152 NATURAL HISTORY OF AQUATIC INSECTS CH. 



end gives rise to the intestine. The stomach is 

 formed out of the cells which immediately inclose 

 the yolk. The hooked feet of the prothorax and of 

 the last segment form comparatively late. The 

 elongate larva is eventually coiled up within the 

 narrow egg ; it escapes by the bursting of what we 

 must call the egg-shell, and begins life on its own 

 account. Until the first moult takes place, the larva 

 differs in some respects from older larvae. The four 

 respiratory tubes on the ventral side of the eleventh 

 segment have not yet been developed, and the blood 

 has not yet assumed its red tinge. Moreover the 

 head is larger in proportion to the rest of the body 

 than at a later time, and contains, as in most Insects, 

 the brain, or fore pair of ganglia. Before long the 

 brain is slowly retracted into the prothorax, where it 

 is found during the rest of the larval period. The 

 relatively small head, which grows very slowly, could 

 not apparently furnish the requisite space for a brain 

 in addition to the powerful muscles of the jaws. It is 

 only after pupation that the head, that is, the new- 

 formed imaginal head, once more incloses the brain. 



TANYPUS. 



Another larva, closely related to that of Chironomus, 

 is found in similar situations. This is the larva of 

 Tanypus. It is nearly colourless, but is often tinged 

 with a reddish colour, which, on closer examination, is 

 found to be due to the contents of the alimentary 

 canal. The heads of Chironomus larvae, remaining 



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undigested in the stomach, show the source of 



