A Queenage of Strepsiptera

Those of you wondering about the significance of the title to this post might want to check out the comments for last week's post on Embioptera. I noted there that a collective noun for Strepsiptera would arguably be one of the most useless concepts in the English language. In making that comment, I was referring to the fact that Strepsiptera, to the best of my knowledge, pretty never occur in noticeable groups. In fact, Strepsiptera are one of the rarest of all insect orders - so rare as to be almost mythical*. As such, their existence is not widely known by non-entomologists, and the discovery of a strepsipteran specimen is usually heralded by an unsuspecting research assistant looking down a microscope at a dish of unsorted survey specimens suddenly exclaiming, "What the f*** is that?"

*If you want a more concrete example, an ecological survey being conducted by colleagues of mine has so far collected tens of thousands of specimens - including about three strepsipterans.

Strepsiptera are endoparasites of other insects. The name means 'twisted wing', and you may also find them being called stylops*. Both sexes are parasitic as larvae, and after pupating the winged males leave the host in search of females (the picture above, from Tree of Life, shows a male Pseudoxenos leaving its wasp host. Ick). Mature males never feed, and may only survive for a few hours. The females, except for one primitive family, never leave the host, but remain in a larva-like form.

*By the way, 'stylops' is both the singular and the plural.



In the extremely unlikely event of ever seeing a strepsipteran, you can rest assured that they cannot be easily mistaken for anything else. The picture above comes from here, and shows a generalised strepsipteran male. Strepsiptera have only one pair of functional wings, with the front pair reduced to balancing organs called halteres. The only other insect order to possess halteres are Diptera (flies), but in Diptera it is the hind pair that has been altered (more on that later). The antennae are branched and antler-like. The so-called 'raspberry eye' of Strepsiptera is actually unique in the insect world, with many disjoint ocelli. It can be seen better in the photo below of Caenocholax fenyesi (by Steve Taylor, from here).



The larvae are produced viviparously by the female, and emerge from the host in large numbers (so maybe there is a use for the collective noun, after all). The first instar larvae (known as triungulins) are surprisingly advanced, with well-developed eyes and legs in order to seek out a new host. Once they have found a host and burrowed in, however, all these mod-cons are jettisoned, leaving the larva legless and grub-like. The presence of such distinct larval stages is referred to as hypermetamorphosis. At least one strepsipteran family, the Myrmecolacidae, has particularly unusual host preferences - the males are parasites of ants, while the females favour grasshopppers and crickets (Kathrithamby et al., 2003). I have not been able to find whether the sex of the larva determines the host, or whether the host determines the sex.

Phylogenetically, the Strepsiptera are arguably the second most difficult insect order - probably, only the Zoraptera can claim to have caused more problems. Still, there are two main competitors for the position of nearest strepsipteran relative. For a long time, the Strepsiptera were associated with the beetles, to the extent that some authors even suggested reducing them to a subgroup of the Coleoptera. This was mainly predicated on similarities between the triungulin larvae of Strepsiptera and certain Coleoptera families, some of which shared the Strepsiptera's branched antennae and hypermetamorphosis. However, these features are also found in other unrelated insect groups, and the chance of convergence cannot be dismissed. Molecular analyses, on the other hand, suggested a relationship between Strepsiptera and Diptera, leading to the radical suggestion by Whiting & Wheeler (1994) that the strepsipteran halteres might actually be homologous to those of Diptera, and their difference in position might be due to a homoeotic reversal switching the identities of the wing pairs! At present, it is difficult to imagine how such a thing could have happened without fatally scrambling the rest of the insect's anatomy in that area, and even if they are sister groups, the Strepsiptera and Diptera may have still evolved their respective halteres independently.



Male Stylops pacificus mating with female parasitic on bee. Photo by Edward Ross, from Tree of Life.

And why should a collection of Strepsiptera be called a 'queenage'? It should be noted that parasitism by Strepsiptera (known as stylopisation), despite the inherent ickiness of having a grub-like parasite protruding from your abdomen, is rarely fatal, and males and larvae can emerge without harming the host. Indeed, stylopised hosts may live longer than they would normally. However, stylopisation can have other significant consequences. Gonad development is reduced, and stylopised hosts may often be sterile. Stylopisation may also have a dramatic effect on secondary sexual characteristics of the host - stylopised individuals may lose their expected secondary sexual features and develop features characteristic of the other sex (Salt, 1927). Hughes et al. (2004) discovered that stylopised individuals of one species of wasp did not work in the colony as normal, but abandoned the colony and formed loose aggregations elsewhere.

Parasite-induced castration is not uncommon in invertebrates, and it is believed that it is advantageous for the parasite to sterilise its host because then time and energy that the host would otherwise waste on finding and winning a mate and producing offspring can instead be focused on feeding the host and hence the parasite (think about the behavioural differences between a neutered and entire cat). Colony desertion by stylopised wasps is probably also induced by the parasite (stylopised individuals were not driven away from the colony by uninfected individuals) as the chance of successful male emergence and mating was greater in the aggregations than within the nest, where healthy wasps would destroy any male strepsipterans they spotted.

REFERENCES

Hughes, D. P., J. Kathirithamby, S. Turillazzi & L Beani. 2004. Social wasps desert the colony and aggregate outside if parasitized: parasite manipulation? Behavioral Ecology 15 (6): 1037-1043.

Kathirithamby, J., L. D. Ross & J. C. Johnston. 2003. Masquerading as self? Endoparasitic Strepsiptera (Insecta) enclose themselves in host-derived epidermal bag. Proceedings of the National Academy of Sciences of the USA 100 (13): 7655-7659.

Salt, G. 1927. The effects of stylopization on aculeate Hymenoptera. Journal of Experimental Zoology 48: 223-331.

Whiting, M. F., & W. C. Wheeler. 1994. Insect homeotic transformation. Nature 368: 696.