But, realistically, given that their stated body weights are in proportion to their size, a living ankheg also needs muscles that are much stronger than those of real-world creatures, perhaps due to a magical effect that ends when they die.
Hand-waving such things away, there are probably still a few general points we can make about likely ankheg anatomy. The lungs, if there are any, are likely to be paired structures in each segment, or every other segment; they open to the outside through spiracles somewhere close by, and not through a nose on the head. The mouthparts, as noted above, are variable between different editions, but unless specifically noted otherwise, we wouldn't expect teeth; that's what the mandibles are for. Otherwise, the digestive system is likely to be fairly straightforward, with an oesophagus, stomach, and intestine. The organs that perform the function of kidneys open directly into the intestine, so there's only one hole for excretion.
Not being a vertebrate, there is no spine, and, in fact, the main nerve cord (probably a paired structure with ganglia along its length) runs along the underside of the animal, not along the back. The heart, on the other hand, is a long muscular tube that does run along at least part of the back - given the animal's size, it probably has proper blood vessels attached, and the blood is likely coloured, although not necessarily red. There is also a large metabolic organ that acts rather like a liver, and, of course, the gonads.
Having got the anatomy out of the way, let's consider a little more about ankhegs in particular. Their habitat is generally stated to be forests and arable land in climates ranging from cool temperate to tropical. At the more polar ends of their range, they hibernate during the winter, suggesting that these reach the sort of climates where there is significant snowfall in winter, but presumably not up to the treeless tundra.
In fact, there seem to be two key requirements for ankheg habitat. The first, of course, is a ready supply of food, which would rule out deserts. The second, however, is deep soil. This is because ankhegs are burrowing animals, and, unsurprisingly, they don't seem to be able to burrow through solid rock. The depth of soil is, of course, a highly variable thing, and this would obviously rule out mountain slopes and hillsides subject to major erosion, among others. Swampland or other heavily waterlogged soil is surely also out of the question for a burrowing air-breather, and, even if there were enough food in the tundra, permafrost is probably also a barrier.
So forests and arable land do make sense, especially in valleys where fertile alluvium can accumulate. Tall trees can, at a pinch, have roots that extend down dozens of metres, and an ankheg's depth limitation is probably about the same. And arable land is, of course, developed in exactly the sort of deep, fertile, soils that ankhegs are likely to prefer. Being sheltered from the extremes of weather below ground, it's also not surprising that ankhegs would favour a wide range of climates from equatorial rainforest to the sparse taiga.
In 1E, the ankheg's diet consists primarily of soil, with only a small amount of meat to supplement it. In later editions, the proportion of meat in the diet seems to increase, and 'soil' is replaced by 'decayed organic matter', which seems to make more sense. Certainly, it's hard to see how an animal of its size could gain much nutrition from soil alone (although, obviously, its metabolism could be magical), so a fair amount of organic matter is going to be required to keep it alive. Presumably, with such an omnivorous diet, it can also eat tree roots, but they may be too woody to provide many nutrients, and it wouldn't want to destroy the forest it lives in.
Ankhegs spend most of their lives underground, burrowing through the earth with (depending on the edition) their mandibles, legs, or both. As an aside, in the illustrations, the 5E version seems to have the best claws for digging, but it's one of the ones that only uses the mandibles... Anyway, it seems to dig a relatively small permanent burrow, including a sleeping chamber, using temporary passages that collapse behind it when moving in search of prey. They seem to be solitary animals, although a particularly rich ecosystem might support a number in close proximity.
Living underground, we'd expect its eyesight to be pretty poor, which would fit best with the small, non-compound eyes of 3E and 5E, although not with the 3E stats (low-light vision, for instance, is completely useless underground). Their sense of hearing, on the other hand, is said to be quite good, although their ears could be almost anywhere on their body; even in closely related groups of insects, they may be on the abdomen, the thorax, or even the legs. Centipedes don't have ears as such, but their antennae are, as in insects, sensitive to vibrations (and, unlike insects, possibly also sound), which fits with the stated habits of the ankheg as an ambush predator that lies just below the ground and attacks when it senses footfalls above.
There's no indication of a strong sense of smell, which would also be a function of the antennae. This doesn't rule out the possibility that they can sense specific pheromones produced by their own kind, but again, it makes sense for something that doesn't come out into the open very often - and, in any event, the lack of a bonus may still mean that it's broadly human-level.
Ankheg reproduction is described only in 2E, in which it turns out that the female kills the male after mating, and lays her eggs in his body to provide a meal for her young. In the real world, this particular combination of traits doesn't occur, and even when females do eat males after mating, it's usually only because they're hungry and the male didn't run away fast enough. Still, there are apparently a few species of spider where sexual cannibalism is almost universal, and, of course, there are insects that lay their eggs inside the bodies of their prey, so, while I suspect it's unlikely from an evolutionary perspective, it may not actually be impossible.
What do newly hatched ankhegs look like? In most insects, the young are born as larvae, often caterpillar or maggot-like in form, and always significantly different from the adult. However, this is not true of all insects, and is certainly not true of centipedes - although some species do grow extra sets of legs as they age. Most likely, ankhegs are born as more-or-less miniature versions of the adult.
The external skeleton of arthropods creates another problem here, since it is too solid to grow as the animal does, requiring it to be shed periodically and replaced by a new one underneath. This new skeleton is initially soft, making the creature temporarily vulnerable, but allowing it to expand in size (by, for example, sucking in a lot of air) and then filling in the new, larger, skeleton with slowly-grown body mass once it hardens. We're specifically told that ankhegs do this in 2E and 5E, but one would suspect that the period of vulnerability, when the animal is both soft and slow-moving, would not last more than an hour or two at the most. How often it occurs depends on how fast they grow; the annual shedding stated in 2E does seem plausible, and implies that ankhegs live for many years.
In most editions, but not 3E, the mandibles of ankhegs are unsuitable for slicing up flesh, so that they have to dribble acidic juices over their prey to render it soft enough to eat. In all editions, they can also eject a spray of stomach juices up to thirty feet (10 metres) as a last resort to drive antagonists away. In the first two editions, this renders them unable to eat for the six hours it takes them to produce sufficient replacement acid; at the opposite extreme, in 5E, this takes no more than about a minute.
Either way (and the former seems more plausible, biologically) the fact that they have developed this ability implies that they must have natural threats to use it against. Quite what would want to eat an ankheg isn't entirely clear, although it's at least possible that they might want to drive say, dire bears, off their kills.
[Photo by Biomechanoid56, from Wikimedia Commons.]