Wild Side: One for the books

Romance among these bee parasites is complex.

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Parasites, seen as the little brown tabs, are on the back of the bee. — Matt Pelikan

There is some really weird stuff out there.

Take the insect order Strepsiptera, commonly known as “twisted-wing insects” because of the bizarre wing form shown by adult males. The front wings are knotted up into an apparently useless, club-shaped structure; the hind wings, which are functional for flight, are fan-shaped, transparent, and lack the veins that are usual in insect wings. While they account for the common name of the order, adult males burn out quickly, living just a few days and having to find a female and mate within that brief window. It’s the even odder females that one generally encounters.

While running into new species, genera, or even families of insects is pretty routine for me, it isn’t often that I encounter a new order. Order is a high-level taxonomic grouping: Think ducks or songbirds in the avian world, or beetles among the insects. So while Strepsiptera is a very small order, with fewer than 100 known members in North America, it is taxonomically distinctive and hence important. Encountering my first examples of this group was a high point of the season so far.

It starts with a bee I found along a fire lane in Correllus State Forest. While the bee was not cooperative, I managed a few decent photographs of it, and later sat down to review the photos and try to identify the insect. It proved to be Andrena vicinia, one of our many types of ground-nesting, solitary bees. But I was startled to notice two ticklike hitchhikers wedged between two of her abdominal segments. I’d never seen anything like that.

A plea for help in a Facebook insect group quickly produced a general ID: members of the parasitic order Strepsiptera. A little research narrowed things down: Among Strepsiptera, families, genera, and even species specialize in parasitizing particular hosts. Since I knew the host’s identity, I worked though the family Stylopidae (which parasitizes Hymenoptera) to the subfamily Stylopinae (parasitizes bees), to the genus Stylops (parasitizes Andrena). So far, so good!

Then things got weird. Although related to the familiar order Coleoptera, the beetles, Strepsiptera has evolved an utterly outlandish life history. Start with the two individuals I found: adult females, probably, but still larval in form. While males in this order mature as winged, free-living insects, females remain in a roughly larval state through maturity — a process called “neoteny” by biologists. It’s an unusual system, even among parasitic insects.

Also unusual is the so-called viviparous reproduction of Strepsiptera: Instead of laying eggs, females expel their young already in a larval state. These first-stage larvae are said to be quite spry, equipped with the usual insect complement of six legs plus a lot of vigor. They find a suitable spot (such as on a flower) to wait for a host, and when one shows up, they step aboard and burrow in.

These parasites live on the tissue and juices of their host, but in contrast to a lot of other insect parasites, rarely kill it. The mobile host helps the Strepsiptids disperse, and the larvae gradually mature inside their host. Males, when mature, fledge into the unusual winged, free-living adults I mentioned earlier. Females remain partly embedded in their host, attracting a male with scent chemicals called pheromones. After mating, which occurs with the female still wedged into the bee’s abdomen, the next generation of larvae develops inside the female. When they’re ready, the female chucks the youngsters overboard to start the cycle again.

The thing that amazes me is that this seemingly random reproductive process manages to work in spite of very fussy host requirements. In a fascinating if somewhat ponderous article in the Zoological Journal of the Linnean Society, a team of Czech and Japanese researchers compared the genetic makeup of Stylops taken from various species of Andrena bees. Their results pointed to a very high degree of specialization, despite little physical difference among the parasites. It appears that most species of Stylops, as determined by their genetic composition, associate with one or at most a very few species of Andrena. Those randomly ejected larvae, that is to say, don’t need to find just any bee; they need to find a bee of a particular species in order to complete their development.

A 2005 article in the journal Zootaxa includes a checklist of the known species of Strepsiptera in the U.S. and Canada, along with known hosts. But among the 30 or species of Stylops, none is listed as associated with Andrena vicina. So the odd parasites I encountered may belong to a species that has not yet been formally described. And with some 1,500 Andrena species known worldwide, one has to wonder how many other species of Stylops are out there, waiting for someone to notice them.

Alerted to the existence of these creatures, I find they’re not rare: I’ve seen several more “Stylopized” bees. And with each one, I marvel that such a complicated, random system of reproduction works. And yet, clearly, it does!

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