INSECTS
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 correlated with the manner of breathing. In most insects, as we have learned (Fig. 70), there is a row of breathing pores, or spiracles, along each side of the body, which open into
Fig. 172. Rat-tailed maggots, larvae of the drone fly, which live submerged in water or mud and breathe at the surface through a long, tail-like respiratory tube
Upper figure, resting beneath a small floating object; lower, feeding in mud at the bottom
lateral tracheal trunks. In the fly larva, however, these spiracles are closed and are not opened for respiration until the final change of the pupa to the adult.
The fly larva is provided with one or two pairs of special breathing organs situated at the ends of the body. Some species have a pair of these organs at each end of the body (Fig. 171, ASp, PSp), and some a pair at the posterior end only. The anterior organs, when present (Fig. 171, Asp), consist of perforated lobes on the first body segment, the pores of which communicate with the anterior 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 fly larva can live submerged in water,
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