Aliens of the Terran Confederation

[Korto]

Why are most insect aliens made into hive critters?

Because I lack imagination.
More seriously, trying to think of something alien, thinks different from human, start thinking "hive mind", and then insects naturally pop into your head.
You know, funny but until now I never actually considered how cliche that is.

Why not reintroduce the warrior breed instead of using mercenaries?

The real advantage the warrior had was size, strength, and large biting mandibles, features that would only have any use of all in direct hand to hand combat. However, the Kerridan mentality is not suited to anything other than strategic combat (the warrior died out before their intelligence developed), and so re-introducing the warriors (which they have the genetic technology to do), with the Kerridan mentality would be a large expense to create a sub-standard warrior. In fact, a trained Kerridan worker armed with modern weapons and armour would be almost as good. They decided it better to hire mercenaries. All Kerridan would fight to defend the Hive, of course, but in general they find it more efficient to leave the fighting to those more suited.
It also gives the Humans something to trade, and the Kerridan calculated that it was more efficient for the Hive for humans (and Tai'Qu) to be able to get Kerridan goods and technology by trade rather then having to resort to force. This was actually a very important part of the decision.
They do take very well thought-out precautions against treachery.

[Ellindsey]

The next stage of their life cycle is parasitical, they infest an adult Annite through a break in the skin. They grow and develop into worms in the bloodstream before migrating to the brain. In the brain, they destroy the Annite's higher reasoning and cause him to want to avoid everyone and hide somewhere dark and remote. From there, they slowly consume him (in a way that keeps him alive for as long as possible). Finally they emerge from the host and form a coccoon around themselves, from which they emerge weeks later as adult Annites. (The first meal of these annites normally consists of any brood-brothers too slow to develop or too weak to defend themselves). Then, following instinct, they seek out other annites to join with.

You have two problems with the biology of this step.

Your first problem is population replacement. If it takes the death of an adult Annite for reproduction to occur, and the first Annite to emerge kills and eats his brood-brothers, then the population of Annites can only ever decrease. This is essentially reproductive cannibalism, and can't be sustained. You need to either have multiple adults emerge and survive from each parasitically infected adult, or have some other host species (with its own more conventional reproductive system) to act as hosts for the Annites.

You second problem is calorie balance. In order for an Annite to grow from spore to full adult, it will have to consume an amount of food significantly greater than the final adult mass. The conversion of consumed food to added body mass is always going to be less than 100%, which gives you some problem with having the parasitic larvae emerge from the cocoon fully adult after having consumed only an adult of equivalent mass. This gets even worse if multiple Annites emerge from each host. This is why real-world parasites that develop this way typically develop in hosts that are much larger than the adult parasite.

It might work if you have the larvae live for a while in the infected victim's body, growing while the host eats enough to support the parasite's growth within them for a while, as a sort of involuntary pregnancy. Alternately you could simply have the newly emerged Annites be much smaller than the adults and have a lot of growth and development left to do.

[Korto]

You need to either have multiple adults emerge and survive from each parasitically infected adult, <snip>

That's what happens, a reasonable number emerge of approximately the same size. It's only those substantially smaller or less developed who get turned upon.

You second problem is calorie balance. In order for an Annite to grow from spore to full adult, it will have to consume an amount of food significantly greater than the final adult mass. <snip> Alternately you could simply have the newly emerged Annites be much smaller than the adults and have a lot of growth and development left to do.

That's what I meant. They're in their adult form, however they are much smaller than a normal Annite. Your idea that the infected annite eats and sustains himself for much of the infection seems quite reasonable. You don't need higher brain functions to scrabble for bugs and worms.

The practise when the higher ups want to replace numbers is to take some underling they're displeased with and deliberately infect him, keeping him in a secure facility. Numbers can be replenished very rapidly in this manner (although there's still all the training, and time and food while they grow to useful size, of course)

[Ellindsey]

Ah, makes sense then.

It does raise the interesting question of whether they're the only creatures in the ecosystem they evolved in to use this method of reproduction, and if they're limited to infecting their own species or if there are closely related animals on their homeworld they could incubate in (and whose spores could incubate in them potentially). Then again, that assumes that the Annites are the result of evolution rather than an especially sadistic bit of genetic engineering or something.

[Korto]

There was an off-the-cuff statement that closely related species used the front pair of limbs to finish off prey, but really I hadn't given the matter much thought. Thinking about it, I would probably say there are a few other species that use a similar method, but they're able to parasitize a wider spread of species. Only the annites have been restricted to only themselves, forcing them to gather together in groups despite the risk entailed.
They are the result of natural evolution (and the sadistic hand of a wanna-be author who wanted a believable reason for an advanced race to have a completely different moral compass from ours.)

I haven't actually thought their world through, whether the spores method is the overwhelmingly common reproduction system, or if it's a bit of an odd branch. I suspect parasitization would not be sustainable as a method used "by most species on the planet", but you don't have to be parasitic to have a microbe stage. If the vast majority of species were to have a spore part in their cycle, then it can be expected life on the microbial level would be exceeding active (more than already), and larger species would have very effective immune systems compared to a human's. It could be that annites became so specialised to whom they infect as a response to the ever-improving immune systems.
It does suggest when they invade another planet, even if they're fought off, people may be cleaning up xenomorphs (from scattered spores) forever after.

And may I welcome you to Stardestroyer, I just noticed it was your first two posts.