MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 31 



gTOKu 2)1(1 re red, itoutstiiiis tbat form, and the uew forces of variatiou are couceiitrated at the other 

 end of the body, resulting;- in tlie hypertrophy of the anal legs. 



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

 culminated in a pair of aual filameuts with their evertible flagella as fully finished as iu Cerura, 

 the larva using these in the same manner as deterrent structures; 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 peuultimate stage. Without doubt by very careful and close»observa- 

 tions in the 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 presence of nine pairs of antlers in H. fjuftivitUi and of but a single pair, 

 restricted to the prothoracic segments, in H. biundata, 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 larviie into account, are the Cerurina'. 



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 Bombyces have led to a lack of variation iu form and structure which does 

 not obtain in the larvte themselves. 



The larva of Cerura is evideiitly a derivation from MacrKrocampa or some lost ally, at least 

 some member of the subfamily Heterocampina?. The prothoracic horns of the young larva of 

 Cerura, owing to the great development and specialization of the first segment succeeding the 

 head, are thiown wide apart and project out laterally. These horns are yet perhaps an heirloom 

 from the dorsal horus of Hcteroeumpa. 



The Cerura larya varied iu 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 putt' adder among the Bombycine caterpillars, as the larva of Chwrocampa 

 is among Sphingid larvse. 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 ott'ensive attitude in order to frighten 

 away other animals, they do not mimic the appearance of other animals; but iu the singular 

 caterpillar of Stauropus 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 up iu the air, as 

 renuirked 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 iu 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. 



THK PROBABLE CAUSES OF VARIATION, LEADINCI 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 iu 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 fiiul them, not so much by virtue of swiftness of flight as by their unusual 

 power of scent, as evidenced by their well-pectinated antennae. 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. 

 Tlie very shoit or vestigial maxilla? of the Saturnians and the reduction in the number of veins of 

 the wings iu that group is the result of disuse; but, on the whole, variations in details of structure, 

 iu the specialization of the scales, of the parts 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 



