34 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



terial spine or hooks are developed. The cremaster affords excellent generic and specific characters.. 

 In the snbterraueau pupa of Dataua it is present, and is of use in aiding the pupa to reach the- 

 surface of the ground. It is very large and acute in the subterranean iDup;e of Ceratocanipidae 

 and Sphinges. It is evident that in the jiresence or absence of the cremaster, and in its shape 

 and in the number of hooks and their shape, we have a set of very plastic characters (though 

 excellent for distinguishing genera and species) whose variability and plasticity is due to the 

 varying habits of the pupa, whether living above or under ground, whether protected by a very 

 thin, loose, net like cocoon or by a solid double one like that of Cerura or of the silkworms. AlsO' 

 whether the thread is continuous and can be readily reeled, as in Bomhyjc mori, or whether the 

 thread is often internrpted at the anterior end, as in Plati/samia cecrojyia, is a feature which was 

 probably the result of a slight change of circumstances and may have been inaugurated as the 

 result of variation in a single individual during a single lifetime, afterwards in succeeding genera- 

 tions becoming fixed by homochronic inheritance. 



3. Imago state. — It is easier to select what may have been acquired characters in caterpillars 

 than in butterflies and moths, and yet the latter have a complicated series of what may originally 

 have been acquired characters. It should be borne in mind that while caterpillars live for weeks 

 and even months, are subject to frequent molts, are active, and are dependent on a proper supi)lyof 

 their food, usually this or that plant, butterflies and moths perish, as a rule, directly after mating, 

 taking little or no food. Of course acquired characters are most marked in the parts which are 

 most used, as the maxilhi?, wings, and external genital armature. 



The absence of maxill* or their very rudimentary condition in Eombycine moths is, with little 

 doubt, a recently acquired character. The very arbitrary distribution in Lepidoptera of scent 

 organs (Audroconia, etc.) are apparently characters recently acquired. The wonderful variations 

 in the markings of the wings, due to a variety of slight causes, may often arise during an indi- 

 vidual's lifetime and become a matter of inheritance, the result of sudden changes in tenqjcrature, 

 moisture, or dryness, and changes in food of the larva. By subjecting individual pupa' to pro- 

 longed cold, -or vice versa, varieties and a greater or less number of broods may be produced 

 artificially, and this may illustrate how seasonal varieties have arisen in nature. 



Many species are only separated by difiereuces in the male genital armature. These, as is 

 well known, are subject to great individual variation, and why should not the characters peculiar 

 to a distinct variety, or even species, arise during the lifetime of two individuals when mated? 

 An unusually vigorous polygamous butterfly may have some new congenital extra development 

 of hooks and processes, and by frequent use develo]i the muscles controlling these to the extent 

 of providing an acquired character, which may be, if useful, inherited in the next and succeeding 

 generations. 



But an especially interesting and fruitful field of investigation would be a study of wingless. 

 Lepidoptera, such as the cankerworni, the autumn moths allied to it, the tussock moths (Orgyia), 

 and especially the sack bearers or Psychidse. 



The loss of wings in these cases seems to be due to disuse in individuals more sluggish thau 

 others, and with little doubt has been the result of inheritance of what were originally acquired 

 characters. It is easy to imagine how this has been induced by a study of a series of forms, 

 beginning with certain European genera, in which thc^ wings of the female are very small, and 

 passing to those in which they become simple pads, as in Orgyia, and ending with those such as 

 Anisopteryx, in wliich their reduction is still further carried out. And then Lepidoptera should 

 be compared with certain of the Ephemera', whose hind wings are so much reduced; with Pezzo 

 tettix and other Orthoptera with aborted wings, and certain Hemiptera in which the wings are 

 aborted, ending with the great order of Diptera, comprising a vast number of species, in which 

 the hind wings have not only undergone a great reduction, but have been transformed through 

 change of function iuto balancers, with their extraordinary sense organs. It is not difficult to see 

 that the disuse of wings may have begun in the life of a single individual, which, losing its wings 

 and having perhaps inherited a tendency to this lesion through corpulency and other bodily 

 changes, became inactive, averse to flight, and finally transmitted the peculiarity to its offspring. 



In a paper in the Proceedings of the Boston Society of Natural History (xxiv, 482), on the 

 life history of Drepana arcuata, I have described the difl'erent stages of this moth, and at the end 



