616 
INSECTA. 
following families they are only composed of two or three joints, the last of which is generally 
fusiform or lenticular, with a small styliform appendage, or hair, either simple or bearded. 
li mouth is only fit for extracting and drawing 
forth fluid matters, and when these are inclosed 
n proper vessels, with an envelope easily pierced, 
\4^ j >0 ^ the pieces of the sucker act as lancets, piercing 
I envelope, and forming a passage for the 
\ liquid, which ascends by the pressure of these 
lancets together, to the pharynx, situated at the 
base of the sucker, the sheath of which serves 
Fiyr- 131— A, iioadofTabaiius; B, hcadofMusea. ouly US a dcfencc to tliese laucets, aud is gene- 
rally folded upon itself in their action. This sheath appears to represent the lower lip of mas- 
ticatory insects, and the setae, at least in those with the most complicated mouth, represent 
the other parts, such as the labrum, mandibles, and maxillae. The clypeus, or epistome as I call 
it, is represented by the basal part of the proboscis preceding the sucker and palpi ; the base 
of the proboscis mostly bears two filiform or clavate palpi, composed in some of five joints, 
but in most of only two. The wings are simply veined, and generally horizontal. As in the 
Hymenoptera, their veins furnish good secondary characters of groups. 
The use of the balancers is not known ; the insect moves them with great rapidity. Many 
species, especially those of the terminal families, have above the balancers two membranous 
pieces, like the two valves of a shell, attached together at one side, and which are termed 
alulets. One of these pieces is united to the wing, and partakes of its movements, at which 
time the two valves are upon the same plane. The size of these winglets is in inverse, propor- 
tion to that of the halteres ; the prothorax is always very short, and often its lateral portions 
prominent, like tubercles. The mesothorax alone occupies the greatest part of the thorax; 
in front of which, on each side, and behind the prothorax, are tw^o spiracles, and two others 
are observed near the base of the balancers. As in the Hymenoptera, those of the meso- 
thorax are hidden or obliterated. 
The abdomen is attached to the thorax only by a portion of its transverse diameter ; it con- 
sists of from five to nine segments, and is generally terminated by a point in the females : in 
those whieh have it composed of the smallest number of joints the terminal ones often form 
a kind of ovipositor, composed of tubular pieces, entering into each other like those of a 
telescope. The male sexual organs are external in many species, and curved beneath the 
abdomen. The legs, which are long and narrow in the majority, are terminated by a 5-jointed 
tarsus with two ungues, and often with two or three vesicular pulvilli. Many of these insects 
do us much damage, either in sucking our owm blood or that of our domestic animals, by 
depositing their eggs upon their bodies, so that their larvae may there obtain nourishment ; 
or by infecting our viands and cereal plants with the same intention. Others, in return, are 
useful, by devouring obnoxious insects, consuming dead carcases, or other decaying animal 
matter, which w'ould otherwise render the air w^e breathe impure, as well as by hastening the 
decomposition of putrid w^ater. 
The duration of the life of dipterous insects arrived at the final state is very short. They 
all undergo a complete metamorphosis, but modified in two material ways. The larvae of 
many change their skin in order to undergo their transformation to pupae, and some spin a 
cocoon ; but the others do not moult ; their skin hardens, contracts, and generally shortens, 
becoming a strong cocoon, of an egg-like form, for the inclosed pupa. The body of the larva 
is detached, leaving its own proper organs attached to the skin within, such as the parts of the 
mouth, &c. : shortly afterwards the inclosed insect assumes the form of a soft and gelatinous 
mass, without any of the parts of the future insect being visible ; some days afterw'ards, how^- 
