276 MESSES. MIALL AND HAMMOND ON THE DEVELOPMENT 



for example, are not yet formed) they are substantially those of the imago, the muscles 

 and other internal tissues being already differentiated, and in some cases highly 

 specialized. With the exception of the prothoracic respiratory appendages and the tail- 

 fin, there is little in the pupa of Chironomus which does not relate to the next stage. 



The ancestral history of the pupa is so completely disguised by adaptive peculiarities, 

 that in this insect it is reduced to little more than a transitional form, effecting the 

 difficult passage from a wholly aquatic to a wholly aerial mode of existence. At the 

 same time there is no reason to doubt the morphological equivalence of the Tipularian 

 and other insect pupa?. All of them represent a stage comprised between two moults, 

 which has become subordinated in various degrees to the succeeding imaginal stage. 

 Their morphological correspondence is as well marked and as interesting as their 

 adaptive differences. 



VIII. Conclusion. 



The most striking feature of the development of the fly of Chironomus is the formation 

 of paired invaginations extending far into the thorax, and giving rise to a great part of 

 the imaginal head. Chironomus furnishes, not the most complex, but the most intel- 

 ligible case of equally extensive invaginations hitherto described. When we inquire, as 

 we cannot help doing, why such invaginations exist at all, the obvious facts 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 12 (male fly) to 11 (larva) ; the breadths as 5 

 (male fly) to 3 (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 : Why should any such disproportion exist between 

 the head of the fly and that of the larva ? We may say in reply that the fly is a nimble 

 aerial insect, requiring keen senses and some degree of intelligence that it may escape 

 clanger, find a mate, and lay its eggs in a suitable position. The larva, on the contrary, 

 is an animal of very simple mode of life, feeding upon dead vegetable matter at the 

 bottom of dark and slow streams. The abundance of its food, and the ease with which 

 it can be appropriated, have led in this, as in many other cases, to some degree of 

 degeneration, which is particularly apparent in the larval limbs and head. 



We should be glad to be in a position to show in what way and to what extent the 

 invaginations of Chironomus lead up to those of the Muscidoe. But this is at present 

 hardly feasible. We look forward to a time when a well connected series of thoroughly 

 investigated Dipterous types can be arranged so as to lead up to and explain the 

 formation of the fly in the Muscidse, and the hope of contributing to such a result has 

 been a principal motive of the present study. 



