Vol 10 Part 14. An introduction to the immature stages of British Flies ...

chromosomes, are being found. The larvae are aquatic and usually found in running
water including rapids and cascades. The three species of Prosimulium are found in hill
streams. The commonest species of Simulium is S. ornatum Meigen, the larvae of which
are found in all types oflowland water courses and are often abundant in small streams
and flowing ditches. Simulium vernum Macquart and S. cryophilum Knoz are often
abundant in many types of small streams. S. equinum (L.) is common, usually on rooted
vegetation in large streams with a steady current and is frequently accompanied by S.
lineatum (Meigen). S. reptans (L.) and S. tuberosum Lundstrom are common in larger
stony rivers but the latter occurs usually above the limit of cultivation of the
surrounding land and the former below, though there is an area of overlap. Other
species are more specialised in their habitat and consequently less common, e.g. S.
costatum Friederichs (spring-fed streams near source), S. noelleri Friederichs (lake or
pond outlets).
The larvae are filter feeders on vegetable debris and diatoms which they strain from
the water with their cephalic fans (figs 68-69). Occasionally they may ingest small
larvae such as Chironomidae or even early ins tars of their own kind. There are six to
eight larval instars in our species. There is a short pharate (see Introduction) stage
(recognised by the much darker gill-spot) when the prepupa, still inside the last larval
skin, spins its cocoon of silk, attached to a suitable substrate, inside which it pupates.
The pupa has long filamentous gills (fig. 1122).
The eggs are eccentric ovoid in shape (fig. 985) and may be laid singly and loosely or
in batches stuck to the substrate; they are white when laid but darker later. Many
species pass the winter as eggs but some do so as slowly growing larvae.
Most adult female Simuliidae suck blood and their biting of man and animals causes
irritation sometimes followed by toxic and allergic reactions. In tropical Africa and
tropical America they transmit the disease onchocerciasis (river blindness). In Britain
they are known to transmit the nematode Onchocerca gutturosa to cattle (without
serious results) and certain Protozoa (e.g. Leucocytozoon) among birds. This medical
and veterinary importance has led to a considerable amount of work on their immature
stages for the purpose of survey and control work throughout the world. The Simulium
fauna of an area is best established by searching for their larvae and pupae in freshwater
systems from small trickles to large rivers. Thorough surveys such as that ofCrosskey
(1985) in the London area would be worthwhile projects and add much to our
knowledge of the ecology and distribution of the family.
L. Davies ( 1968) keys the British species in the larval, pupal and adult stages, but the
keys to early stages are difficult to use and are currently under revision by the
Freshwater Biological Association. Ladle & Bass (1975) have described the larva of a
new Metacnephia. The Diptera Check List in the Handbook series should be consulted
for nomenclatorial changes.
Anisopodidae
(Figs: larvae 70-73, egg 986, pupa 1123)
The family Anisopodidae contains about 100 species in six genera of which 5 species
in 2 genera occur in Britain. Two subfamilies are recognised, the larvae of which may be
separated as follows.
Anal segment rather truncate posteriorly; posterior spiracular disc larger and with distinct
lobes (fig. 72); lines drawn through the axes of the diagonal posterior spiracles would
intersect on disc . . . . . . . . . . . . . . . . . . Anisopodinae
One genus, Sylvicola ( = Anisopus)
Anal segment elongated and more rounded posteriorly; posterior spiracular disc small,
without distinct lobes; lines drawn through axes of posterior spiracles would not intersect
on disc (fig. 73) . . . . . . . . . . . . . . . . Mycetobiinae
One genus and species, Mycetobia pallipes Meigen
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