Paraxenos

Paraxenos
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Pancrustacea
Class:	Insecta
Order:	Strepsiptera
Family:	Xenidae
Genus:	Paraxenos; Saunders, 1872

Paraxenos is a genus of insects in the family Xenidae (Order: Strepsiptera) with 14 species. The genus Paraxenos is defined as having males with four antennal segments, a thin sclerotized prementum separate from the mouthfield sclerites, a relatively wide postfrons not separated from the vertex plates, and a hindwing with a CuP vein.^[1]. All known Paraxenos species parasitize wasps in the family Bembicidae. The type species is Paraxenos erberi.

Distribution

This genus is found in Eurasia, Africa, and Australasia, owing to the distribution of bembicid wasps, its hosts. It is likely found in even more places than is known from specimens as insects in this order are notoriously cryptic and difficult to identify^[2]. Countries with known specimens include Algeria, Australia, Cyprus, Czech Republic, Democratic Republic of the Congo, Egypt, Germany, Greece, Hungary, India, Italy, Japan, Jordan, Mali, Mongolia, New Guinea, Senegal, Spain, Sri Lanka, and Tajikistan. Many species formerly considered to be in Paraxenos are found worldwide, wherever there are sphecid wasps.

Taxonomic history

First described in 1872 by Sydney S. Saunders as a member of Stylopidae (now divided into multiple families) as “Antennal branches distorted, swollen, compressed; 3rd joint prominent at basal elbow, rectangularly recurved; apex obtuse. Wing nerves insulated on both sides, duplicated” ^[3] (written originally in Latin). At the time, he described four species: Paraxenos erberi, P. corcyricus, P. sieboldii (synonymous with Xenos sphecidarum), and P. westwoodii. The type species is P. erberi. It is now considered a member of the family Xenidae separate from Stylopidae. Several other genera were formerly considered separate but are now in Paraxenos: Eupathocera, Ophthalmochlus, Homilops, Sceliphronechthrus, Tachytixenos, and Isodontiaphila. Historically considered sister to the genera Pseudoxenos and Xenos, genetic studies showed it to be polyphyletic and scattered across the phylogeny of Xenidae^[4]. As a result, a number of species were moved to other genera and some were placed into entirely new genera, such as Tuberoxenos. It was previously thought that Strepsiptera largely co-evolved with their hosts, but phylogenetic studies have revealed a significant amount of host-switching and thus a reconsideration of the relationships of the genera within Xenidae was necessary5. The family as a whole evolved approximately 54 million years ago. The basal clade of Xenidae, containing the first Paraxenos group of species, is approximately 41 million years old and of Palearctic origin.

Species list

Cook provides a list of all known hosts for Paraxenos in his 2019 catalog. Due to the polyphyly of this genus, Benda et al. proposed a new set of classifications for these. As such, a second list of species formerly in this genus is provided below. There are currently 14 recognized species in Paraxenos:

Paraxenos araibicus (Benda et al. 2023)
Paraxenos australiensis (Kifune & Hirashima 1987)
Paraxenos beaumonti (Pasteels 1951)
Paraxenos biroi (Székessy 1956)
Paraxenos erberi (Saunders 1872)
Paraxenos hofenederi (Pasteels 1956)
Paraxenos hofenederianus (Luna de Carvalho 1978)
Paraxenos hungaricus (Székessy 1955)
Paraxenos krombeini (Kifune & Hirashima 1987)
Paraxenos nagatomii (Kifune 1985)
Paraxenos novaeguineae (Székessy 1956)
Paraxenos occidentalis (Kifune & Hirashima 1987)
Paraxenos polli (Pasteels 1956)
Paraxenos rieki (Pasteels 1956)

Species formerly in Paraxenos:

