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

Biology and morphology of the immature stages
Diptera are holometabolous insects with a complete metamorphosis of four stages:
egg, larva, pupa and adult.
During growth the larvae of most Diptera pass through a series of moults, the period
between the moults being called ins tars. Moults that take place soon after hatching and
just prior to pupation are easily overlooked but those that take place during the active
feeding period are easily observed.
Nematocera usually have the largest number of moults, e.g. four instars in Culicidae
and six or seven in Simuliidae. Among Brachycera, some Tabanidae have up to nine
instars and may take up to two years to complete their development. The maggots of
Cyclorrhapha usually have three instars and most feeding and growing is done in the
third instar.
Some larvae of Cecidomyiidae reproduce by paedogenesis, i.e. reproduction by an
animal that becomes sexually mature before reaching the adult form . In this method of
reproduction a few (up to about 35) large eggs are produced which can be seen through
the larval cuticle. These become larvae and feed on their parent larva from inside (fig.
118). This condition can continue for several generations without the appearance of an
adult. These midges (e.g. Miastor) thus reproduce like aphids and remain in their
birthplace producing a great mass oflarvae which may be found under the bark oflogs.
Eventually the daughter larvae will cease to produce eggs and pupate normally. Both
sexes are produced in this way though of course all offspring of one larva are of the
same sex as the parent larva. These paedogenetic larvae do not develop a sterna! spatula
but it reappears when pupation is imminent which confirms the function of this structure
as a prerequisite to pupation. Pupal paedogenesis is also known in the British
species Henria psalliotae Wyatt, a pest of mushrooms. This reproductive pupa is called
a hemipupa (Wyatt, I. J., 1961).
Some Chironomidae appear to be paedogenetic but actually the adult female lays her
eggs while she is still inside the pupal skin and is thus a pharate adult.
In most Nematocera and Brachycera the fully fed larvae form pupae but in
Cyclorrhapha the last (third) instar larva forms a puparium from the last larval skin
inside which it pupates. Stratiomyidae are exceptional in the Brachycera in not shedding
the last larval skin when they pupate but this appears to be a protective device unrelated
to the cyclorrhaphous puparium. Some Nematocera also pupate inside the last larval
skin, e.g. Mayetiola and Chortomyia (Cecidomyiidae), some Scatopsidae, Penthetria
holosericea Meigen (non-British- Bibionidae) and the non-British Perissomatidae.
Eggs
The eggs of flies are generally oval or spindle-shaped, white or pale yellow and
relatively featureless to the naked eye. Under the microscope many are seen to have a
sculptured surface and some also have horns, stalks or a raised network of ridges. These
surface features are largely respiratory adaptations to prevent the eggs from drowning
after rain and function as a respiratory plastron which holds a thin film of air, as in
many aquatic beetles and bugs (Hinton, 1961 ). The ornamentation of an egg increases
surface tension and may cause it to float, and the eggs of aquatic species may have more
elaborate floatation devices (e.g. mosquitoes, figs 976-978). At the head end of the egg
is the micropyle (fig. I 023), an opening for the admission of sperm.
The largest number of eggs (c. 2,000) is produced by some aquatic flies whose larvae
suffer enormous mortality from predation (e.g. Chironomus plumosus L.). Female
blow-flies (Calliphora) lay about 300 eggs, usually in several small batches. The female
housefly (Musca domestica L.) lays eggs singly in small or large batches at the rate of
I 00-150 per day up to I ,000. At the other extreme, Hippoboscidae and other Pupipara
mature only one egg at a time, nurture it internally and eventually deposit a fully
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