22 MEMOIRS OF THE NATIO^WL ACADEMY OF 8ClENt'ES. 



Prothoracic scgmeni. — Witli a large subsplieriual tubercle on each tside bearing uumerous 

 radiating hairs (Lasiocampid;e of lirst stage) or peueils of hairs (Parorgyia); two antlers (H. 

 guttivltta, biundata and H. obliqua). 



iSccond thorarif srgmcni. — Two high slender spines. First stage of Anhota .senatoria, A. sfirjma, 

 and JJryocampd rithicunda. 



Third thoracic segment. — Two spinulose pappose flaps, Empretia stimulca. 



First, second, and third thoracic segments. — Each with a pair of high spines, Cithcronia regnlis 

 and Elides imperinlis. 



Second and third thoracic segments.— \^:\(i\i with a pair of long horns, Sphingicampa bicnlor. 



First and third thoracic segments. — In Stage I of the European Aglia tan (Poultou). 



First abdominal segment. — Movable tubercle in Schizura and Xylinodes. 



Eighth abdominal segment.— Tim caudal horn of Sesia and most Sphingid;e, Pheosia, and 

 Eudrouiis, Bomhy.v mori, and other species— Sphingicampa, Eacles, Citheronia, and Aglia tan 



(Stage 1). 



So far as I am aware no one has suggested why these horns and high tubercles, and often 

 pencils of hairs, are restricted to these particular segments. As a partial explanation of the reason 

 it may be stated that the presence of these high tubercles, etc., is correlated with the absence of 

 abdominal legs on the segments bearing the former. It will also be noticed that in walking the 

 apodous segments of the caterpillar are more elevated and i)rominent tlian those to which the legs 

 are appended. They tend to bend or hump up, particularly the tirst and the eighth abdominal, 

 the ninth segment being reduced to a nHninium,and the tenth simply represented by the suranal 

 £Ci<l paranal plates, together with the last pair of legs. 



As is -well known, the loopers or geometrid worms, while walking, elevate or bend up the part 

 of the body situated between the last thoracic and iirst pair of abdominal legs, which are appended 

 to the seventh uromere. Now, in the larva of Xematocampa Jilamentarin, which bears two pairs of 

 remarkable fllamental tubercles rolled up at the end, it is certainly very suggestive that these are 

 situated on top of the loop made by the caterpillar's body duiing progression, the first pair arising 

 from the second and the hinder pair from the fourth abdominal segment. 



It seems, therefore, that the humps or horns arise from the most ])romineut portions of the 

 body, at the point where the body is most exposed to external stinuili; and the force of this is 

 especially seen in the conspicuous position of those tubercles which are voluntarily made to nod or 

 so move as to frighten away other creatures. Perhaps the teiulency of these segments to loop or 

 hump up has had a relation of cause and effect in inducing the hypertrophy of the dermal tissues 

 entering into the composition of the tubercles or horns. 



Analogous positions are in the verteljrates utilized, as in spiny, osseous fishes, or the sharks, 

 the horned Amphibia, or horned reptiles and horned mammals. The prominence of I he foundation 

 parts, from which the tubercles arise, may lead to a determination of the blood toward such places, 

 and thus in well fed or overfed (possibly underfed) individuals induce a tendency to hypertrophy, 

 ■which once set up in early generations led to the production of incipient humps which became more 

 developed as they proved useful and became preserved in this or that form by natural selection. 

 On the other hand, the hypertrophy of certain piliferous warts would tend to cause an arrest of 

 development or a tendency to atrophy iu the piliferous warts of adjoining segments. And in like 

 manner may the simple setai have become hypertrophied on account of their great utility as 

 deterrent organs, and become wonderfully modified iu this and that direction in such and such 

 forms, until they became in recent geological times the common and normal inheritance not only 

 of scattered species but of certain genera in scattered families, and even of entire families. 



It is to be observed, as one will see by referring to the special larval histories and the recapit 

 Illations which we have appended, that in the species of Sehizura the evolution or hypertrophy of 

 the movable or uutant tubercles begins in the third stage at about the time when the young 

 ■caterpillars leave their common birthplace on the underside of the leaf and seek more conspicuous 

 feeding grounds on the outer edge or on the upper side of the leaf, where they are exposed to 

 the visits of ichneumons, or Tachin;^, or carnivorous Hemiptera, or to the onset of open-mouthed 

 insectivorous birds. At the same time arise the bright colors, spots, and stripes, the very peculiar 

 V-shaped silver or yellowish-white mark characteristic of the species of Sehizura— these are per- 



