Dark, black and buzzing around where you don't want them to. Until the light catches them just right and the realisation hits that they aren't plain at all. A green, almost metallic shimmer radiates from the body. A deep red from its eyes. As difficult as it is to see them in detail, if they land on a plant or flower, very quick but small movements make them almost jitter across the leaf.
What you can just about see with the eye is the green bottle, Lucilia sericata, working the plant with its proboscis. Stopping to sample. Taste and assess the chemical signal as they hunt for food. Nectar. Honeydew. Maybe even just moisture and some microorganisms. Or maybe they suck up the mass left from their extraoral digestion.
The green bottle tastes with its tarsi as much as with its proboscis, reading the chemical language of the plant. Sugars, amino acids, the markers of something worth consuming. It has existed like this for millions of years, refining that sensory precision, that ability to detect and assess in seconds what will sustain it. The green bottle feeds on carrion and decay. Its larvae consume rotting flesh, breaking it down.
Then, for a long second, it stops moving altogether. Whatever it has found is being read in detail, by senses we do not have. A small shift, a smaller resettle, and the inspection continues.
Welcome to the vomitorium
The fly does not chew. It cannot. Its mouthparts are designed to drink, and what it cannot drink it has to first dissolve.
It vomits. A drop of digestive enzyme is laid down on the surface of whatever it is feeding from — a leaf, a fruit, a piece of carrion — and the surface begins to break down externally. After a few seconds the fly drinks the resulting liquid back up.
There is a quiet historical irony in this. The popular image of a Roman vomitorium — a room into which the wealthy retreated to vomit between courses — is, in fact, wrong. A vomitorium was a passageway in an amphitheatre, the place through which the crowd was spewed out at the end of a show. The Romans did not do what we have spent two thousand years accusing them of. The green bottle does it constantly. The creature is more accurate to the myth than the people the myth was about.
The work it does
The maggots of Lucilia sericata are used in NHS hospitals today for maggot debridement therapy. The larvae are bred under sterile conditions, applied to a wound inside a small mesh dressing, and left for two or three days.
They will not touch healthy flesh.
The chemistry is wrong, and the same senses that let an adult fly tell sugar from amino acid at the lip of a flower let a larva tell living tissue from necrotic. They clean a wound at a level of detail no surgeon can match. They lay down antibacterial secretions while they work.
The angler kneeling at a tackle box on the bank is using the same insect, sold as gentles or as casters by the pint. A man may fish with the worm that hath eat of a king, Hamlet says quietly to his uncle, and the king and the maggot and the trout all run through one quiet kitchen.
What it asks of you
The adult lives two to four weeks. Every conscious second of it is spent finding the next thing to break down, the next surface to read with the tarsi, the next clutch of eggs on the next piece of carrion.
See the green in the body, and the red in the eye. The Romans were not who we said they were, the fly is not what we said it was, and the wound on a stranger's leg in a hospital across town is, this afternoon, being made clean by a creature we have just spent an essay refusing to admit is beautiful.
