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The Corethrellidae are a small family of flies,
both in size and numbers of species, but in spite of this they have
some fascinating habits and represent a very ancient lineage of Diptera.
The family presently includes 64 extant and 4 fossil species and all
are placed in the single genus Corethrella. Species are best
represented in tropical and subtropical regions worldwide but some do
get as far north as southern Canada.
Corethrellidae are close relatives
of the Culicidae (mosquitoes) and Chaoboridae (phantom midges) and were
actually first placed in the Culicidae along with the chaoborids, then
just with the chaoborids and have only recently been recognized as a
separate family (since 1986)."Adults" share
a similar
"Wing venation"
with the mosquitoes and chaoborids but are easily distinguished by the
presence of a short first radial vein (R1). In addition, Corethrella
are small, with a wing length of about 0.6 - 2 mm."Larvae"
are also easily recognized: they have a mosquito-like siphon but the
"Head" is very peculiar, with the bases of antennae close to one
another medially and when at rest, lying laterally against the head
capsule. The larval head capsule also has a row of strong lateral spines.
For many years, specimens of
Corethrella were quite rare in collections and most species were
represented by only 1-3 specimens.
Then, in 1975 Sturgis McKeever made a great
discovery in Georgia, USA: female adults were attracted to calling frogs
and fed on their blood! Furthermore, many females could be collected
using a cassette tape of the frog calls. It is now possible to collect
female adult Corethrella in large numbers (sometime over 500
individuals in 30 minutes!) using a CDC trap (with the bulb removed)
placed in front of a cassette player (CDC traps are basically a cylinder
containing a fan which blows passing insects into a collecting bag).
The immatures of Corethrella live in very small water bodies
and the very margins of larger aquatic habitats. Although never common,
in Costa Rica they are consistently found in such habitats as tree holes,
leaf axils, old bamboo stems and epiphytic plants (especially in tank
bromeliads), in small pools isolated on the margins of streams and sometimes
in the heavily vegetated margins of larger aquatic habitats. The larvae
are predaceous and feed on other small aquatic organisms. Their siphons
have specialized spines which allow the larvae to anchor themselves
amongst detritus or another substrate and they only come to the water
surface to breath on an irregular basis. While they are anchored below
the surface they wait for passing prey and when one comes close by,
a hungry larva will slowly turn its body in the direction of the prey
and stealthily extend itself by about a quarter of its length to come
within reach. At that point the larva will strike its prey with a lightening
quick grab of its antennae and mouthparts and ingest the victim with
the help of its strong mandibles.
Pupae float at the water surface and are quite inactive. A few days are sufficient for the adults to emerge and fly away. As noted above, females are easily collected with a cassette player and I have found that running a nearby UV light trap may attract numbers of males (but only when the cassette player is running). Female adults readily lay eggs (which are beautifully sculptured and float singly on the water surface) and larvae are easily reared on small mosquito larvae or other moving aquatic organisms in small petri dishes. Overcrowding, though, will result in cannibalism.
In Costa Rica, I have identified 20 species of Corethrella (some unnamed and new to science). All of these are restricted to lower altitudes. Generally, adults may be very abundant (100 adults trapped in 1 hour) in lowland habitats and become scarce at 2400 meters. I haven't yet found any species above this elevation.
The relationship between Corethrella species and frogs is likely an ancient one. The most primitive Corethrella species lives in New Zealand, where some of the most primitive frogs in the world also reside. Furthermore, we now know of a fossil Corethrella in Lebanese amber 120-122 million years old which looks very similar to those living today.
Because specimens of Corethella were so difficult to collect for so many years, our knowledge of the group is actually quite poor. However, as mentioned above, it is now possible to easily collect and study the different species. The larvae and pupae are particularly rich in structural variation and this genus would be a fascinating group for someone interested in interpreting their phylogeny, behaviour and biology.
Although no one has yet investigated this aspect of their biology, it is quite possible that species of Corethrella transmit diseases among their frog hosts."Although no one has yet investigated this aspect of their biology, it is quite possible that species of Corethrella transmit diseases among their frog hosts" a "Very recently, McKeever and French have reported that some Corethrella in the eastern United States transmit diseases between male frogs." .Considering the global concern with the health of frog populations, this will be a fruitful line.
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