BBC Wildlife Magazine
Nick Baker’s hidden Britain
Hitchhiking swift louse flies
Swifts provide the classic sight and sound of summer. Swift by name and also by nature, they hit about 115kph in level flight. But spare a thought for their passengers. Each bird can be carrying several strange insects, which are arguably even more spectacular.
Swift louse flies, Crataerina pallida, have a confusing name – yet they’re most definitely a fly, and a weird one at that. For a start, they can’t – fly, that is. They don’t have operational wings. They are a parasite of common and pallid swifts, and when you have such specialist hosts, you have to be an equally specialist parasite.
Swift louse flies are not something many folks will get to see. But if you’ve been lucky enough to hold a swift, or have peered into a swift nestbox, one might come scuttling out. They’re tough, unsquashable, cling like Velcro and have a fast, furtive sprint.
If a nest is heavily infested, it’s virtually impossible to stop them running up your sleeve. For a day or so, these odd insects will keep appearing – in underpants, scarves, hair. The only saving grace is that they don’t much like human blood.
Louse flies are small – even with legs spread, they just about cover a little fingernail. Look closely, however, and you’ll notice a suite of features designed for hanging on to the fastest birds in level flight. They are incredibly flat, as if someone has squished a regular fly between finger and thumb. Each of their leg sections is also flattened, perfect for sliding deep between feathers.
When the flies get close to the warmth and blood-rich skin of their unwitting host, they simply hang on, even during rigorous preening. The tips of their feet bristle like a Swiss Army knife, including a pair of wicked-looking, blade-like, curved hooks. These are further divided into three ‘teeth’, between which the swift’s feather barbs become clamped. Meanwhile, the foot pads are covered in microscopic, petal-shaped split hairs, and the body and legs have a pelage of black bristles.
There is little a swift can do to free a determined louse fly – a fact that makes sense when you think about it. Detachment is essentially a death sentence for such a specialist.
When swifts return to their nests in May, their body heat stimulates the almost spherical, glossy pupae, lodged in the moth-nibbled fibres of last year’s nest, to hatch. The flies then spend the next two months sipping blood, getting their strength-up for mating. Infestation rates vary with the age of the nest and colony, but nearly all established nests will have between 3 and 20 of these insects scuttling around.
The abdomen of a swift louse fly is a large, expanding bag of blood and babies. You see, these insects don’t go in for egglaying, and there is almost no maggot as such. Instead, they practise ‘adenotrophic viviparity’ – that is, they give birth to one maggot at a time, which is nurtured in a womb-like cavity and fed nutritious milk from a specialised gland. Every week or so, assuming a plentiful supply of bird blood, the female fly pops out a fully grown prepupa, which will wiggle off into the nest to pupate almost immediately, ready to sit out a cold and swiftless winter.