Mysterious parasites peeking from paper wasp abdomens

A large number of us may complain about an eccentric relative of ours. Similarly, if beetles (Coleoptera) could talk, they would perhaps complain about the peculiar beings that comprise their sister group. Strepsiptera, also known as twisted-wing insects, are obligate parasites of other insects, such as bees, wasps, or mantises. Parasitic adult females of most strepsipteran species spend their entire life wedged between abdominal segments of the host. Due to its parasitic lifestyle, the female no longer resembles a ‘proper’ insect. Rather, it looks like a mere sack with disrupted organs, coated in multiple cuticular layers. To add to this oddity, the female is viviparous, and young larvae freely walk around inside its body. By contrast, adult males are free-living and short-lived, and a layperson would probably consider them to be unusual flies.

Strepsipterans of the advanced families Stylopidae and Xenidae parasitise a plethora of aculeate Hymenoptera and can manipulate their host’s morphology and behaviour. Thus, Strepsiptera are of immense interest, however their rarity (and the rarity of strepsipterologists) impedes their study, and most species are thought to be undescribed.

Parasitised paper wasp of the genus Mischocyttarus with Xenos bicolor protruding from between abdominal segments.
Source: original paper

Entomologists Daniel Benda and Jakub Straka from the Department of Zoology, Faculty of Science, recently participated in the description of two new species of strepsipterans. They focused on Xenidae, a family with problematic taxonomy that varies from author to author. Furthermore, individual genera have been defined based on host specificity rather than morphological and/or molecular data.

 A new perspective on the taxonomy of Xenidae is provided by the continuous work of entomologists from our faculty along with their foreign colleagues. Their most recent study focuses on the species-rich genus Xenos parasitising social wasps. New World Xenos parasitises only paper wasps of the genera Polistes and Mischocyttarus, the latter of which was known for over a century to be parasitised by only one species: Xenos americanus from Bolivia. The holotype of X. americanus may have been lost, but luckily the author who wrote about the species provided a detailed description and photographs of the specimen. The two newly described species (Xenos bicolor from various host species from Mexico and USA, and Xenos pallens from one host species from Costa Rica) could thus be compared with this enigmatic species.

Front part of the body of female Xenos bicolor.
Source: original paper

Furthermore, the authors provide a Xenos identification key based on colouration and morphology that should facilitate the identification of new and decades-old museum specimens alike. Moreover, they provide a list of characters suitable for species identification, and a list of characters that vary even between individuals of a single species, and hence are of little diagnostic value. The methods developed by the authors as well as the overview of characters pave the way for easier species identification and description: this is a valuable contribution, given the presumed high number of undescribed Xenos species. The undiscovered diversity of Strepsiptera surely offers much to scientists, and enhancing our knowledge of strepsipteran taxonomy is an important step towards better understanding of this enigmatic group.

Benda, D., Pohl, H., Beutel, R., & Straka, J. (2022). Two new species of Xenos (Strepsiptera: Xenidae), parasites of social wasps of the genus Mischocyttarus (Hymenoptera: Vespidae) in the New World. Acta Entomologica Musei Nationalis Pragae, 62(1), 185-195.

Kateřina Bezányiová