[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.’

The broad end of the maggot is the tail, while the narrow extremity marks the position of the mouth.  Above this are a pair of very short feelers (fig. 21 c), while from the aperture project the tips of the mouth-hooks (fig. 21 e, f), formidable, black, claw-like structures, articulated to the strong pharyngeal sclerites and moved by powerful muscles, tearing up the fibres of the flesh.  On either side of the prothorax is an anterior spiracle, a curious branching or fan-like outgrowth (fig. 21 b), with a variable number of tiny openings which are probably of little use for the admission of air to the tubes.  In many maggots the mouth-hooks and the front spiracles become more and more complex in form in the successive instars.  The cuticle, white and smooth to the unaided eye, is seen on microscopic study to be set with rows of tiny spines which assist the maggot’s movements through its food-mass.  At the tail-end the large hind spiracles are conspicuous on a flattened dorsal area of the ninth abdominal segment; each shows a hard brown plate, traversed by three slits.  And as we watch this curious degraded larva thrusting its narrow head-end into the depths of its ofttimes loathsome food-supply, we understand the advantage of access to the air-tube system being mainly confined to the hinder end of the body.