Tachytixenos indicus: formerly Paraxenos indicus
Eupathocera argentina: formerly Paraxenos argentinus
Eupathocera auripedis: formerly Paraxenos auripedis
Eupathocera bucki: formerly Paraxenos bucki
Eupathocera duryi: formerly Paraxenos duryi
Eupathocera fasciati: formerly Paraxenos fasciati
Eupathocera fuliginosi: formerly Paraxenos fuliginosi
Eupathocera inclusa: formerly Paraxenos inclusus
Eupathocera luctuosae: formerly Paraxenos luctuosae
Eupathocera lugubris: formerly Paraxenos lugubris
Eupathocera mendozae: formerly Paraxenos mendozae
Eupathocera piercei: formerly Paraxenos piercei
Eupathocera striati: formerly Paraxenos striati
Eupathocera taschenbergi: formerly Paraxenos taschenbergi
Eupathocera westwoodii: formerly Paraxenos westwoodii
Sphecixenos abbotti: formerly Paraxenos abbotti
Sphecixenos astrolabensis: formerly Paraxenos astrolabensis
Sphecixenos dorae: formerly Paraxenos dorae
Sphecixenos erimae: formerly Paraxenos erimae
Sphecixenos esakii: formerly Paraxenos esakii
Sphecixenos gigas: formerly Paraxenos gigas
Sphecixenos kurosawai: formerly Paraxenos kurosawai
Sphecixenos laetus: formerly Paraxenos laetum
Sphecixenos orientalis: formerly Paraxenos orientalis
Sphecixenos reticulatus: formerly Paraxenos reticulatus
Sphecixenos simplex: formerly Paraxenos simplex
Sphecixenos vanderiisti: formerly Paraxenos vanderiisti
Pseudoxenos corcyricus: formerly Paraxenos corcyricus
Tuberoxenos altozambeziensis: formerly Paraxenos altozambeziensis
Tuberoxenos sinuatus: formerly Paraxenos sinuatus
Tuberoxenos sphecidarum: formerly Paraxenos sphecidarum
Tuberoxenos teres: formerly Paraxenos teres
Tuberoxenos tibetanus: formerly Paraxenos tibetanus
Paraxenos crassidens: synonymized with Paraxenos erberi

Biology

Reproduction

All Paraxenos parasitize wasps in the family Bembicidae2, and like all Strepsiptera, are obligate parasites. The females remain inside their host’s body until they die and males emerge as free-flying adults to find and inseminate a female. Like many other Strepsiptera, Paraxenos mate by traumatic insemination, in which the male inserts sperm directly into the body of the female by piercing her integument with his penis^[5], although some evidence suggests they may mate by brood canal insemination^[6]. In Paraxenos erberi, the male penis is hook-shaped and lacks spines and the female lacks a paragenital organ^[7]. The male possesses long, fine, spatulate microtrichia on his tarsomeres in order to adhere to the female during copulation^[8]. The larvae emerge mobile (often called planidia in this mobile stage) and must find a host to complete their life cycle. In Paraxenos lugubris (now Eupathocera lugubris), the larvae can only infect wasps in their vulnerable larval stage. In this case, the adult wasps carry the parasites to their offspring (known as vertical transmission) after picking them up on a flower without being infected themselves. Infected individuals (those infected as young) are sterile and do not invest in offspring care at all, instead spending the majority of their time drinking nectar, allowing the Paraxenos lugubris (now Eupathocera lugubris) larvae to jump on to flowers and be transmitted to another adult wasp^[9]. The larger the host, the larger the eggs laid by the female and the larger the female. The number of eggs depends on the gregariousness and foraging behavior of the host; females tend to lay fewer eggs when their species’ hosts are more gregarious and more frequent flower-visitors^[10].

Host species list

A list of hosts parasitized by Paraxenos sp. and species formerly in Paraxenos before 2019 is provided below.

