616 



INSECTA. 



m^^. 



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. 



The mouth is only fit for extracting and drawing 

 forth fluid matters, and when these are im-losed 

 in proper vessels, with an envelope easily pierced, 

 the pieces of the sucker act as lancets, piercing 

 this 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 

 Fig. 131— a, head of Tabanus ; b, head of Musca. only as a defence to these lancets, and is gene- 



rally folded upon itself in their action. This sheath appears to represent the lower lip of mas- 

 ticatory insects, and the sets, 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 

 Ilymenoptera, 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 

 are alone visible. In some species of Scenopinus, Culicidce, and Psychoda, they are very 

 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 two 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 k 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 which 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 own blood or that of our domestic animals, by 

 depositing their eggs upon their bodies, so that their larva; 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 would otherwise render the air we breathe impure, as well as by hastening the 

 decomposition of putrid water. 



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 afterwards, how- 



