Insectgirls are generalists with average stats across the board. They only have strong stat growth in mana capacity, which helps with efficient use of their fixed skill, ExtractMana. With their three open skill slots, they can be bred to serve a variety of combat roles while maintaining their mana/energy transfer abilities.
Their TransferMana onFuck passive allows them to recharge spellcasters, which is helpful especially in combination with Brood Queens/Lava Brood Queens (who receive double mana from being fucked), units like the Succubus with onGettingFucked passives, and any units with Abdomens like other insectgirls.
With Abdomens, they don't lose energy when they get fucked, and they have a 75% chance to gain a point of energy if their partner lost energy, so they can collect energy in combat from both allies and enemies. An insectgirl can move adjacent to an enemy with high lust and then use Seduce on itself to present itself and try to entice the enemy into fucking it. Their mediocre beauty stats don't give them much of a bonus to this, though, so they'll probably need assistance from other units with good seduction abilities to raise the target's lust.
Their Collector trait means that if they Service a male or futa enemy to completion, they can drain double the semen and energy, so they can choose between attempting to drain energy quickly and attempting to steal it.
Note that all of these abilities require an insectgirl to be adjacent to an enemy to be used, so choose their equipment, skills, etc. expecting them to see frontline combat.
Their Abdomen gives them the TransferEnergy overworld ability, which allows them to increase another character's energy by 1 at the cost of 2 of their own energy. This allows them to recharge the energy of non-Abdomen units, and it's less efficient but definitely safer than fucking another Abdomen unit during combat. This can help keep your main recruiter units from running out of energy on portal expeditions.
They can produce eggs at the cost of 10 energy and stamina, effectively breeding with themselves to determine the genes and traits of the egg. The stability of eggs produced this way is halved for non-queens. Newly hatched/hired insectgirls start with 10 energy, so they can immediately produce an egg right after they join your team if you want them to.
Their base fertility and virility is mediocre, but they have an energy capacity of 20, which gives them an additional +200% bonus to those stats when breeding while at 20 energy. A stock insectgirl takes 20 days to completely refill on its own, though, so they're dependent on their energy stealing and transferring abilities to reasonably function for breeding.
Brood queens and lava brood queens have amazing fertility, good virility, and even higher energy capacity than insectgirls, so instead of breeding insectgirls with each other, it may be more effective to have your insectgirls transfer their energy to brood queens so they can breed with each other. They could also transfer their energy to a particularly virile male or futa insectgirl for breeding with the queen, for a 50/50 ratio of insectgirls and queens in their offspring.
Because their base fertility and virility can be multiplied so much at high energy totals and insect breeding is so energy-intensive, having a variety of sizes for your insectgirls may have a major impact on their breeding ability. A small female and a large male might produce significantly more eggs than two equal-sized units for the same amount of energy.
Insectgirls have a low base beauty stat, but they have something else going for them when it comes to working at the brothel - they have four hands and they use all four at once during sex training sessions, which makes it very easy for them to gain massive Hand xp and cap-breaking bonus xp, with little stamina expenditure required from the main character. They're a great addition to a gloryhole room where beauty isn't considered but sex skills count double, where their godlike quadruple handjob skills can attract large numbers of clients to help train new brothel workers' sex skills.
Their Collector trait doubles the amount of semen they drain when working on the farm, making them an important part of production of resources for magical items and potions.
The Evolution MC in insectgirl form can take advantage of the doubling of semen collection from Collector in some situations, such as while using the Collect card during dates.
It may be useful to have a team of insect-type units to clear out low-level Arena battles that otherwise wouldn't grant xp or be worth the stamina cost, just so they get the chance to drain energy from their opponents.
Basic supporter for a Broodmother.
They can collect/store huge amounts of semen in their abdomen, to later gift to their queen.