454 DIPTERA CHAP. 



traced back to the embryo ; how many centres for the new 

 growth there may be is uncertain, for though there are upwards 

 of sixty for the outer body, the number of regenerative centres 

 for the internal organs is not ascertained. The peculiar central 

 nervous mass, mentioned in our remarks on the larva, consists of 

 two kinds of tissue mixed together in a complex manner ; one 

 of these kinds is functionally active during the larval life and 

 at the metamorphosis undergoes histolysis, while the other, or 

 embryonic, portion develops into the nervous system of the fly. 



It forms no part of our task to deal with general subjects, 

 but we may be pardoned for calling attention to the bearing the 

 metamorphosis of the higher Diptera has on our ideas of heredity 

 in Insects. The fly bears no resemblance whatever to the larva, 

 and is only obtained by the organic destruction of the latter, 

 which occurs before the perfection of the sexual organs takes 

 place, and yet the fly reproduces itself only secondarily, but 

 primarily gives rise to the totally different larva. It is supposed 

 that the larval structures have been gradually acquired, and yet 

 they are transmitted with the utmost faithfulness by the totally 

 different fly. We can only conclude that that which is bequeathed 

 in each species is the early state of a particular process of develop- 

 ment from which the subsequent stages follow necessarily if the 

 developing organism be placed in conditions having on it in- 

 fluences like to those that influenced the ancestors. 



Classification. The classification of Diptera is as yet very 

 imperfect. Formerly they were divided into two great groups, 

 Nemocera and Brachycera, according to the structure of the 

 antennae, as previously mentioned. This division has been 

 abandoned, and the term Brachycera is now applied to only a 

 small part of the old section that bore the name. The primary 

 division usually adopted at present is into Orthorrhapha and 

 Cyclorrhapha. The characters of these two groups are based 

 on the nature of the metamorphosis, and have been gradually 

 elaborated by Brauer in various memoirs. 1 The Orthorrhapha 

 includes the forms with obtected pupae, the Cyclorrhapha those 

 with a nymph-compound, as previously described. This distinction 

 is of great importance, but unfortunately it is difficult to apply 

 to the fly itself; the only character that can be used in connec- 



1 Monograph of Oestridae, Verh. Ges. Wien, 1863, and other papers op. cit. 

 1864, 1867, 1869 ; also Denk. Ak. Wien, xlii. 1880, xlvii. 1883. 



