222 INSECTA. 
ORDER XII. 
DIPTERA(1). 
The distinguishing characters of dipterous Insects consist 
in six feet ; two membranous, extended wings, with, almost 
always, two movable bodies above them called halteres(2)3 a 
sucker composed of squamous, setaceous pieces, varying in 
number from two to six, and either enclosed in the superior 
groove of a probosciform sheath terminated by two lips, or 
covered by one or two inarticulated lamine which form a 
sheath for it(3). 
Their body, like that of other Hexapoda, is composed of 
three principal parts. The number of ocelli, when any are 
present, is always three. The antenne are usually inserted 
(1) Anthiata, Fab. 
(2) In order to be conyinced that these organs do not represent the second 
wings, we must compare the thorax of a large Tipula with that of some Hymenop- 
terous Insect, and particularly of a female Cryptocerus, where the posterior stig- 
mata are very-apparent. Here, as in all the Hymenoptera, the segment bearing 
the second pair of wings, is but very slightly developed or incomplete, and merely 
follows a small, very narrow, transverse, linear, and extremely short piece imme- 
diately under the scutellum. Next follows the metathorax, which forms that semi- 
segment which in my Memoir on the articulated appendages of Insects I have 
called mediate. On each side of it isa spine with two stigmata, more exterior 
than the spines, and situated at but a little distance from them. The thorax of 
these Tipule exhibits the same disposition, except that the semi-segment, which in 
the Hymenoptera gives insertion to the second wings, is here somewhat less dis- 
tinct, and that no trace of wings can be perceived at either of the ends. The 
halteres (balanciers) occupy the precise situation of the spines, and the stigmata 
in like manner ‘are exterior. It is evident then, that this posterior extremity of 
the thorax bearing the halteres corresponds to the mediate segment, that in which 
the musical organs of the male Cicadz are placed, and which in several Acrydia 
of the same sex presents analogous peculiarities. 
(3) This proboscis is elongated, in several species of the same family, in the 
manner of a long siphon. 
