34 MEMOIKS OF THE XATIOXAL ACADEMY OF SCIENCES. 



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

 In the subterranean pupa of Datana it is present, and is of use in aiding the pupa to reach the- 

 surface of the ground. It is very large and acute iu the subterranean pupie of Ceratocampidio 

 and Si)hinges. It is evident that in the presence or absence of the cremaster, and in its sliape 

 and in the number of hooks and their shape, we have a set of very phistic 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 Bomhyx mori, or whether the 

 thread is often interrupted at the anterior end, as iu Platymmia cecropia, 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 homochrouic inheritance. 



3. Imayo state. — It is easier to select what may have been acquired characters iu 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 mouths, are subject to frequent molts, are active, and are dependent on a proper supply of 

 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 cluxracters are most marked in the parts which are 

 most used, as the maxilL^, wings, and external genital armature. 



The absence of maxilhe or their very rudimentary condition in Bombycine moths is, with little 

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

 organs (Androconia, etc.) are apparently characters recently ac(iuired. 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 temperature, 

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

 longed cold, or vice verm, 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 differences 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 develop the nuiscles 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 woidd be a study of wingless 

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

 and especially the sscck bearers or I'sychidte. 



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

 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 the wings of the female are very small, and 

 passing to those in which they become simple pads, as in Orgyia, and eiuling Mith those such as 

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

 be compared with certain of the Ephemerae, whose hind wings are so nuich reduced; with Pezzo- 

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

 aborted, ending with tlie 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 into balancers, with their extraordinary sense organs. It is not ditUcult 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 tliis lesion tiirough corpulency and other bodily 

 changes, became inactive, averse to flight, and finally transmitted the i)eculiarity to its offspring. 



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

 life history of Brepana arcuata, 1 have described Mie different stages of this moth, and at the end 



