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 cor- 

 related with the manner of breathing. In most insects, as 

 we have learned (Fig. 70), there is a row o{ 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 pos- 

 terior 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 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 fly larva can live submerged in water, 



[326] 



