II FLIES WITH AQUATIC LARVAE 139 



The head is apparently protruded by blood-pres- 

 sure, set up by the contraction of other parts of the 

 body. When the head, still covered by the pupa- 

 skin, is once protruded, matters go on more slowly. 

 The old cuticle of the rest of the body is worked off 

 bit by bit, and it is not until some hours have passed 

 that the pupa gets completely rid of it. 



It is interesting to the biologist to find various de- 

 grees of complication in the method of development 

 of new parts by infolding. In some Dipterous and 

 other larvae the folds which give rise to the imaginal 

 head are shallow, and the new parts are in close con- 

 tact with the corresponding organs of the larva, where 

 such exist. In the Muscidae, i.e. the family to which 

 the House-fly and Blow-fly belong, the formation of 

 the imaginal parts is incredibly complicated, though 

 the process is in principle the same as in Chironomus. 

 The degree of correspondence between the larval and 

 imaginal organs is one circumstance which affects the 

 extent of the infoldings. In Muscidae the fly differs 

 in almost every particular of external and internal 

 structure from the footless and almost headless larva, 

 and here the folds attain their maximum both of 

 number and complexity. 



Nearly allied species of Insects, though closely 

 similar in the winged state, may differ considerably 

 as larvae or pupae. We may take an example of this 

 from Chironomus. In this genus there are two types 

 of larvae, each of which is succeeded by a special kind 

 of pupa. One group, which includes most of the 

 larger species, such as Chironomus plumosus, has two 

 pairs of respiratory tubules on the eleventh segment 



