The Twisted-Wing Parasites (Insecta: Strepsiptera)

Source: Grimaldi & Engel (2005)
Morphology of adult male Strepsiptera. Source: Grimaldi & Engel (2005)

The Strepsiptera, commonly called twisted-wing parasites, are an enigmatic order of obligately parasitic insects discovered by the “founder of entomology”, William Kirby, and characterised by him in Kirby (1813). Unrelated, this paper also contains the first mention of another order of insect, the Trichoptera (caddisflies). There are over 600 Recent Strepsiptera species in 10 families, according to Kathirithamby’s database, and this is most likely a tremendous underestimation. Most strepsipterans are grouped together in the Stylopidia, which includes all strepsipterans besides fossil-only families and the basal Mengenellidae (Pohl & Beutel, 2005).

It’s very difficult to find them out and about in the field. Your best chance is to find an infected host and dissect the female out, or leave it in there and wait for the males to show up. Adult males range in size between 1-5 mm (largest up to 30 mm). They are recognisable by their possessing only one pair of hind wings; the male’s front wings are reduced to flight balancing organs, as seen in the detail above. Females tend to be paedogenic, resembling larvae: wingless and spending their entire lifespan as internal parasites of other insects, with the major exception being females of the Mengenellidae family, which are free-living as adults, presumably the state representative of the ancestor of the Strepsiptera (Pohl & Beutel, 2005).

Triungulin larva of Mengenilla chobauti. Source:
Triungulin larva of Mengenilla chobauti. Source: Osswald et al. (2010)

The larvae of strepsipterans count as among the smallest animals known, especially the first instar larvae, dubbed the triungulin larvae: they range between 80 – 850 µm (Beutel et al., 2005); for comparison, the typical Amoeba is around 600 µm. Everything in them is geared to allow them to enter their hosts: they have colour vision (Kirkpatrick, 1937), a jumping ability, and pads on their feet to stick to the host (Pohl & Beutel, 2004). With such a small size, you might think they have a tiny brain, but in fact, they invest so much in their brain that the 200 µm larva of Mengenilla chobauti has a brain that is larger than its head and extending into the prothorax – it takes up 5% of the body volume (Beutel et al., 2005). That’s a much better percentage than humans!

Host specificity of major Strepsiptera families. Source: Kathirithamby (2009)
Host specificity of major Strepsiptera families. Source: Kathirithamby (2009)

Hosts include silverfish, crickets, planthoppers, hymenopterans, cockroaches and mantises, flies, and caddisflies. As the diagram above shows, there is a degree of host specificity at the family level. Once the triungulin finds and enters the larval host, it will go through three more stages as a larva, staying inside the host through its entire life cycle (Kathirithamby, 2009). An infected host is referred to as being stylopised, and the main symptom is infertility, including by extreme reduction or disappearance of the host’s genitals. In remarkable cases of coevolution, some strepsipterans manage to avoid any reaction from the immune system of their hosts by making a house out of the tissue of their hosts (Kathirithamby et al., 2003). Interestingly, it could be that they also induce behavioural changes in their hosts: parasitised Polistes dominulus wasps leave their colonies and aggregate together, behaving more like gynes than workers (Hughes et al., 2004).

Exemplary developmental cycles of Strepsiptera. Source: Kathirithamby (2009)
Exemplary developmental cycles of Strepsiptera. Source: Kathirithamby (2009)

The developmental cycle of Strepsiptera is not very well known. Two summaries are presented above, and they show how variable their cycle can be. If you’re interested in this aspect of the strepsipterans, you need to get Kathirithamby (2009), the most comprehensive review of strepsipteran parasitism I know of, and the source of several stolen diagrams in this post. The general plan goes like this:

  • The larvae attack and grow in the hosts.
  • Females remain in the host. Upon reaching maturity, they extend their cephalothorax (fused head and thorax) to the outside.

Males emerge from the hosts and have to use their up to six hours of adult lifespan to find and mate with the females.

Examplary life cycle of a strepsipteran species. Source: Kathirithamby (2009)
Examplary life cycle of a strepsipteran species. Source: Kathirithamby (2009)

As an example of how weird their parasitism can be, look at the diagram above, or consider the myrmecolacid Caenocholax fenyesi, in which males and females have different hosts: males infect ants, females infect crickets (Kathirithamby & Johnston, 2004).

Head of Mengenilla moldrzyki. Source:
Head of Mengenilla moldrzyki. Source: Pohl et al. (2012)

The life of the adult male is geared towards finding a female to mate with, which their well-developed sense organs can take care of (Beutel & Pohl, 2006). As can be seen above, their antennae are shaped rather like antlers, allowing them to sense more than simple antennae. The antennae are also covered in sensillae. Their compound eyes are relatively large, and have a unique structure. There are around 150 lenses in each. Each lens has its own independent retina that is sampled by a spate of photoreceptors (Buschbeck et al., 1999). Adult males also have microtrichia (small hairs) on the bottom of their legs to grab on to the host/female for mating (Pohl & Beutel, 2004)

The male’s quest is helped by the female releasing pheromones through Nassonow’s gland in the cephalothorax (Dallai et al., 2004).

