DIPTERA. 4.99 
The thorax in these insects is short and robust, the distinctness 
of the various portions being much diminished, and the size of 
many of them reduced, in pursuance of the rules of proportionate 
development, resulting from the existence of only one pair of wings; 
hence the prothorax is very minute, the collar only being slightly 
visible, whilst the mesothorax extends nearly over the whole tho- 
racic region: at the sides of this segment a pair of naked membran- 
ous wings * are placed, having, in many species, attached to their base 
behind a pair of small membranous appendages, termed alule, or 
winglets, which vary in size in an inverse proportion to the size of 
the halteres, and which have been regarded by some entomologists as 
the analogues of the lower pair of wings in the other orders ; at the 
base of the halteres, as well as behind the collar, there exists a pair 
of spiracles. (See fig. 126. 1., and 128. 8.) 
The halteres +, as Dalman well observes (Analect. Ent. sub Chionea), 
are the most characteristic organs of the order, being present even 
when the wings themselves are wanting. ‘They are generally kept 
in constant vibration; respecting their uses{, however, as well as 
their analogies, dependent upon the analogous formation of the pos- 
terior portion of the thorax, entomologists are at variance ; some au- 
thors regarding them as representatives of the posterior pair of wings, 
whilst others, including Audouin and Latreille, deny them this cha- 
* Whilst many entomologists have endeavoured with very great pains to reduce 
the variable position of the nerves of the wings of the Hymenoptera to one typical 
form, the typical neuration of the wings of the Diptera has been but little in- 
vestigated. Latreille has partially attempted its elucidation ( Genera, §c. tom. iv. 
p- 237.); and see MacLeay, in Zool. Journ. Nos. 2 and 16. (in paper on Ceratites), 
An observation worthy of consideration in respect to this subject, has been made 
by M. Maecquart (Hist. Nat. Ins. Dipt. tom. i. p.11.), that if we compare the 
wing of one of these insects with the fore wing of one of the Hymenoptera, 
we cannot trace any analogy between the neuration of the two; but if both wings 
of the latter are thus compared with the single wing of the Dipterous insect, we 
are easily able to distinguish in the latter the various cells of the former ; and 
hence, that the single wing of the Diptera represents both wings of the Hymen- 
optera, and that the halteres consequently cannot represent the hind wings. 
t+ See Robineau Desvoidy, on these organs, in Ferussac’s Bulletin for May 1827. 
¢ From being connected with the metathoracie spiracle, they have been regarded 
as appendages of the respiratory system. See Schelyer (in Wiedemann’s Zool. Ar- 
chiv. ), Burmeister (in Poggendorff’s Annalen, translated inTaylor’s Scientific Memoirs, 
vol. i. pt. 3., and in Silbermann, Rev. Entomol. No. 4.), on the noise made by insects 
in flight ; and see Kirby and Spence (Introd. vol. ii. p. $60.), on the variations in 
the alary organs and flight of the Diptera. 
Keke OD 
