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The brief lives and loves of male strepsipterans


This wasp has a problem! Three relatively enormous parasitic strepsipterans are occupying her abdomen…Photo by Sean McCann.

Who wouldn’t want to get to know the Strepsiptera? These animals are extremely odd, being obligate endoparasites of other insects, with a free-flying male and an eyeless, wingless female that never leaves the abdomen of her host. Different families of these parasites infect different hosts, ranging from silverfish and cockroaches to solitary and social wasps, leafhoppers, and froghoppers.

Allow me to introduce Xenos peckii, a strepsipteran parasite of Polistes fuscatus, the Northern Paper Wasp.  As an entomologist, I have long been interested in these little-studied insects, so I was thrilled to get to help my colleague Mike Hrabar in his investigation their life history and reproduction.

Mike collected a several colonies of infected wasps from Maine and brought them back to the lab to observe their emergence, flight and mating behaviour in a systematic way. We used high speed videography and careful record keeping to document their life history in closer detail than had ever previously been recorded.


Not really bling. This wasp sports a heavy infestation of four developing Xenos, costing vast amounts of resources.  Photo by Sean McCann.

From my perspective, one of the coolest things we learned is that the free-flying male opens his puparium by means of blade-like mandibles, which are used to cut along a zone of weakness in the pupal cap, functioning like a tiny can opener!

new head

Head of male Xenos peckii. Note the scissor-like mandibles and the large and unusual compound eyes. Photo by Mike Hrabar. Figure 3G from Hrabar et al. 2014.

Check out the video below to see the male’s little mandibles working the cap open.

[youtube http://www.youtube.com/watch?v=aCeEsXVNiOY?rel=0&w=560&h=315]

These little troopers fly immediately upon emergence, in stark contrast with most other insects, which need time to inflate and harden their wings. In fact, once the males begin beating their wings, they remain in flight continuously except for a brief period during mating.

Before our study, biologists had assumed that female Strepsiptera were completely immobile and passively waited for males to find them, but we observed that they move to adopt a distinct calling posture, elevating their cephalothorax up from the wasp’s abdomen, likely emitting a pheromone plume.

new calling

Female Xenos peckii in the abdomen of a Polistes fuscatus. This female is in the calling posture, elevating her cephalothorax. Photo by Mike Hrabar. Figure 4D from Hrabar et al. 2014.

The males smell this pheromone plume and fly toward it rapidly, in a zig-zag fashion reminiscent of pheromone-questing moths. As soon as a male reaches the female-infected host, he lands on her abdomen and walks down to where the female protrudes, using backwards steps with his heavily-modified tarsi.

new leg

Mid leg of Xenos peckii male. The tarsi are highly modified for gaining a strong grip on a wasp abdomen while searching for and mating with a female.  Figure 8 from Hrabar et al. 2014.

Mating occurs rapidly, with typical copulation time being 3-5 seconds. As soon as mating is finished, the male is once again in flight, presumably in search of another female. After copulation, the female immediately withdraws from the calling posture and ceases calling other males. The following video was taken at 1000 frames/second with a high-speed video camera and shows the sequence from just after landing by the male through the majority of copulation.

[youtube http://www.youtube.com/watch?v=QPiG8AV0XWY?rel=0&w=560&h=420]

S. McCann Polistes and Strepsipteran sm

This male Polistes fuscatus was weakening, and died while we were watching. Mike pinned the host, and we forgot about it for a while, until glancing at it we realized that one of the males was emerging! This shot was snatched quickly while the male had just popped off his cephalotheca. Photo by Sean McCann, Figure 2E from Hrabar et al. 2014.

We have shown that female Xenos are not just a passive receptacle or bag of eggs, but rather play a physically active role in soliciting mates. The male emergence is facilitated by using sharp mandibles to cut around an ecdysial suture line, and navigating the surface of his prospective mates host is aided by his extremely modified tarsi.

The short-lived males face a great challenge to locate and fly to a host with a calling female in the short amount of time they live (on average 2-2.5 hours). They are in constant flight from emergence until death with only a very short pause for mating. The female, by contrast, remains alive in her host, maturing a brood of eggs which she retains in her body until they hatch and crawl from her brood canal as motile planidial larvae.

These larvae will exit the brood canal at some point, but it is unclear exactly where they manage to find new hosts. It is possible they « deplane » at flowers and wait for a ride on a Polistes to a new nest of victims. Much more research will need to happen to fully understand these fascinating insects, but we have made a start at uncovering some of the mysteries of their emergence, communication and reproduction. Many more questions remain unanswered and provide opportunities for any natural historian to explore.

If you would like to read the whole paper, you can find it on the Canadian Entomologist site here, or if you are not a subscriber, I am hosting a corrected proof here.

The full citation for this paper is:

HRABAR, M., DANCI, A., MCCANN, S., SCHAEFER, P. W., and GRIES, G. 2014. New findings on life history traits of Xenos peckii (Strepsiptera: Xenidae). The Canadian Entomologist doi: http://dx.doi.org/10.4039/tce.2013.85 pp.1–14.

This post is also available in: English