[200] [The grub-formed Hymenopterous larvæ, like the larvæ of all other holometabolous insects, thus represent an acquired degenerative stage in the development, _i.e._ an adaptation to the conditions of life at that stage. Bearing in mind the above-quoted observations of Bütschli and the caterpillar-like form of the Terebrantiate group of Hymenopterous larvæ, the following remarks of Balfour’s (_loc. cit._ p. 353), appear highly suggestive:--“While in a general way it is clear that the larval forms of insects cannot be expected to throw much light on the nature of insect ancestors, it does nevertheless appear to me probable that such forms as the caterpillars of the Lepidoptera are not without a meaning in this respect. It is easy to conceive that even a secondary larval form may have been produced by the prolongation of one of the embryonic stages; and the general similarity of a caterpillar to _Peripatus_, and the retention by it of post-thoracic appendages, are facts which appear to favour this view of the origin of the caterpillar form.” See also Sir John Lubbock, _loc. cit._, pp. 93 and 95. R.M.] [201] [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 Fossores_, and _Mellifera_. (See, for instance, F. P. Pascoe’s “Zoological Classification,” 2nd ed. p. 147.) Of these groups the larvæ of the _Terebrantia_ as thus restricted are all of the caterpillar type (_Tenthredinidæ_ and _Siricidæ_), whilst those of the other groups are maggot-shaped. For a description of the development of the remarkable aberrant larva of _Platygaster_, see Ganin in Zeit. f. wissenschaftl. Zool., vol. xix. 1869. R.M.] [202] [For recent investigations on the structure of the thorax in Diptera, see a paper by Mr. A. Hammond, in Journ. Linn. Soc., Zoology, vol xv. p. 9. R.M.] [203] I am familiar with the fact that the two sub-orders of true Diptera, the short-horned (_Brachycera_), and the long-horned (_Nemocera_), are not sharply limited; and I am likewise well acquainted with the circumstance that there are forms which connect the two larval types. The connecting forms of the imagines do not, however, always coincide with the intermediate larval forms, so that there here arises a second and very striking incongruence of morphological relationship which depends only upon the circumstance that the one stage has diverged in form more widely than the other through a greater divergence in the conditions of life. The difficulty is in these cases aggravated because an apparent is added to the true form-relationship through convergence, so that without going into exact details the form and genealogical relationships of the Diptera cannot be distinguished. It would be of great interest for other reasons to make this investigation, and I hope to be able to find leisure for this purpose at some future period.