MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
31 
grown it outstrips that form, and the new forces of variation are concentrated at the other 
end of the body, resulting in the hypertrophy of the anal legs. 
This tendency once initiated, it became accelerated, until in the larva of Macrtirocamim it 
culminated in a pair of anal filaments witb their evertible flagella as fully finished as in Cerura, 
the larva using these in the same manner as deterrent structures j and yet nature holds on to the 
prothoracic armature, rudimentary to be sure, through all the stages of larval development up to 
and including the fourth or penultimate stage. Without doubt by very careful and close«observa- 
tions in tbe past geological times of the Tertiary, the courses of the variation along this line would 
have been worked out had there been an eye and trained mind behind it to observe. 
Attention should also be called to the remarkable incongruence in the first larval stages of 
this subfamily, the iweseuce of nine pairs of antlers in U, (futtlvitia and of but a single pair, 
restricted to the prothoracic segments, in if. bumdatciy though the moths are very closely allied. 
In the succeeding and what we regard as the latest and most highly modified or specialized 
group, taking the larvte into account, are the Cerurime. 
The imago of Cerura is structurally quite distinct from Macrurocampa, but apparently the 
sluggish habits, the infrequent, weak, and more or less curtailment of the power of flight common 
to the entire family of Eombyces have led to a lack of variation in form and structure which does 
not obtain in the larvm themselves. 
The larva of Cerura is evidently a derivation from Macrurocampa or some lost ally, at least 
some member of the subfamily Heterocampinm. The protlioracic horns of the young larva of 
Cerura, owing to the great development and specialization of the first segment succeeding the 
head, are thrown wide apart and project out laterally. These horns are yet perhaps an heirloom 
from the dorsal horns of Ileterocampa, 
The Cerura larva varied in the direction of the enlargement of the prothoracic segment to 
form a sort of hood to admit the head, serving to make a visage calculated to frighten away any 
assailant. It is the puff iidder among the Bombyciue caterpillars, as the larva of Chocrocampa 
is among Sphingid larvic. The stematopoda, which seemed to have proved very useful in 
Macrurocampa^ were retained in Cerura, being apparently too useful to be lost. 
While the Cerura caterpillars assume a defensive and offensive attitude in order to frighten 
away other animals, they do not mimic the appearance of other animals; but in the singular 
caterpillar of Stauropus there is such a mimicry, the thoracic legs being much longer than in any 
other known lepidoi^terous larva and the stemapods being thickened and shortened, so that when 
the creature throws itself into a sprawling, grotesque attitude, with the tail up in the aii*, as. 
remarked by Hennaun Miiller, it resembles a great spider. At the same time the style of coloration 
is changed; it has not the green and red tints of Cerura, but is tinted light and dark horn-brown, 
like the bodies of many large spiders. 
In the case, then, of Stauropus, variation has gone on in a novel and determinate direction, 
the process of natural selection ending in a result not to be observed in the case of any other 
lei>idopterous larvie, the initial cause of variation being apparently the result of protection due to 
a resemblance to members of another class of arthropods. 
THE PROBABLE CAUSES OF VARIATION, LEADING TO INCONGRUOUS LARVAL CHARACTERS. 
We have seen that the moths of the Bombyces are far less active, have a weaker flight, are 
more sluggish, and hence are more uniform in color and markings than any other superfamily of 
Lepidoptera. The females remain stationary on the bark of trees and in similar situations, while 
the males seek and find them, not so much by virtue of swiftness of flight as by their unusual 
power of scent, as evidenced by their well-pectinated antennai. Variation, then, is the result more 
of disuse of the wings and of themaxilhe than any other cause, these suffering more or less reduction. 
The very shoit or vestigial maxilhe of the Saturuians and the reduction in the number of veins of 
the wings in that group is the result of disuse; but, on the whole, variations in details of structure, 
in the si)ecializatioii of the scales, of the partvS and appendages of the legs, of the palxn, and other 
parts so striking in the Noctuina are very noticeable. 
On the other hand, from causes potent though obscure, the degree of variation in the larval 
forms is most striking. We have every reason to believe that this great degree of modificatioa 
