22 
MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
Prothoracic segment — AVitli a large subsplierical tubercle on eacli side bearing numerous 
radiating hairs (Lasiocampidic of first stage) or pencils of hairs (Parorgyia); two antlers {11. 
guttivitta^ binndata and 11. obliqiia). 
Second thoracic segment . — Two high slender spines. First stage of Anisota senatoria, A. stigma^ 
and Bryocam])a ruhicnnda. 
Third thoracic segment — Two spinulose pappose flaps^ Empretia stimulca. 
Firsts second^ and third thoracic segments . — Each with a pair of high spines, Cltheronia regalis 
and Fades imiyerialis. 
Second and third thoracic segments , — Each with a pair of long horns, Sphingicampa bicolor. 
First and third thoracic segments . — In Stage I of the European Aglia tau (Poulton). 
First abdominal segment . — Movable tubercle in Schizura and Xjdiuodes. 
Eighth abdominal segment . — The caudal horn of Sesia and most Spliingidm, Pheosia, and 
Endroniis, Bombyx mori^ and other species — Sphingicain]>a, Eacles, Citheronia, and Aglia tau 
(Stage 1). 
So far as I am aware no one has suggested why these horns and high tubercles, and often 
I)encils of hairs, are restricted to these particular segments. As a partial exi)lanation 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 tlie former. It will also be noticed that in walking the 
apodous segments of the caterpillar are more elevated and iwominent than those to which the legs 
are appended. They tend to bend or hump up, particularly the first and the eighth abdominal, 
the ninth segment being reduced to a minimum, and the tenth simply represented by the suranal 
a:::^d 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 x>art 
of the body situated between the last thoracic and first pair of abdominal legs, which are appended 
to the seventh uromcrc. Now, in the larva of Eematocampa jUamentaria^ M'hich bears two pairs of 
remarkable filameutal 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 dining 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 prominent portions of the 
body, at the point where the body is most exposed to external stimuli; 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 tendency of these segments to loop or 
hump uj) has had a relation of cause and effect in inducing the hypertrophy of the dermal tissues 
entering into tlie composition of the tubercles or horjis. 
Analogous positions are in the vertebrates utilized, as in spiny, osseous fishes, or the sharks, 
the horned Amphibia, or horned rei)tiles and horned mammals. The prominence of the 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 
develoi)ed as they jiroved 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 in the piliferous warts of adjoining segments. And in like 
manner may the simple setm have become hypertrophied on account of their great utility as 
deterrent organs, and become wonderfully modified in this and that direction in such ai»d such 
forms, until they became in recent geological times the common and normal iidieritance 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- 
ulations which we have api^ended, that in the species of St^hizura the evolution or hypertrophy of 
the movable or niitant 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 Tachium, or carnivorous Hemiptei’a, or to the onset of open-mouthed 
insectivorous birds. At the same time arise the bright colors, spots, and stripes, tlie very i)eculiar 
•y-shaj)ed silver or yellowish- white mark characteristic of the species of Schizura — these are per- 
