ment of extension may be repeated. In this fashion the soft, legless larva moves forward; or, by a reversal of the process when occasion demands, it goes backward. A special feature in the construction of fly larvae is the arrangement of their breathing apertures, which is cor- re!ated with the malmer of breathing. In most insects, as we have learned (Fig. 70), there is a row of breath]ng pores, or spiracles, along each side of the body, which open into .3 -- , Fro. ?7 ?E, Rat-tailed maggots, larvae of the drone ?y» whkh lire submerged in water or mud and breathe ?t the surface through a long» tail-like respiratory tube Upper figure, resting beneath a small ?oating obiect; lower, feeding in mud at the ?ottom lateral tracheal trunks. In the tir larva, however, these spiracles are closed and are hot opened for respiration until the final change of the pupa to the adult. The tir larva is provided with one or two pairs of special breathing organs situated at the ends of the bodv. Some species have a pair of these organs at each end of t?e body (Fig. 171 , .J.ç?, P«ç?), and some a pair at the pos- terior end only. The anterior organs, when present (Fig. 171 , «J.ç?), consist of perforated lobes on the first body segment, the pores of which communicate with the an- terior ends of a pair of large dorsal tracheal trunks (DTra). The posterior organs (PSp) consist of a pair of spiracles on the rear end of the body, which open into the posterior ends of the dorsal tracheae. By means of this respiratory arrangement, the tir larva can lire submerged in water, [3261
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