IMAGO OF PRIVET HAWK MOTH. 161 



The origin of a perfect metamorphosis, such as that of Sphinx, with three well- 

 marked stages, larva, pupa, and imago, is probably due to the operation of more 

 than one cause. The Cockroach is an example of an insect which always lives 

 from birth to the adult state in the same manner and under the same conditions ; 

 and this is the case with all Ametabolous Insecta. At birth such insects differ from 

 the adult only in a few points, and the differences are gradually abolished. On the 

 other hand, the differences between a caterpillar and a butterfly are very great, 

 and the change from one state to the other is effected in an abrupt manner. It 

 has been pointed out by Balfour (Comp. Embryology, i. p. 353) that a pupa-stage 

 might easily arise (i) from a change at first small, then greater, in the character of the 

 mouth-parts which would necessitate a more or less prolonged period of quiescence, 

 the more prolonged the greater the change ; (2) from the operation of climate and 

 other natural causes on such a period of quiescence. The question however is 

 a very difficult and complex one, impossible to treat in a short compass, and the 

 student must refer to the list of works given below relating to the Metamorphosis 

 and Genealogy of Insecta. But it must be carefully borne in mind that, among 

 the Micro-Lepidoptera, certain minute moths are known with mouth-parts con- 

 formed to the type of biting mouth-parts (note, p. 159); that it is possible 

 that the period of pupation is accompanied by more than one ecdysis, and is 

 therefore essentially a period of abbreviated development ; that in some Hymenop- 

 tera Aculeata there is, as it were, a preparatory stage previous to the true pupa- 

 stage (Packard, Guide to Study of Insects, 1872, p. 66) ; that the changes from the 

 larval mouth-parts to those of the adult are not so very great in all Metabola, 

 e. g. in the Tenthredinidae and that grades may be traced among Metabola, e. g. 

 the Trichoptera and some Neuroptera afford examples of a comparatively simple 

 perfect metamorphosis, the Lepidoptera a more complex example ; whilst in some 

 Hymenoptera, and especially in the Diptera as a class, the metamorphic changes 

 are of a very profound character. Nor must it be forgotten that our knowledge of 

 many details of anatomy, embryology and of life-histories is still very imperfect. 



British Butterflies and their Transformations, Humphreys and Westwood, 

 London, 1841. British Moths, lid., 2 vols., 1843-45. Tineina (Insecta Britan- 

 nica\ Stainton, London, 1842. Natural History of Tineina, Id., 13 vols., 

 London, 1855-73. Butterflies, &c., Scudder, New York, 1881. 



Anatomy of Danais Archippus. Burgess, Anniv. Memoirs, Boston Soc. Nat. 

 Hist. 1880. Cossus. Lyonet, Ouvrage posthume (see p. 156, ante), Paris, 1832. 



Sphingidae. Butler, Tr. Z. S. ix. 1877. Anatomy of, &c., Baltzer, A. N. 

 30, 1864. 



Mouth-parts. Kirbach, A. N. 50, 1884. Walter, Beitrage, J. Z. xviii. 1884. 

 Antlia. Breitenbach, J. Z. xv. 1882. In Ophideres fullonica. Darwin, Q. J. M. 

 xv. 1875. 



Mode of Action of mouth-parts. Burgess, Amer. Naturalist, xiv, 1880. 



Palpus maxillorum. Walter, J. Z. xviii. 1884. 



Wing and Scales. Semper, Z. W. Z. viii. 1857; Deschamps, A. Sc. N. (2) 

 iii. 1835; cf. Dimmock, Psyche, iv. 1883. On Wings in Insecta. Adolph, Nova 



three facts point strongly to the conclusion stated. The vaginal aperture below the anus is therefore 

 secondary, but the mode in which it arises has not been worked out. It should be noted that in the 

 caterpillar the somites in this posterior region are much abbreviated. 



M 



