II FLIES WITH AQUATIC LARV^ 193 



There are no limbs. The larva moves in the water 

 by vigorous flexion of its body, first to one side and 

 then to the other. It can also creep on earth or 

 mud. Swammerdam observes that, like other Dip- 

 terous larvae, it drags itself along by its mouth when 

 out of the water. Alternate contraction and exten- 

 sion of the segments, aided by the stiff hairs which 

 stand out from the body, are also used as a means of 

 propulsion. 



The wider part of the body lodges the alimentary 

 canal, which is very long and thrown into complicated 

 folds. The salivary or silk glands are tolerably large. 

 A pair of very large air-tubes run along the body. 

 Beginning as narrow tubes in the head, they dilate 

 until they attain half the width of the body, gradu- 

 ally narrowing after this until they terminate in 

 oval spiracles which occupy the centre of the 

 coronet. There are nine pairs of spiracles on the 

 sides of the body, which are not open, but take the 

 form of black, circular plates. Branches from the 

 longitudinal air-tubes pass to them on the inside. 

 The second segment, which will ultimately carry the 

 wings, has no spiracles. In the same way the second 

 and third segments of Insect-larvae which are destined 

 to acquire two pairs of wings bear no spiracles, though 

 closed tracheal tubes running to the surface may 

 often be discovered by dissection in the places where 

 spiracles would be expected. 



Stratiomys changes to a pupa in early summer. 

 The larvse prefer to leave the water at this time, and 

 bury themselves in the earth or in the floating vege- 

 tation of a ditch. Swammerdam says that even in a 



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