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What Bugs Lay Their Eggs Into Others?

Insects that lay their eggs in other arthropods are known as parasitoids. A parasitoid is distinct from a parasite in that parasitoids kill their host, while parasites usually leave their hosts alive. Parasitoid insects prey mostly on other insects, as insects that lay eggs in larger animals are usually classified as parasites. Some estimates classify up to a quarter of all insect species as parasitoids. However, the vast majority of these species consist of wasps, flies and strepsiptera.

How Parasitoids Work
- Most parasitoids lay their eggs directly in insect hosts. They may target a stationary developmental stage such as an egg or a pupa, or they may actively attack other insects to serve as hosts. Once the parasitoid egg hatches, it feeds on the tissue of the host insect until it is ready to undergo metamorphosis. At this point, the parasitoid emerges from the host insect's body as an adult, typically killing the host in the process.
Wasps
- Parasitoid wasps target the eggs, pupae and adults of other insect species. They lay their eggs in insect hosts using a specialized structure known as an ovipositor. The ovipositor pierces the host and injects venom along with one or more eggs. The venom may paralyze an adult host and in all cases alters the tissue around the injection site. Most commonly, this tissue alteration serves to dampen the immune response of the host insect, which would ordinarily attack and encapsulate the parasitoid larvae. In some cases, the venom also pre-digests the surrounding tissue to make it easier for the newly hatched larvae to feed.
Flies
- The majority of parasitoid flies belong to the family tachinidae, which commonly prey upon moths and butterflies, especially in their caterpillar form. Though some parasitoid flies will directly attack their victims, inserting their eggs into the body of a caterpillar when they are about to hatch, others will lay their eggs on leaves and branches where caterpillars are likely to encounter them. Once the eggs hatch within the host, they feed on non-vital organs until they mature. At this point, they exit the host's body, usually killing the host in the process.
Strepsiptera
- Strepsiptera, or twisted-wing parasites, are an isolated order of parasitoids that are not closely related to any other major order. They are interesting in that the female of most strepsiptera species does not have a free-living external form. Instead, the female remains within the body of the host, with only the reproductive organs protruding. The males are flying insects that escape from their hosts upon reaching adulthood, killing them in the process. The females mate and lay eggs without ever emerging from the host insect.

Insects & Science