Stylops melittae female parasitising an Andrena vaga bee. Source: Soon et al. (2011)
Stylops melittae female parasitising an Andrena vaga bee. Source: Soon et al. (2011)

The male strepsipteran genital apparatus is highly simplified. Unlike most holometabolans, strepsipteran males do not use spermatophores, instead having a sperm pump to transfer liquid sperm to the females (Kinzelbach, 1971). Only Diptera, Mecoptera, and Siphonaptera have the same condition, although it evolved separately in the Strepsiptera. The female is buried in the host, with her cephalothorax sticking out for the male to enjoy himself, as pointed out by the red arrow in the picture above. That’s right, they mate while the female is still in the host, like a sexually mature human male baby going into a delivery room and mating with a female baby as it’s coming out of the vagina.

In the Stylopidia there is a brood canal in the cephalothorax, but in the basal Mengenellidae, the female genitals are reduced to practical non-existence, so the male has to stab her and ejaculate, a mating method known as traumatic insemination. Sperm travels into the female, which has cells that engulfs it (Beani et al., 2005). Fertilisation occurs and several weeks later, the female gives birth to live young free to infect unparasitised hosts when they exit the host through the female’s brood canal.

Protoxenus janzeni (Protoxenidae). Source: Pohl et al. (2005)
Protoxenus janzeni (Protoxenidae). Source: Pohl et al. (2005)

Despite their tiny size and cryptic lifestyle, Strepsiptera do have a fossil record, including a parasitised halictid bee in Dominican amber (Poinar, 2004). The oldest specimen known comes from the Burmese amber (Grimaldi & Engel, 2005), and Pohl et al. (2005) describe a very basal Baltic amber strepsipteran, pictured above, that has both tell-tale strepsipteran features as well as more plesiomorphic characters, making it an important fossil for the study of the evolutionary history of strepsipterans.

Strepsipterans are mysterious for many reasons, but the most historically wild debate concerns their phylogenetic placement, so much so that we refer to it as the “Strepsiptera Problem” (Kristensen, 1981), and it was often used as a way to pit molecular phylogenetic methodologies against each other, especially in molecular phylogenetics where the variable substitution rates and shifts in base composition in the genes used for phylogenetic reconstruction provide great examples of long branch attraction (Huelsenbeck, 1997). The very first total evidence analysis combining morphological and molecular data conducted for elucidating insect phylogeny was done in order to solve the Problem (Whiting et al., 1997).

In his major reviews on insect classification, Kristensen even had doubts as to whether they could be placed in the Holometabola, the insects that undergo a complete metamorphosis in their life cycle (think caterpillar – butterfly, or maggot – fly) (Kristensen, 1981), but these doubts are now quelled. In his groundbreaking 1953 article, Willi Hennig left them floating free as holometabolans, not having any leads to associate them with any sister group (Hennig, 1953).

Within the Holometabola, the Strepsiptera are sometimes allied to the Diptera (flies). Lamarck (1816) placed them as such, a hypothesis later reprised by early molecular phylogenies (Whiting et al., 1997) and dubbed the “Halteria” hypothesis due to the major morphological commonality between Strepsiptera and Diptera being a reduced pair of wings. In Diptera, the hind wings are reduced to form the characteristic halteres, balancing organs for flight. The reduced forewings of male Strepsiptera have the same function (Pix et al., 1993). The fact that the Strepsiptera have reduced forewings and the Diptera reduced hindwings was trumpeted as a perfect example of a homeotic mutation in which master developmental control genes, Hox genes, got mutated, causing the second and third thoracic segments to be flipped around in the Strepsiptera – in effect saying that the halteres and reduced forewings are homologous, even though they are on different segments (Whiting & Wheeler, 1994). The precise mutation enabling this supposed reversal has not been found, and these molecular analyses are not very reliable anyway: they were done using ribosomal sequences, and a major problem for insect molecular phylogenetics is that these sequences evolve too fast to be useful (Hwang et al., 1998). Another feature is the presence of a sperm pump (Whiting, 1998), but the lack of structural homologies makes this unconvincing (Kristensen, 1999).

Another hypothesis states that the Strepsiptera are a weird cucujiform beetle group (Crowson, 1960), but this is highly unlikely since any traits they share also evolved convergently in other beetle groups, so the evidence for it is solid at all.

The general feeling for a long time was that the Strepsiptera are close to the Coleoptera, the beetles (Hinton, 1958), and the hypothesis that they are the sister group to the Coleoptera is currently the most supported (Niehuis et al., 2012). Crowson (1960) also suggested that the Strepsiptera are a beetle group, but this is not viewed as viable. The bulk of the morphological data supports a Strepsiptera+Coleoptera grouping, especially when it comes to hindwing venation (Kukalová-Peck & Lawrence, 2004). The main functional argument is that both use their hind wings for flight. Both of them have lost their dorsal pulsatile organs, as well as several muscles in the mesothorax (Beutel & Haas, 2000). As in beetles, the ventral segments of a strepsipteran are more sclerotised than the dorsal segments. There are several recent molecular phylogenies that also support this hypothesis, e.g. Ishiwata et al. (2011) or Longhorn et al. (2010).

Evolution of the Strepsiptera. Source:
Evolution of the Strepsiptera. Source: Pohl & Beutel (2008)

The main reason for all this confusion is the key trait of the Strepsiptera: their convoluted parasitism, with extreme sexual dimorphism. The unsteady molecular evolution rates make molecular phylogenetics tricky, and the parasitism makes morphological datasets difficult to assemble properly, since features are either reduced to oblivion (females) or possibly modified to an extent too large for reliable homologisation (males). What is clear is that the evolutionary success of the Strepsiptera was determined to a large part by the key novelty of having a completely endoparasitic female, as seen in the summary diagram above (Pohl & Beutel, 2008).

References:

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