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