II 



FLIES WITH AQUATIC LARVAE 



169 



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doubt serve for the admission of air to the trachea ; 

 their minute size must constitute an effective pro- 

 vision against the entrance of water or dirt. Along 

 the back of the pupa are a number of transverse rows 

 of minute spines. Almost the entire surface of the 

 thorax is roughened in the same way. Five of the 

 abdominal segments bear paired prominences on the 

 ventral side. By means of these 

 spines and fleshy processes, the pupa 

 can travel through the mud, and come 

 to the surface, when the fly is ready 

 to emerge. A pupa, when laid upon 

 damp mud, readily moves about, and 

 in no long time establishes itself in 

 a convenient position just below the 

 surface. Many other pupae of the 

 most diverse sorts, which bury them- 

 selves in earth or wood, have some 

 provision of the same kind. The 

 pupa of the Daddy-long-legs is armed 

 like that of Dicranota. Every col- 

 lector of Moths will recollect the spiny 

 rings of the pupae of the Clear-wings, 

 the Goat-moth, the Wood Leopard, 

 and the Swifts. The roughening of 

 the abdomen in such cases gives it a 

 sufficient hold on surrounding objects 

 to enable the Insect to creep to the 

 surface before the final change takes 

 place, thus avoiding the damage to gauzy or plumed 

 wings, which would inevitably result if the moth or 

 fly emerged in a narrow gallery, or below the surface 



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FIG. 55. Pupa of 

 Dicranota, ventral 

 surface. 



