#trichogrammatidae

The degree to which they live their life in water varies, but almost all of them parasitise aquatic insect larvae – with the exception of the spider wasp Anoplius depressipes, which parasitises fishing spiders instead – and their offspring have to swim back out once they emerge as adults. A lot of them have longer claws than other wasps to help with gripping vegetation underwater and hairs that can trap air and make them somewhat hydrophobic. And they're all largely unrelated, they're thought to have evolved being aquatic at least 50 times separately!

Anoplius depressipes

One neat one I want to point out is Caraphractus cinctus. It's a fairyfly (Mymaridae), and if you've heard of them, you've probably heard they're small enough that they essentially swim through the air with their feather-like wings. C. cinctus takes that a little further by. Well. Swimming. They emerge as adults from the parasitised eggs of diving beetles (Dytiscidae) and the females immediately begin searching for new eggs, moving through the water using their wings and taking in oxygen from the water. They are seemingly able to survive for up to 15 days below the surface, and if beetle eggs are available then most females never do leave the water. Mating usually occurs underwater, although not within the beetle eggs as the Bennett paper says. I find it really cool that they can break through the surface tension by diving in head first, although they tend to just use vegetation to help them down.

From Jackson (1958)

Another really cool one is Hydrophylita emporos, which sometimes locates its host's eggs by catching a ride on the adult female damselfly's (Psolodesmus mandarinus mandarinus) abdomen and running down to lay their own once she starts ovipositing. Some, like C. cinctus, stay below the water their whole lives.

Hydrophylita emporos on the back of Psolodesmus mandarinus mandarinus, CC BY 2.5

Also, one of them is named after Godzilla (Microgaster godzilla). I think that's very important

Bennett, A.M.R. (2008) 'Global diversity of hymenopterans (Hymenoptera; Insecta) in freshwater', Hydrobiologia, 595, pp. 529–534

Jackson, D.J. (1958) 'Observations on the biology of Caraphractus cinctus Walker (Hymenoptera: Mymaridae), a parasitoid of the eggs of Dytiscidae (Coleoptera) – I. Methods of rearing and numbers bred on different host eggs', Transactions of the Royal Entomological Society of London, 110(17), pp. 533–554

Jackson, D.J. (1961) 'Observations on the biology of Caraphractus cinctus Walker (Hymenoptera: Mymaridae), a parasitoid of the eggs of Dytiscidae (Coleoptera) – II. Immature stages and seasonal history with a review of Mymarid larvae', Parasitology, 51(3–4), pp. 269–294 (not really referenced here but this isn't an essay it doesn't need to be one to one right)

Jackson, D.J. (1966) 'Observations on the biology of Caraphractus cinctus Walker (Hymenoptera: Mymaridae), a parasitoid of the eggs of Dytiscidae (Coleoptera) – III. The adult life and sex ratio', Transactions of the Royal Entomological Society of London, 118(2), pp. 23–49 (these three are unfortunately not open access but if you can use sci-hub then 👀)

Shih Y.T. et al. (2013) 'Hydrophylita (Lutzimicron) emporos Shih & Polaszek (Hymenoptera: Trichogrammatidae) from Taiwan, parasitising eggs, and phoretic on adults, of the damselfly Psolodesmus mandarinus mandarinus (Zygoptera: Calopterygidae)', PLoS One, 8(7), e69331

Copidosoma floridanum is an Encyrtid wasp whose brood is a fascinating example of polyembryony, in which multiple embryos form from a single egg. It is primarily a parasitoid of Noctuid moths in the subfamily Plusiinae. The Encyrtid egg will produce thousands of clone embryos. The mother C. floridanum will sometimes lay one male egg per host, or one female egg, or sometimes both. What's even more fascinating is that the wasp larvae have a caste system: the reproductives and the (precocious) soldiers. The reproductive larvae emerge during the caterpillar's last instar, consume it, and pupate into adult wasps. The soldiers emerge earlier, but never molt and die when the host dies. Their sole purpose is to protect their reproductive siblings from other parasitoid rivals and to kill their own opposite-sex precocious siblings. Since male soldiers compete for resources (caterpillar), by killing some of them off, the sterile soldiers are helping their genes survive by securing more resources for their clone reproductive siblings. In mixed-sex broods, the male larvae will try to hide and encyst themselves in the caterpillar's fat body to escape their murderous sterile sisters. Copidosoma competes with Microgastrine species such as Microplitis demolitor and Glyptapanteles pallipes. C. floridanum demonstrates haplodiploidy where unfertilized eggs become haploid males and fertilized eggs become diploid females. There are both male and female soldiers, but male soldiers tend to be less aggressive towards competitors. However, C. floridanum isn't invincible. There is one competitor, Trichogramma, that is also an egg parasitoid, and only uses the eggs of the moths. The adult wasps emerge long before the caterpillars hatch. When Trichogramma and Copidosoma end up in the same egg, the former usually ends up victorious.