After last week's fairly nominal effort at Taxon of the Week, I'm happy to report that the ecology labs are over and done with*, and I can present you with something a little more this week. Many of you will probably be aware of the existence of parasitoid** wasps - Hymenoptera that lay their eggs inside insects and other animals so that when the larvae hatch out they can devour the unfortunate host from the inside out (in some situations, you can't help but say "devour"). The most well-known examples of parasitoid wasps are the large ichneumons***, but I'll be dealing today with a different group - the micro-wasps of the Proctotrupomorpha.
*So I can stop explaining to students that their chances of having actually found a dragonfly in a pitfall trap are fairly minimal.
**Not a typo. Technically speaking, "parasitism" implies that the parasite feeds off the host without (ideally) actually killing it. "Parasitoid" Hymenoptera are referred to as such because the growth of the larva almost invariably results in the death of the host. As such, they are better described as internal predators rather than parasites. All the same, I apologise in advance for when I'm going to inevitably slip back into referring to them as parasites later on.
***Not to be confused with the mongooses also known as ichneumons. The two are easily distinguished - mongooses are much harder to fit into a collection vial.
Proctotrupomorphs are a spectacularly diverse group. The image at the top of the post from Natural History Museum shows an array of examples from only one of the component superfamilies, the Chalcidoidea. Proctotrupomorphs also include the Proctotrupoidea, Platygastroidea and Cynipoidea. Most are exceedingly small - according to the website just linked, the smallest chalcidoid (also the world's smallest insect) reaches a maximum adult size of 0.11mm. There are proctotrupomorphs with wings, there are ones without. There are species with relatively enormous 'horns' arising from the front of the abdomen that allow space for ovipositors considerably longer than the remainder of the insect (as shown above in an image from here). Most emerge from eggs or juveniles of other arthropods, but some have taken to living in galls or pollinating figs. Some are even parasitoids of other parasitic wasps. And a few are even aquatic.
A number of proctotrupomorphs exhibit what is called polyembryony. A single egg is laid within a host which then divides into a number of larvae - up to two thousand in Copidosoma floridanum. The latter species also has a remarkable characteristic in that some of the polyembryonically produced individuals, the precocious larvae, develop enlarged mandibles and seek out and destroy other larvae of the same species but from different eggs (Zhurov et al., 2004). These precocious larvae never mature and die along with the host, leaving their identical siblings (the reproductives) to emerge as adults. In another species, Encarsia formosa (shown above in a picture from Cornell University), the gift from one larva to another is even more significant, though perhaps less willing. Most E. formosa larvae are female, and develop within greenhouse white-flies. Males are much rarer, and actually have a different host - they develop as hyperparasites of the female larvae! (Askew, 1971) As with the marine fly Pontomyia, this demonstrates the dangers potentially inherent in reading morals of human society into the biology of other organisms.
In many cases, however (particularly with egg parasites), there is often no room at the inn for more than one larva - if two larvae attempt to grow within the one host, food supplies would be exhausted before either could complete development. Therefore, most parasitic wasps have measures to prevent competition within the host. As already mentioned for Copidosoma, larvae may kill each other off within the host. There are a number of cases where development of supernumerary larvae halts terminally once one has hatched out or pupated (Askew, 1971), though the mechanisms of this termination may be unclear. Some species act to prevent supernumerary oviposition from happening at all. Trissolcus basalis (image above from SARE) is a parasite of shield bug eggs. After the female has laid within an egg, she scratches the ovipositor over the cap of the egg in a figure-eight movement to leave a mark indicating that the egg has already been parasitised. As a contrast to all this, though, Tetrastichus giffardianus is an obligate superparasite of the fruit fly Dacus cucurbitae. Larvae of T. giffardianus can only avoid encapsulation* by the host if said host has already been parasitised by another wasp, the braconid Opius fletcheri.
*Encapsulation is the formation of a hard capsule around the parasite larva by the host's natural defenses, which isolates and kills the parasite.
Finally, a number of proctotrupomorphs have abandoned parasitism to become herbivores. Fig wasps are a number of families of chalcidoids that lay their eggs within fig flowers. Fig flowers are produced entirely enclosed within an immature fig, and can only be accessed by a single small hole in the fig. The female wasp crawls within the fig and lays her eggs in the flowers. The hatching larvae feed on the inside of the fig (though ovules are deep enough to escape the depredations of the larvae) before maturing. Once mature, they mate within the fig, and the males chew an exit path for the females before expiring without dispersing. The females become covered in pollen as they escape the fig (some species apparently actively collect pollen into special pockets), which they carry to the fig they will lay in. Figweb is a website with all the information on the fig-wasp interaction you could possibly want, as well as some pretty good images.
Askew, R. R. 1971. Parasitic Insects. Heinemann Educational Books: London.