Stylopized, emasculated and zombified: the risks of visiting a flower

By Athayde Tonhasca

‘Bizarre’ and ‘weird’ are overused adjectives for describing many characters and events of the natural world. Life is way too complex and varied to conform to familiar patterns, so the out-of-the ordinary is all around us, even though we not always see it. But when the discussion turns to stylopids, it’s difficult to avoid talking about the bizarre and the weird.

Stylopids (or stylops) are small, seldom seen and poorly known insects with about 700 described species, 10 to 16 of them in Britain; the true figures are likely to be much higher. They are parasites of other insects such as bees, wasps, plant hoppers and leaf hoppers. From a distance, male stylopids can be mistaken for flies, but their ruffled wings give them away and explain the name of this group of insects: the order Strepsiptera, from the Greek strephein (to twist) and pteron (wing). Twisted-wing insects is another common name for them. 

Males have branched antennae, and their eyes are berry-like structures comprising dozens of image-forming eyelets. This unusual array inspired the development of new cameras of reduced size and sharp images, which are handy for smartphones. Males cannot feed because their mouthparts are not developed. But never mind going hungry; they don’t live for more than a few hours. Their only objective in life is to use their fancy eyes to find a female and mate.

A male stylopid © Mike Quinn,, Mckenna & Farrell, 2010, PLoS ONE 5(7):e11887; and detail of a male head © CSIRO, Australian Insect Families

Females look nothing like the males. In fact, they don’t look like your ordinary insect at all because they don’t have wings, antennae, legs, mouthparts or eyes: they are neotenic, i.e., they retain their larval features. An adult female does not need a fully-formed body since she never leaves her host: she will develop and die semi-buried in another insect. ‘Semi-buried’ because the tip of her cephalothorax (the head and the thorax fused together) protrudes from the host’s abdomen. 

A bee carrying a female stylopid. Scale bars = 1 mm © Soon, V. et al. 2012. Entomologica Fennica 22: 213-218.

Through this exposed area, the female releases a pheromone to attract males. Once suitors finds her, they face an anatomical challenge. Only parts of her head/thorax are exposed, which doesn’t bode well for conventional insect romance. But this setback is nothing compared to the facts that she doesn’t have genitalia, and her eggs float in the haemolymph (‘blood’). So a male has only one course of action: the disturbingly sounding traumatic or hypodermic insemination. He pierces the female’s cuticle with his penis and injects his sperm into her haemolymph.

Two male stylopids going after a female tucked in a bee © W. Rutkies at Peinert et al. 2016. Scientific Reports 6: 25052

The deed done, males soon die. The fertilised eggs hatch inside the female, giving birth to thousands of tiny six-legged, very active and agile larvae called planidia (just like blister or oil beetles). The planidia feed on mum’s innards and eventually crawl out of her body to disperse and start looking for a host of their own. 

A stylopid planidium, and planidia emerging from a female stylopid. Scale bars = 0.1 mm © Kathirithamby, J. 2018. Biodiversity of Strepsiptera

A wandering planidium climbs a flower to wait for an unsuspecting visitor. When a bee or wasp lands, the planidium somehow hitchhikes a ride to their nest. We are not sure how it does this: it could hide in the pollen, or possibly be swallowed with the nectar sucked up by the flower visitor, then released when the host regurgitates nectar inside the nest. Chances are it will end up in the wrong nest, so most planidia are done for. But the staggering fecundity of female stylopids compensates bad odds: they can dish out 750,000 planidia, so a few are likely to find a suitable host.

Once inside the right nest, the planidium burrows into a host’s egg or larva and transforms into a traditional legless, grub-like larva. It is followed by other larval stages, pupation, and finally adulthood – by then the host has also become an adult. If it’s a male stylopid, it squeezes out of the host and flies away, usually leaving a big gap behind and killing the host. If it’s a female, it will park itself in the host’s abdomen. 

A male stylopid emerging from a wasp © Kathirithamby, J. 2009. Annual Review of Entomology 54: 227–49

The story above is a peep at stylopids’ life histories, as there is considerable variation depending on the species and type of host. And if all this sounds outlandish, there is more to come.

Stylopization (parasitism by stylopids) causes all sorts of physical and behavioural eccentricities in the host, all for the parasite’s benefit. Such as infertility. Reproduction involves mating, nest building, nest provision, etc., which are risky and energy-consuming, therefore not beneficial for a parasite. Stylopids solve this problem by disabling the host’s reproductive organs, functionally castrating them. Some stylopized bees have reduced scopae (pollen-carrying structures) and seldom if ever carry pollen: there’s no point, as they don’t have a brood to provide for. Contrary to what happens to most parasitized insects, stylopization often lengthens hosts development; they live longer so that stylopids have more time to mature. Some stylopized bees are led to stand still on a grass or flower stems with their head downwards. Such a zombie state greatly facilitates stylopids’ mating business. 

In Britain, furrow bees (genera Halictus and Lasioglossum), yellow-face bees (genus Hylaeus) and especially mining bees (genus Andrena) are victims of stylopids, but we have little information about their interactions and no idea about consequences of parasitism. 

Stylopids are odd and enigmatic, but they are also one of the most complex and intriguing groups of animals. They are evolutionary marvels that have puzzled and awed generations of entomologists and naturalists, and more surprises should be revealed from future research. It seems quite fitting then for the august Royal Entomological Society to have adopted a stylopid (Stylops kirbii) as a representative of the organisation.

Royal Entomological Society badge © Wikipedia Creative Commons