Common Greenbottle (Lucilia sp.)

Dear Readers, I was sitting on the patio after eating a Waitrose Lemon Curd yoghurt (and if there’s a more middle-class statement than that I’d like to hear it) when I noticed that three Greenbottles were attracted to the empty yoghurt pot. And being the curious person that I am, I wondered why they came when no other insects did.

Well, first up Greenbottles have very superior taste (joke!) but actually they are attracted to the colour yellow, and also to the scent of many flowers. Notoriously, they love the smell of flowers like the Dead Horse Arum Lily, which stinks of carrion, and this makes sense, as Greenbottles are blowflies – they lay their eggs in carrion, and their larvae devour the corpses. However, when the adults can’t find carrion, they eat pollen, which is also very high in protein – females in particular are drawn to flowers, especially pregnant ones, who need the protein for their eggs to develop.

I’d noticed this attraction to flowers on many occasions – Greenbottles are often the first insects to arrive on newly open blooms, especially ones like wild carrot or cow parsley, with lots of tiny flowers. These flies are not specialist feeders like bees, but they are important pollinators of lots of plants, transferring pollen from one plant to another as they search for something to eat.

Greenbottle cleaning its hind legs

The love life of a Greenbottle is a complicated thing. First, the male pushes a potential mate several times with his head, and then taps her with his front legs. If all is going well, the male will mount the female whilst continuing to tap her with one front leg. If she’s amenable, the mating will take place, and the result is possibly 200 tiny maggots laid in a dead rabbit or some other corpse. However, if she’s not happy she will kick the male away with her back legs. Apparently, male Greenbottles have ‘handedness’, with some preferring to tap with their left foreleg, and others being ‘right-handed’.

How do the males recognise the females in the first place, though? Scientists have found that Greenbottle males can recognise a healthy female by the rate at which their iridescent green bodies ‘flash’ through their wingbeats – 178 Hz in case you wondered. This is actually slower than  the rate of other males, and is also slower than the rate for older flies of either sex. Furthermore, Greenbottles mate less on cloudy days, which seems to indicate that they are reliant on the flashes to identify a mate.

I am utterly gobsmacked, and not for the first time. The smallest, most maligned and commonest plants and animals that surround us often have the most complicated and nuanced of lives.

Let’s return briefly to the laying of eggs and the development of larvae, though. Anyone who has ever had a maggot-filled dustbin can attest to the speed and sheer numbers of blowfly larvae that can manifest themselves when conditions are right. But how? First up, when a suitable carcass is found, many females will lay their eggs on it – scientists aren’t sure quite what signal is given, but it may be that the sight of a female ovipositing encourages other females to do the same, or there may be some kind of chemical signal. But whatever the reason, this means that all the larvae are about the same age, and will all hatch together. This mass hatching means that the larvae thermoregulate, controlling and raising the temperature of the whole group, which means that they all develop more quickly. Furthermore, each larva secretes digestive enzymes which make the carrion easier to digest, benefitting the group as a whole. The group also appears to be able to make collective decisions – they will congregate in one area if there is lots to eat, but will split into smaller groups as the food runs out. It appears that each larva secretes a chemical signal, which encourages other larvae to follow.

To us, this might all seem a bit disgusting, but it’s worth bearing in mind that without flies getting rid of dead matter, we would be in trouble. And, finally, Greenbottles are one of the first insects to colonise dead matter, and as such are extremely important to forensic entomologists, who can calculate time of death from the degree of infestation by Greenbottle larvae. Larval secretions have also been found to be effective against antibiotic-resistant bacteria, including MRSA.

While Greenbottles are vectors of human disease (due to their habit of landing on contaminated or rotting food and then transmitting the bacteria elsewhere), they are so much more – detritivores, pollinators, ‘smoking guns’ in murder cases, and possible sources of medical treatment. I don’t begrudge them a few mouthfuls of Waitrose yoghurt one bit.