488 Studies in the Theory of Descent. 



We have here therefore another case like that 

 which we met with among the Rhopalocerous 

 Lepidoptera, in which the imagines appear to be 

 capable of being formed into a higher group than 

 the larvae, because the former live under conditions 

 of life which are on the whole similar whilst the 

 latter live under very divergent conditions. 



The old division of the Hymenoptera into two 

 sub-orders has certainly been abandoned in the later 

 zoological text-books ; they are now divided into 

 three : saw-flies, parasitic, and aculeate Hymen- 

 optera ; but even this arrangement has been 

 adopted with reference to the different structure 

 of the larvae. Whether this system is better than 

 the older, i.e. whether it better expresses the 

 genealogical relationship, I will not now stop to 

 investigate. 5 



DIPTERA. 



The imagines of the Diptera (genuina), with the 

 exception of the Aphaniptera and Pupipara, agree 

 in all their chief characters, such as the number 



6 [In the most recent works dealing with this order six groups, 

 based on the character of the imagines are recognized, viz. : 

 Tubulifera, Terebrantia, Pupivora, Heterogyna Fossorcs, and Mel- 

 lifera. (See, for instance, F. P. Pascoe's " Zoological Classifi- 

 cation," 2nd ed. p. 147.) Of these groups the larvae of the 

 Terebrantia as thus restricted are all of the caterpillar type 

 (Tenthrcdinida and Siricida), whilst those of the other groups 

 are maggot-shaped. For a description of the development of 

 the remarkable aberrant larva of Platygastcr, see Ganin in 

 Zeit. f. wissenschaftl. Zool., vol. xix. 1869. R.M.] 



