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

Posterior spiracles are lacking in the hemipneustic system which is represented by
Sciaridae and most Mycetophilidae.
The amphipneustic system has only the anterior and posterior pairs of spiracles. It
is the commonest system in Diptera and typical of Anisopodidae, Psychodidae,
Thaumaleidae, Trichoceridae (and the non-British Axymiidae and Tanyderidae) in the
Nematocera and most other Diptera.
In the propneustic condition only anterior spiracles are present. This respiratory
system occurs only in some Mycetophilidae (Diadocidia species and some Sciophilinae).
The metapneustic system in which only the posterior spiracles remain is found in
some larvae living in aquatic or semi-aquatic conditions in the families Culicidae,
Dixidae, Ptychopteridae, and Tipulidae in the Nematocera, and the Tabanidae in the
Brachycera. The Tabanidae do have anterior spiracles but they are extruded just before
pupation and it is not known if they are functional.
When spiracles are completely absent (although tracheae are always welldeveloped)
the condition is said to be apneustic. Such larvae are aquatic (except
Keroplatinae, Mycetophilidae) and include the Ceratopogonidae, some Chaoboridae
(Chaoborus}, most Chironomidae, Simuliidae (and the non-British Blephariceridae,
Deuterophlebiidae and Nymphomyiidae) in the Nematocera; Athericidae and
Empididae in the Brachycera; and some representatives of other families.
The respiratory system may differ between instars, e.g. Mycetophilidae and
Sciaridae are metapneustic in the first ins tar, propneustic in the second and third ins tars
and hemipneustic in the fourth (final) ins tar. In Cyclorrhapha the first ins tar larvae are
metapneustic and the second and third (final) instar are usually amphipneustic.
Spiracles vary in structure and Keilin (1944) recognised three types (in final stage
larvae). In the first type the spiracular opening through which the trachea of the
previous instar is withdrawn becomes the spiracular opening of the following instar
(e.g. Culicidae, Tabanidae). In the second type the spiracular opening is closed and in
the following instar forms a scar around which perforated oval or linear air intakes
open (e.g. most Nematocera and Brachycera). In type three the spiracular opening of
the second instar forms a scar near the inner margin of the spiracular plate and the
number of air intakes opening around the scar is usually reduced (e.g. Cyclorrhapha).
A very useful 'spot character' of the larvae of most Cyclorrhapha is that in the three
instars each posterior spiracle has one, two or three respiratory slits respectively (figs
786, 790). The spiracular plate upon which the posterior spiracles are situated has a
margin or peritreme. The arrangement and shape of the spiracular slits and the state of
the peritreme (complete or incomplete) provide valuable diagnostic characters in the
Cyclorrhapha.
In the majority of the Nematocera the posterior spiracles are usually sessile but in
the Brachycera and Cyclorrhapha they are raised above the body surface on short
processes. Larvae living in liquid or semi-liquid media may have the spiracles each
situated at the end of two short processes (Scatopsidae) or together at the end of a
relatively short respiratory siphon (e.g. Psychodidae, Culicidae, Stratiomyidae,
Tabanidae and Ephydridae) or at the end of a long and sometimes retractable siphon
(e.g. Ptychopteridae; Eristalis and some other Syrphidae; Aulacigastridae).
Some Stratiomyidae exhibit another adaptation to an aquatic existence: they have
the posterior spiracles enclosed in a small chamber (atrium}, the opposing edges of
which are fringed with hairs which enclose a bubble of air when the larva submerges
(fig. 135). The same effect can be achieved by the anal lobes especially when they are
fringed with hairs (e.g. Tipulidae (fig. 27) and Dolichopodidae (figs 208- 209). The
spiracular pit in Sarcophaga (fig. 781) may have a similar function.
The posterior spiracles of Chrysogaster (Syrphidae, fig . 258) and Notiphila
(Ephydridae, figs 615-616) are modified into a respiratory spine used to pierce and
draw air from tissues of aquatic plants. Structurally similar spines are found in some
Tabanidae but are not situated at the point of air intake, and spiracular spines of
28