MEMOIKS OF THI<: NATIONAL ACADEMY OF SCIENCES. 31 



grown 2)iilrer(o. it outstrips that form, and the new forces of variation are concentrated at theotiier 

 end of the body, resaltiny in the hypertrophy of the anal le<^s. 



This tendency once initiated, it became accelerated, until in the larva of MacrurocampaM 

 cnlminated in a pair of anal filaments with their evertible Mayella as fnlly tinishcd as in Cernra, 

 the larva using these in the same manner as deterrent strnctnres; and yet nature holds on to the 

 prothoracic armatnre, rudimentary to be sure, through all the stages of larval development up to 

 and including the fdurth or penultimate stage. Without doubt by very careful and close«observa- 

 tions in the past geological t iuies 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 obser\e. 



Attention should also be called to the remarkable incongruence in the first larval stages of 

 this subfamily, the presence of nine pairs of antlers in H. nutticiiia and of but a single pair, 

 restricted to the prothoracic segments, in 77. hitmdata, 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 larvie into account, are the Cerurin;i^. 



The imago of Cerura is structurally quite distinct from Macrnrocampa, but apparently the 

 sluggish habits, the infrequent, weak, and more or less curtailment of the power of flight common 

 to the entire family of Bombyces have led to a lack of variation in form and structure which does 

 not obtain in the larva' themselves. 



The larva of Cerura is evidently a derivation from Macruiocampa or some lost ally, at least 

 some member of the subfamily Heterocampinje. The prothoracic 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 perhaijs an heirloom 

 from the dorsal horns of Heterocampa. 



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 t(j frighten away any 

 assailant. It is the puff adder among the Bonibycine caterpillars, as the larva of Vhceyocampa 

 is among Sphingid larvse. The stematopoda. which seemed to have proved very useful in 

 Macrnroiampu, 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 Statiropus there is such a mimicry, the thoracic legs being much longer than in any 

 other known lepidopterous larva and the stemapods being thickened and shortened, so that when 

 the creature throws itself into a sprawling, grotesque attitude, with the tail ajj in the air, as 

 remarked by Hermann Midler, 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 

 lepidopterous larva', 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 OP 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 autennne. Variation, then, is the result more 

 of disuse of the wings and of the maxilhe than any other cause, these suffering more or less reduction. 

 The very shoit or vestigial maxilhe of the Saturnians 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 specialization of the scales, of the i)arts and appendages of the legs, of the palpi, 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 modification. 



