134 Development within the Larva 



Compari- 

 son -with 

 otlier 

 Dipterous 

 insects. 



Possible 

 motive of 

 invagina- 

 tions. 



larva changes to a pupa. This is more fully described 

 in the next chapter (p. 138). 



Comparison with allied insects shows that the forma- 

 tion of a new head is not accomplished in the same way 

 in all Diptera. In a gnat the invaginations are shallow, 

 and the comjDOund eyes and antennae form within the larval 

 head, though the base of the new antenna is telescoped 

 into the head, and its shaft becomes folded. In Corethra 

 the same process is carried a little further. Chirononius 

 dorsalis comes next in the series, while in the Muscidae 

 we reach the maximum of complexity. The invaginations 

 are deep, and apparently, though not really, unconnected 

 with the larval epidermis. In the Muscid pupa the epi- 

 dermis, the muscles, the intestinal epithelium, and even 

 a great part of the nervous system are regenerated ; the 

 old tissues are devoured by j^hagocytes, and only nests 

 of cells persist as rudiments from which the new organs 

 are developed. 



Chironomus furnishes a ^particularly accessible and 

 easily understood case of the development of what is 

 practicall}^ a new head within the larva. When we 

 inquire, as we cannot help doing, why the new head 

 should be formed by imaginal folds in the thorax of the 

 larva, the obvious tacts suggest themselves that the head 

 of the fly is utterly unlike the larval head in shape and 

 that it is of larger size. The lengths are as twelve (male 

 fly) to eleven (larva) ; the breadths as five (male fly) to 

 three (larva). As a mere matter of dimensions, such 

 a head as that of the male fly of Chironomus could not 

 be developed within the larval head. This explanation 

 at once provokes a further question : AVhy should any 

 such disproj^ortion exist between the head of the fly and 

 that of the larva ? "VVe may say in reply that the fly is 

 a nimble aerial insect, requiring keen senses and some 

 degree of intelligence that it may escape danger, find 



