24 ME3I01RS OF THE NATIONAL ACADEMY OF SCIENCES. 



and the above-iiieutioiieil group, Plieosia, Leiocaiupa) sliould properly, by their smooth, uoctui- 

 form shape, stand at the bottom of the family, as beiiiy,- nearest related to the primitive form of the- 

 group. But until we know more of the earliest stages it is best to suspend our judgment. 



1. The more prominent tubercles and spines or Ijristles arising from them are hy])crtr()phied 

 piliferous warts, the warts with the seta or hair which they bear being common to all caterpillars. 



2. The hypertrophy or enlargement was i)robably primarily due to a change of station from 

 herbs to trees, involving better air, a more equable temperature, perhaps a diHereut and better 

 food. 



3. The enlarged and specialized tubercles developed more rapidly on certain segments than 

 others, especially the more prominent segments, because the nutritive fluids would tend to more 

 freely supply parts most exposed to external stimuli. 



4. The stimuli were in great part due to the visits of insects and birds, resulting in a mimicry 

 of the spines and projections on the trees; the colors (lines and spots) were due to light or shade, 

 with the general result of prote(!tive mimicry or adaptation to tree life. 



5. As the result of some unknown factor several of the hypodermic cells at the base of the 

 spines became in certain forms specialized so as to secrete a poisonous fluid. 



r>. After such primitive forms, members of different families, had become established on trees, 

 a process of arboreal segregation or isolation would set in, and intercrossing with low feeders, 

 would cease. 



7. Heredity, or the unknown factors of which heredity is the result, would go on uninter- 

 ruptedly, the result being a succession of generations perfectly adajited to arboreal life. 



8. Finally the conservative agency of natural selection would operate, constantly tending 

 toward the elaboration and preservation of the new varieties, species, and genera, and would not 

 cease to act in a given direction so long as the environment remained the same. 



9. Thus, in order to account for the origin of a species, genus, family, order, or even a class, 

 the first steps, causing the origination of variations, were in the beginning due to the primary 

 (direct and indirect) factors of evolution (Neolaniarckism), and the tinal stages were due to the 

 secondary factors, segregation and natural selection (Darwinism). 



f 



111.— OX CERTAIN POLXTS IN THE EXTERNAL ANATOMY OF BOMBYCLXE LARV^. 



Homology of the ^•JJafjcUiim'" of Ccruru, etc., ivith the planta of the other abdominal legs. — We 

 have in a former ' article, in describing the larvie of Macrurocampa marthesiu and of certain species of 

 Cerura, called attention to the nature of the stemapoda- or filamental legs of those caterpillars, 

 aud their generally undisputed homology witli the anal legs of other Notodoutiaus. PI. XXXX'II, 

 fig. !», represents the anal legs of r><milophla anf/iiina in its first larval stage. It is intermediate in 

 form between the normal leg and the stemapod. It has no crochets, but the plauta, of which the 

 "flagellum" of Cerura and IT. marthesia seems to be the homologue, is retracted and the retractor 

 muscles, one of which is divided, are much as in the filamental legs of Cerura, etc. 



Note on the modijications in the tenant or (jlandular hair.s of the thoracic feet. — ^^As is well known, 

 the thoracic feet of caterpillars are five jointed and end in a single claw, with apparently a 

 rudimentary one at the base. Usually, besides the unguis or claw, there is a tenant hair, which 

 is generally spine like, but besides these appendages there are sometimes more or les.s flattened, 

 lamellate seta;, which are curious and worthy of notice. In Parorgyia parallela., besides the unguis 



'Proceedings Boston Soc. Nat. Hist.,xxiv, 1890. 



= TIio term "tails" or caudal filaments is too vague for these highly modified anal legs; hence we propose th& 

 term stemapoda or stemapods for those of Cerura aud Macrurocampa. The derivation is Gr. ani/in, filament, -koIc, 

 TToiWr, leg or foot. Mr. J. Hellius, referring to these organs in Buckler's Larvie of the British Butterllies aud 

 Moths (Koy. Soc, ii, 138), remarks: "But now through Dr. T. A. Chapman's good teaching, I regard them as dorsal 

 appendages, somewhat after the fashion of the anal spines of the larva' of the Satyridie.'' This, I am satisfied, is au 

 error. After repeated comp.irisons of the filamental anal legs of Cerura with those of Mucriirocampa marthetiid, aud 

 comparing these with the greatly elongated aual legs of young FT. iinirolor as figured by Popenoe, aud taking into 

 account the structures aud homologies of the supraanal and par.inal Haps, one can scarcely doubt that those of 

 Cerura are modified anal legs. 



