July 2020 BBC Wildlife 23WILD JULY
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.Next generation
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 egg-
laying, 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 pre-
pupa, which will wiggle off
into the nest to pupate almost
immediately, ready to sit out
a coldandswiftlesswinter.
Iustrat
ons
by
Peter
Dav
dScott/The
Art
Agency
NICK BAKER
is a naturalist, author and TV presenter.Revealsafascinating
worldofwildlifethat
weo enoverlook.onlysavinggraceis thatthey
don’tmuchlikehumanblood.
Lousefliesaresmall– even
withlegsspread,theyjust
aboutcovera littlefingernail.
Lookclosely,however,and
you’llnoticea suiteoffeatures
designedforhangingon to
thefastestbirdsinlevelflight.
Theyareincrediblyflat,as
if someonehassquished a
regularflybetweenfinger
andthumb.Eachoftheir leg
sectionsis alsoflattened,
perfectforslidingdeep
betweenfeathers.
Whentheflies get
closetothewarmth
andblood-rich skin
oftheirunwitting
host,theysimply
hangon,evenduring
rigorouspreening. The
tipsoftheirfeetbristle 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
S
wiftsprovidetheclassic
sightandsoundof
summer.Swiftbyname
andalsobynature,
theyhitabout115kphinlevel
flight.Butsparea thoughtfor
theirpassengers.Eachbird
canbecarryingseveralstrange
insects,whicharearguably
evenmorespectacular.
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
NICK
BAKER
N
Hidden
MILKING IT
Female louse flies nurse their young internally.
Fly milk is similar to mammalian
milk in many ways. It is a creamy
concoction of fats, proteins
and symbiotic bacteria.
That last ingredient
is particularly
important in
the gut of
parasitic flies.
This is because
the bacteria
can synthesiseessential vitamins and other
nutrients not found freely in
the blood of the host.
Once the maggot
has completed its
development,
nourished entirely
by the milk, it is
ejected into the
world by its mother.
She is, e ectively,
a fly in labour!The swift louse fly’s legs
each grip with a force of
200mN – equivalent
to 1,000 times
the weight of
the fly.D I D Y O U
KNOW?SWIFT LOUSE FLY
Swift louse flies
make themselves
at home in nests.