For me, the most captivating thing about insects is their perplexing life histories. That is why I was so intrigued when I first learned about twisted-winged parasitoids (Strepsiptera).  Strepsiptera is a small insect order and modern consensus places it as a sister group of beetles (Niehuis et al. 2012). Only about 640 extant species are known worldwide and 9 from Finland. However, since these miniscule creatures are sometimes hard to find, and some species are so similar that you must use DNA to identify them, in reality the number of species is higher. For example, for a long period of time it was thought that Europe has only one Strepsiptera species from the genus Stylops. However, molecular methods revealed that this species, Stylops melittae, is a species complex of 30 species (Straka et al. 2015).

After this taxonomical jargon, let me tell you about the bizarre ways Strepsiptera live their lives. Strepsiptera are parasites of other insects and have hosts from 7 different insect orders (Kathirithamby 2018). Finnish species are parasites of bees (No, not honeybees but of some of our wonderful wild bees, mining bees and sweat bees) and planthoppers. 

In all but one family (not occurring in Finland), females remain as endoparasites their whole life whereas adult males are free-living. The short-lived adult males have characteristic “raspberry” eyes, halter-like forewings and fan-shaped hind wings. Adult females remain larvae- like and only have chitinized cephalothorax (fused head and thorax). The abdomen is soft. Because in most cases only the cephalothorax of the female is protruding outside of the host body (picture on left), common ways of copulation are out of question. Instead Strepsiptera have so-called traumatic insemination. The name is really fitting, since the males basically shove their hooked penis through the female’s head. The egg cells are floating freely in the females’s haemolymph, where they are fertilized by the male sperm. The eggs hatch inside of the female and motile first instars called planidia come out of the female (picture on bottom right). Then the adventure begins for the plandia. They need to find a host. In most cases the exact ways they do it, is not known. However, many of them use phoresy, meaning that they lurk in places where they ought to come across members of the host species and then they attach to the host mama, who then takes them to its nest. After successful hitchhiking to the nest, planidia invade the larva (their host) and often cover themselves with the host tissue to hide from the host immune system. This is followed by 3 endoparasitic larval stages after which the male pupates and hatches, and female goes through semi-pupation and extrudes its cephalothorax out of the host body.

By definition, parasitoids kill their host before they reach maturity. Strepsiptera are an exemption. They actually sometimes lengthen their hosts life span (Kathirithamby 2009). Then why are they parasitoids? Because they castrate their host leading to a reproductive death. Castration is not the only way they manipulate their host. They even alter the host behaviour. For example, mining bee females usually emerge after the males, but parasitization by Stylops makes both sexes emerge at the same time to facilitate the mating of Stylops (Straka et al. 2011).

I started working with Strepsiptera through the red list assessment of the order in Finland. At the time, the amount of Stylops species had just recently increased and the occurrence of the species in Finland was not known. So, before the actual assessment, I used ecology, morphology and DNA barcodes (see below) to determine what species occur in Finland. Unfortunately, many of our Stylops species are endangered. Their host are bees which need the type of pastures and meadows that the modern agriculture doesn’t create.

I continued with the subject in my master thesis and suprisingly, it will be the subject of my PhD as well.

Finnish species:


Niehuis, O., Hartig, G., Grath, S., Pohl, H., Lehmann, J., Tafer, H., Donath, A., Krauss, V., Eisenhardt, C., Hertel, J., Petersen, M., Mayer, C., Meusemann, K., Peters, Ralph S., Stadler, Peter F., Beutel, Rolf G., Bornberg-Bauer, E., McKenna, Duane D. & Misof, B. 2012: Genomic and Morphological Evidence Converge to Resolve the Enigma of Strepsiptera. — Current Biology 22: 1309-1313.

Kathirithamby, J. 2009: Host–Parasitoid Associations in Strepsiptera. ––– Annual Review of Entomology 54: 227–249.

Straka, J., Juzova, K. & Nakase, Y. 2015: Nomenclature and taxonomy of the genus Stylops (Strepsiptera): an annotated preliminary world checklist. ––– Acta Entomologica Musei Nationalis Pragae 55: 305–332.

Straka, J., Rezkova, K., Batelka, J. & Kratochvíl, L. 2011: Early nest emergence of females parasitised by Strepsiptera in protandrous bees (Hymenoptera Andrenidae). ––– Ethology Ecology & Evolution 23: 97–109.

Järvi, J., Lähteenaro, M. & Sihvonen, P. 2019: Suomen hyönteiset – Lahkojen tunnistusopas. — Luonnontieteellinen keskusmuseo Luomus, Helsingin yliopisto. Helsinki. 56 s.

Lähteenaro, M. & Paukkunen, J. 2019: Twisted-winged parasites.In: Hyvärinen, E., Juslén, A., Kemppainen, E., Uddström, A. & Liukko, U.-M. (eds.) 2019.The 2019 Red List of Finnish Species. Ministry of the Environment & Finnish Environment Institute.Helsinki. P. 509-512.

Lähteenaro, M. 2019. Strepsiptera, Twisted-winged parasitoids. – In: FinBIF 2019: The FinBIF checklist of Finnish species 2018. – Finnish Biodiversity Information Facility, Finnish Museum of Natural History, University of Helsinki, Helsinki

Lähteenaro, M. 2019: Twisted-wing parasitoid genus Stylops (Strepsiptera: Stylopidae) in Finland and the suitability of adult female morphology for species identification  (Master thesis, University of Helsinki.)

Duplouy, A., Minard, G., Lähteenaro, M., Rytteri, S. & Saastamoinen, M. 2018: Silk properties and overwinter survival in gregarious butterfly larvae. Ecology And Evolution 2018; 00:1–13 DOI: 10.1002/ece3.4595

Kirjat/Tutkija suosittelee: Ensimmäinen ekologi Maria Sibylla Merian Yliopisto/Helsingin yliopisto Y/07/19: 27.9.2019

Pasi Sihvonen, Jani Järvi & Meri Lähteenaro: Kansallinen hyönteiskokoelmadigikuvataan verkkoon. Yle TV 1, Luonto lähellä 26.11.2018

Meri Lähteenaro: Twisted-Wing Parasitoid Genus Stylops (Strepsiptera: Stylopidae) in Finland. Spring Symposium 4.-6.3. 2019. Helsinki. (Best Poster -award)