STRUCTURE 



445 



exhaustively every method of inquiry : and -from this point of 

 view the development is of great importance. This has, however, 

 as yet thrown but little light on the subject, this study being a 

 very difficult one owing to the profound changes that take place 

 during metamorphosis, the diversity of the parts in the early 

 stages of Diptera, and the possibility that the larval conditions 

 may themselves have been greatly changed in the course of the 

 phylogeny. Miall informs us, however, that in Chironomus as 

 well as in Goretlira the new parts of the mouth of the imago are 

 developed within those of the larva.-^ This may permit of an 

 identification of the main divisions of the mouth, at any rate in 

 these cases. Lowne has to some extent traced the development 

 in the blowfly, and he does not agree with the usual interpreta- 

 tion of the parts in the adult. 



The mouth is of considerable importance in the classification 

 of Diptera. The Nemocera ave remarkable from the linear de- 

 velopment and flexibility of the palpi, which are nearly always at 

 least three- or four -jointed ; this condition occurring in no other 

 Diptera. The palpi attain an extraordinary development in some 

 CuHcidae ; in the genus IlegarrMna they are nearly as long as 

 the body, and project in front of the head after the fashion of 

 the palpi of Lepidoptera. In the Brachycera the sclerites or 

 hard parts of the mouth reach a maximum of development, and 

 in Tabanidae (Fig. 214), Xemestrinidae and Bombyliidae are 

 often quite disproportionate to the size of the Insect.- In many 

 of the Eumyiid flies the soft parts are greatly developed, and 

 capable of a variety of movement, the proboscis as a whole being 

 protrusible, and having an elbow-joint in the middle. 



The thorax is remarkable from the absence of distinct separa- 

 tion into the three divisions that may usually be so easily dis- 

 tinguished in Insects. The perfect combination of the three 

 segments adds much to the difficulty of arriving at general con- 

 clusions as to the identification of the parts ; hence considerable 

 difi'erence of opinion still prevails. It was formerly supposed 

 that a segment from the abdomen was added to the thorax of 

 Diptera as it is in Hymenoptera, but this has been shown by 

 Brauer to be erroneous. Indeed, according to Lowne, the abdo- 

 minal cavity is increased by the addition of the small posterior 

 area of the thorax ; it being the mesophragma that separates the 



1 Tr. Linn. Hoc. London (2) v. 1892, p. 271. 