Sphecixenos abbotti, host: Sphex sp.
Tuberoxenos altozambeziensis, host: Ammophila sp.
Paraxenos arabicus, host: Bembix kohli
Eupathocera argentina, host: Prionyx thomae
Paraxenos astrolabensis, host: Sphex cognatus
Eupathocera auripedis, host: Isodontia auripes
Paraxenos australiensis, host: Bembix musca
Paraxenos beaumonti, host: Stizus marthae
Eupathocera bucki, host: Ammophila sp.
Paraxenos biroi, host: Bembecinus antipodum
Pseudoxenos corcyricus, host: Odynerus spinipes
Paraxenos crassidens, host: Bembecinus tridens
Sphecixenos dorae, host: Sphex nigrohirtum
Eupathocera duryi, host: Prionyx atratus
Paraxenos erberi, hosts: Bembecinus hungaricus, Bembecinus peregrinus, Bembecinus tridens, Bembecinus sp., Stizus marthae
Sphecixenos erimae, host: Sphex fumicatus
Sphecixenos esakii, hosts: Isodontia nigella, Isodontia maidli
Eupathocera fasciati, host: Sceliphron fasciatum
Eupathocera fuliginosi, host: Sphex servillei
Sphecixenos gigas, host: Sphex schoutedeni
Paraxenos hofenederi, hosts: Stizus biclypeatus, Stizus ruficornis, Stizus rufiventris
Paraxenos hofenederianus, host: Stizus distinguendus
Paraxenos hungaricus, hosts: Bembex oculata, Bembex rostrata
Eupathocera inclusa, host: Ammophila sp.
Tachytixenos indicus, host: Tachyetes xenoferus, Tachyetes maculicornis, Tachyetes modestus, Tachyetes vischnu
Paraxenos krombeini, host: Bembix orientalis
Sphecixenos kurosawai, host: Sphex madasummae
Sphecixenos laetus, host: Sceliphron laetum
Eupathocera luctuosae, host: Sphex luctuosus
Eupathocera lugubris, hosts: Ammophila extremitata, Ammophila gracilis, Ammophila pruinosa, Ammophila kennedyi, Ammophila fernaldi, Ammophila aberti, Ammophila nasalis, Ammophila breviceps, Eremnophila aureonotata, Ammophila arvensis, Ammophila urnaria, Ammophila fernaldi, Ammophila pictipennis
Eupathocera mendozae, host: Prionyx neoxenus
Paraxenos nagatomii, host: Bembecinus bimaculatus
Paraxenos novaeguineae, host: Bembecinus gazagnairei
Paraxenos occidentalis, host: Bembix atrifrons
Sphecixenos orientalis, host: Sceliphron madraspatanum formosanum
Eupathocera piercei, host: Isodontia costipennis
Paraxenos polli, host: Nysson braunsii
Sphecixenos reticulatus, host: Sphex tomentosus
Paraxenos rieki, host: Stizus basalis
Sphecixenos simplex, host: Isodontia praslinia
Tuberoxenos sinuatus, host: Ammophila punctaticeps
Tuberoxenos sphecidarum, hosts: Podalonia affinis, Podalonia ebenina, Ammophila apicalis, Ammophila campestris, Ammophila heydeni, Ammophila holosericea, Ammophila nasuta, Ammophila pubescens, Ammophila sabulosa, Ammophila sp., Eremochares dives, Prionyx kirbii, Prionyx viduatus, Prionyx niveatus
Eupathocera striati, host: Prionyx striatus
Eupathocera taschenbergi, host: Prionyx pumilio
Tuberoxenos teres, hosts: Ammophila beniniensis, Ammophila ferrugineipes
Tuberoxenos tibetanus, host: Ammophila sp.
Sphecixenos vanderiisti, host: Isodontia pelopoeiformis
Eupathocera westwoodii, host: Sphex ichneumoneus

Morphology

The larvae of Paraxenos (as in other Strepsiptera) are highly miniaturized^[11]. In Paraxenos kurosawai (now Sphecixenos kurosawai), the first instar larvae are 0.85mm long, one of the largest among the order in fact. The miniaturization and parasitic lifestyle of this order has resulted in a number of unusual morphological traits. Many muscle cells in the head have their nuclei in the posterior end of the head region due to limited space and other muscles typically found in the head in other insects are in their thorax. Their brains are also in their thoracic region. They have a reduced tentorium, reduced and fused mouthparts primarily used to pierce their host’s integument, reduced antennae and maxillary palps, no heart, and reduced eyes that are still capable of color vision. The ventral surfaces of the larvae have outgrowths that appear to assist in adhering to surfaces. The legs also have adhesive pads to stick to their hosts. Paraxenos larvae lack a jumping mechanism due to their phoretic nature. Adult males have four antennal four antennal segments, a thin sclerotized prementum separate from the mouthfield sclerites, a relatively wide postfrons not separated from the vertex plates, and a hindwing with a CuP vein. As in all Strepsiptera, the males have reduced forewings and longer hindwings that appear “twisted”, giving them their name “twisted-wing parasites”. They only live a few hours after emerging from their host to mate. Females are neotenic (also called larviform), meaning they retain larval characters. They spend their entire adult lives inside their hosts.

