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= Xenos vesparum =

Xenos vesparum is a parasitic insect species of the genus Strepsiptera that are endoparasites of paper wasps in the genus Polistes, most commonly Polistes dominula.

Morphology
Males and female of this species show remarkable sexual dimorphism according to their mating system. Both develop inside the abdomen, where males pupate and emerge, and females permanently reside.

Males
Males are short lived (less than 6 hours), free-living flying insects whose adult life is solely dedicated to finding a mate.

-Sentence about unique eyes

Females
Females are markedly different than males, and remain in a neotenic larval-like form and permanently reside in their hosts, only partially emerging for the purpose of mating.

Morphology
Males and female of this species show remarkable sexual dimorphism according to their mating system. Both develop inside the abdomen, where males pupate and emerge, while females permanently reside inside.

Males
Adult males are free-living, flying insects, whose extremely short (<5 hours) adult lives are solely dedicated to finding a mate, which they are thought to locate by scent. After locating a mate (who is protruding from the host wasp's abdomen, the male lands on the wasp's abdomen, holding on with its legs and wings, while avoiding the brushing of the wasp's hindlegs, which could potentially dislodge it. The male then inseminates the female by either spreading sperm around the female's genital opening, where it eventually the haemocoel, or by directly penetrating the female's cuticle (hypodermic insemination), injecting the sperm directly into the haemocoel. The male then dies several hours after mating. Their small size coupled with their extremely short lifespan has made male X. vesparum very difficult to study.

Males also develop very unusual eyes compared to other insects. The eyes consist of a very small number of ommatidia (around 65, but each eye can vary by 10-15), while most insect eyes have thousands per eye which are closely packed together. The ommatidia are irregularly distributed across the eye and are well separated by cuticle. The function of these eyes is unknown, because mate finding, which is their only purpose as adults, seems to be done by scent, and the structure of the eyes indicate that they are modified larval eyes. Interestingly, they are externally quite similar to the eyes of phacopid trilobites.

The forewings of these insects are modified into small, club-like organs called pseudohalteres. These are to help the insect maintain equilibrium in flight, and function similarly to halteres found in Dipterans.

Females
Female X. vesparum are markedly different from their male counterparts. They display a high degree of neoteny, and are permanent endoparasites of their hosts. They reside in the wasp's body cavity and never develop mouthparts, legs, eyes or wings, and their only form of genitalia is the ventral opening where males can inseminate them, as well as being the point of larval escape. Females can often survive overwinter inside hibernating female wasps, which will emerge the following spring with underdeveloped ovaries, and will only be able to serve are vessels to spread the parasite's larvae as they are now effectively castrated.

They dwell their entire lifespan in the abdomen of the wasp. Two particular species of social wasp it has been known to infect are Polistes gallicus and Polistes dominula.Younger members of a wasp colony tend to be targeted, although X. vesparum displays parasitic behavior in all stages of this host.

According to Fabio Manfredini of Pennsylvania State University, co-author of an animal behaviour study of this insect's odd life cycle, published on 8 October 2011, the parasite infects a European paper wasp and completely alters its worker caste behaviour. The infected wasp begins to suffer nutritionally, then flies to meet with other infected wasps. The male parasite, which pupated in the infected wasp, exits the wasp's abdomen as a winged insect and mates with the female parasites which stay inside their host. Females are larviform and do not pupate. Wasps infected with the male parasite die. Wasps infected with the female parasite then fatten themselves up much like queen wasps do. They then fly to meet with other uninfected queen wasps. Then when the parasite is mature, the infected wasp flies to mingle with other uninfected wasps, thereby spreading brood and larvae into new environments