prothorax, the latter possibly closed up and useless. This restriction of the breathing-holes to a front and hind pair (amphipneustic condition) or to a hind pair only (metapneustic type) is highly characteristic of the larvae of Two-winged flies. [8] Known as the Orthorrhapha and the Cyclorrhapha; these terms are derived from the manner in which the larval or pupal cuticle splits, as will be explained in the next chapter (p. 88). [Illustration: Fig. 20. Crane-fly (_Tipula oleracea_), _a_, female; _b_, larva ('leather-jacket' grub). Magnified twice.] [Illustration: Fig. 21. Maggot of House-fly (_Musca domestica_), _a_, side-view, magnified 5 times; _b_, prothoracic spiracle; _c_, feeler; _d_, hind-region with posterior spiracles; _e_, _f_, head-region with mouth-hooks; _g_, head-region of young maggot; _h_, eggs. All magnified. After Howard, _Entom. Bull._ 4, _U.S. Dept. Agric._] Turning now to the _maggot_, characteristic of the House-fly section (fig. 21) of the Diptera, we see the greatest contrast between the larva and the imago that can be found throughout the whole class of the insects. The Bluebottle's eggs, the well-known 'fly blow' laid in summer time on exposed meat, not unnaturally arouse feelings of disgust, yet they are the prelude to one of the most marvellous of all insect life-stories. The fly--with its large globular head, bearing the extensive compound eyes, the highly modified feelers with their exquisitely feathered slender sensory tips, and the complex suctorial jaws; with its compact thorax bearing the glassy fore-wings alone used for flight, though the hind-wings modified into tiny drumstick-like 'halters' are the organs of a fine equilibrating sense--is perhaps the most specialised, structurally the 'highest' of all insects. Yet in a week or two this swift, alert, winged creature is developed from the degraded maggot, white, legless, headless, that buries itself in putrid flesh, 'feeding on corruption.'