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Show INSECTA. 224 . . d to the wing and partt• alulre. One of t h ese pt· eces is umteh two parts are nearly 1· n · · but then t e · Cipates in all1ts motiOns, 1 }en is in an inverse ratio Th · of these a u ""' the same plane. e s1ze th x is always very short The pro ora . to that of the halteres. d' •rer its lateral portions. mere I y 1SCO T p h and frequently we can . . ertain Culices, and syc o- In some, such as the Scenopd1m,bc culous The greater part . nt an tu er . B dre they are promtne . d of the mesothorax. e- ' h xIS compose . of the trunk or t ora h' d the prothorax are two stigmata; fore on each side, or be tnd the origin of the hal teres; ' b obser-ve near two others may e . the Hymenoptera, are con-· those of the mesothorax, as m cealed or obliterat_ed. uentl attached to the thorax by a The abdomen 18 freq 1Y d·ameter. It is composed of . 1 f its transversa 1 . . portiOn on Y o 111. and usually termtnates m a fi t · ne apparent annt , u1· . from ve o nt . h where the number of ann 118 point in the females; tnt 1 os; m a sort of ovipositor present-less, the last ?nes fr~q~e~~~e;:liding into each other like the ing a success1on of htt e h al organs of the males are ex· . 1 s T e sexu . j01nts of a spy-g as : d b nt under the abdomen. Their . . y spectes an e f tertor In man 1 'd legs are terminated by a tarsus o II 1 g and s en er c. usua Y on f h'ch has two hooks, and very 01ten · · t the last o w 1 five JOln s, . r or membranous pellets. two or three vesiCu~a . db M Leon~Dufourwere provided 11 h D'ptera dtssecte Y · ~ . . A t e 1 d h cter according to him, common 1. lan s a c ara ' W1th sa tvary g . h' d "th a sucker· their structure, how· to all Insects furrus e Wt (l)' . d'ng to the genus · ever, varies accor I • both by sucking our f h Insects are noxious, . Many o t esef domestic animals, by depositing their blood and that o ou_r d th t their larvre. may feed on h · body tn or er a · eggs on t etr . ed meats and cerealia. d b . nfecttng our preserv . them, an y I • ful to us by devouring noXIous Others in return are. h1gdhlydu~e d' and animal substances left Insects, and consuming ea o tes . . , i obos ue des ChevaUJt" AwL (1) See his" Recherches A.natonuques sw 1 H pp q des Sc. Nat., VI, 301. DIPT.EUA. 225 on the surface of the earth that poison the air we breathe, and by accelerating the dissipation of stagnant and putrid water. The term of life assigned to the perfect Aptera is very short. They alJ undergo a perfect metamorphosis, modified in two principal ways. The larvre of several change their skin to become nymphs. Some even spin a cocoon, but others never change their tegument, which becomes sufficiently solid to form a case for the nymph, resembling a seed or an egg. The body of the larva is first detached from it leaving on its internal parietes the external organs peculiar to it, such as the hooks of the mouth, &c. It soon assumes the form of a soft or gelatinous mass, on which none of the parts that characterize the perfect Insect can be seen. After the lapse of a few days, those organs become· defined and the Insect is a true nymph. It extricates itself from confinement by separating the anterior extremity of its case which comes off like .a cap. The Iarvre of the Diptera are destitute of feet, though appendages that resemble them are observable in some. This order of Insects is the only one in which we find ]arvre with a soft and variable head. This character is almost exclusively peculiar to the larvre of those which are metamorphosed under their skin. Their mouth is usually furnished with two hooks that enable them to stir up alimentary substances. The principal orifices of respiration, in most of the larvre. of the same order, are situated at the posterior extremity of their body. Several of them, besides, present two stigmata on the first ring, that which immediately follows the head or re-· places it. ' Messrs Fallen, Meigen, Wiedemann, and Macquart have la~ely rendered great service to this part of entomological ~Cience, l>oth by establishing various new genera, by describmg ~vast number of new species, and by rectifying errors relative to several of those previously known. They have also employed the characters presented by the disposition of the nervures of the wings, to which I first resorted, with a corresponding nomenclature in my Genera. M. Macquart, in par- VoL. IV.-2 D |