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

Orthorrhapha-Brachycera which are mostly predaceous. In these Brachycera the
pharynx is sclerotized and in some families (e.g. Dolichopodidae, Empididae, Stratiomyidae)
may also be fused with the internal skeleton of the head. In the Cyclorrhapha
the head consists of an outer membranous segment and the internal cephalopharyngeal
skeleton, a characteristic feature of the familiar maggot type of larva. The cephalopharyngeal
skeleton is normally divisible into three main parts: the basal or pharyngeal
sclerite (ps); the intermediate or hypopharyngeal sclerite (hs) (tentoropharyngeal) and
the mouth hooks or mandibles (m) (fig. 788). The pharyngeal sclerite consists of a pair
of roughly U-shaped sclerites on either side of the pharynx. The two arms of each of
these sclerites are called the dorsal (de) and ventral cornua (vc) (singular cornu) or
'wings' and the latter are fused to the pharynx on each side. The pharyngeal sclerites
may be joined by a bridge anterodorsally and a pair of slender parastomal bars may
project from the anterior margin above the hypostomal sclerite (fig. 788, p). Below the
basal part of the mandible there may be a dental sclerite (ds) and beneath (Muscidae,
fig. 911, ob, ar) or between (Calliphoridae, fig. 788) the apices of the mandibles other
accessory oral sclerites (os) may be present. Papers by Roberts (1969- 197la) should be
consulted for a detailed discussion of the mouthparts of larval Diptera in relation to
feeding habits. A knowledge of the functional morphology of mouth parts may provide
important clues as to whether or not an unidentified larva is the cause of primary
damage to its host.
The antennae (fig. 2) are usually close to the anterior mandibular articulations near
the anterodorsal corners of the genae (cheeks). They may be very small (e.g.
Anisopodidae, Bibionidae, Ptychopteridae, some Psychodidae, many Mycetophilidae)
and are normally subdivided into three divisions (though in some Chironomidae seven
may be evident). In Chaoboridae the antennae have evolved into prehensile structures
with apical spines used for capturing prey (fig. 45, an).
The eyes are also located on the genae. In the Nematocera and orthorrhaphous
Brachycera the eyes are usually simple (double in many Chironomidae) but in some
Culicidae and Chaoboridae a compound eye is present, in front of the simple eye
(ocellus). Little work has been done on the eyes oflarval Diptera but Roberts (1970b) is
useful. An egg-burster or hatching spine may be present on the head of the first instar
larva of some Diptera (fig. 984) (see Edwards, F. W., 1919; Madwar, 1934; Hinton,
1981 ). Egg-bursters in Diptera require further investigation (see Smith, 1955a).
Body
This is very variable in shape. Most Nematocera and some Brachycera are slender
and subcylindrical; Therevidae, Scenopinidae and some Ephydridae and Canacidae
are spindle-shaped (fusiform); Cyclorrhapha and Xylophagidae are markedly
narrowed anteriorly; Fanniidae, Lonchopteridae, Platypezidae, Stratiomyidae and
Xylomyidae are dorsoventrally flattened; Chaoboridae and Culicidae have a swollen
thorax; Simuliidae are swollen posteriorly while some Syrphidae and parasitic groups
are generally stout; Conopidae are pear shaped and Microdon (Syrphidae) species are
hemispherical.
Nematocera usually have 12 body segments, other Diptera 11. Three of these
segments comprise the thorax and the remainder the abdomen. Variation in the
numbers of abdominal segments nearly always involves a decrease in Nematocera and
an increase in other Diptera.
In the Chaoboridae, Culicidae and Simuliidae the three thoracic segments are fused.
In the Anisopodidae and Psychodidae subdivision of the segments, or pseudosegmentation,
occurs (figs 40, 70, 71). The Therevidae and Scenopinidae have 20 segmental
divisions.
The cuticle ofDiptera larvae is usually only weakly sclerotized and non-pigmented, a
condition probably explained by their mainly concealed mode of life in a humid terrestrial
atmosphere or by the adoption of an aquatic or semi-aquatic environment. Larvae
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