References

↑ Cook, J.L. (2019). "Annotated Catalog of the Order Strepsiptera of the World". Trans. Am. Entomol. Soc. 145 (121).
↑ Cook, J.L. (2019). "Annotated Catalog of the Order Strepsiptera of the World". Trans. Am. Entomol. Soc. 145 (121).
↑ Saunders, S.S. (1872). "Stylopidarum, ordinem Strepsipterorum Kirbii constituentium, mihi tamen potius Coleopterorum Familiae, Rhiphidiphoridis Meloïdisque propinquae, Monographia". Trans. Entomol. Society London. 1872: 1–48.
↑ Benda, D., Nakase, Y. & Straka, J. (2019). "Frozen Antarctic path for dispersal initiated parallel host-parasite evolution on different continents". Mol. Phylogenet. Evol. 135: 67–77.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Jandausch, K., Van De Kamp, T., Beutel, R. G., Niehuis, O. & Pohl, H. (2023). "'Stab, chase me, mate with me, seduce me': how widespread is traumatic insemination in Strepsiptera?". Biol. J. Linn. Soc. 140: 206-223.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Millena, R. J. A., Lähteenaro, M., Riegler, M. & Kathirithamby, J. (2025). "Strepsiptera systematics: past, present, and future". Insect Syst. Divers. 9: 1.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Jandausch, K. (2025). "The Paragenital Organ of Stylopidae (Insecta: Strepsiptera) and the Functional Incorporation of the Secondary Larval Exuvia". J. Morphol. 286.
↑ Pohl, H. & Beutel, Rolf. G. (2004). "Fine structure of adhesive devices of Strepsiptera (Insecta)". Arthropod Struct. Dev. 33: 31–43.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Millena, R. J. A. & Rosenheim, J. A. (2022). "A double-edged sword: parental care increases risk of offspring infection by a maternally vectored parasite". Biol. Lett. 18.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Maeta, Y., Takahashi, K. & Shimada, N. (1998). "Host body size as a factor determining the egg complement of Strepsiptera, an insect parasite". Int. J. Insect Morphol. Embryol. 27: 27–37.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Pohl, H. & Beutel, R. G. (2019). "Effects of miniaturization in primary larvae of Strepsiptera (Insecta)". Arthropod Struct. Dev. 48: 49–55.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[1] Cook, J.L. (2019). "Annotated Catalog of the Order Strepsiptera of the World". Trans. Am. Entomol. Soc. 145 (121).

[2] Cook, J.L. (2019). "Annotated Catalog of the Order Strepsiptera of the World". Trans. Am. Entomol. Soc. 145 (121).

[3] Saunders, S.S. (1872). "Stylopidarum, ordinem Strepsipterorum Kirbii constituentium, mihi tamen potius Coleopterorum Familiae, Rhiphidiphoridis Meloïdisque propinquae, Monographia". Trans. Entomol. Society London. 1872: 1–48.

[4] Benda, D., Nakase, Y. & Straka, J. (2019). "Frozen Antarctic path for dispersal initiated parallel host-parasite evolution on different continents". Mol. Phylogenet. Evol. 135: 67–77.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[5] Jandausch, K., Van De Kamp, T., Beutel, R. G., Niehuis, O. & Pohl, H. (2023). "'Stab, chase me, mate with me, seduce me': how widespread is traumatic insemination in Strepsiptera?". Biol. J. Linn. Soc. 140: 206-223.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[6] Millena, R. J. A., Lähteenaro, M., Riegler, M. & Kathirithamby, J. (2025). "Strepsiptera systematics: past, present, and future". Insect Syst. Divers. 9: 1.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[7] Jandausch, K. (2025). "The Paragenital Organ of Stylopidae (Insecta: Strepsiptera) and the Functional Incorporation of the Secondary Larval Exuvia". J. Morphol. 286.

[8] Pohl, H. & Beutel, Rolf. G. (2004). "Fine structure of adhesive devices of Strepsiptera (Insecta)". Arthropod Struct. Dev. 33: 31–43.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[9] Millena, R. J. A. & Rosenheim, J. A. (2022). "A double-edged sword: parental care increases risk of offspring infection by a maternally vectored parasite". Biol. Lett. 18.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[10] Maeta, Y., Takahashi, K. & Shimada, N. (1998). "Host body size as a factor determining the egg complement of Strepsiptera, an insect parasite". Int. J. Insect Morphol. Embryol. 27: 27–37.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[11] Pohl, H. & Beutel, R. G. (2019). "Effects of miniaturization in primary larvae of Strepsiptera (Insecta)". Arthropod Struct. Dev. 48: 49–55.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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