MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 19 



common foundation or wart-liko tubercle. To quote from my account: '" On the eighth segment 

 is a single central dorsal, black, oval, moderately prominent wart, which is twice as large as the 

 largest on the ninth segment; it is transverse, bearing a bristle at each end, thus having plainly 

 originated fi'oni what was once two separate warts." At the end of this stage, before the tirst 

 molt it becomes a double, large black tubercle, still ending in a pair of setse; after molting 

 (stage II) it is " now well developed, high, conical, and fleshy-, slightly inclined backward, dark 

 at tip, and still bearing two bristles, though the dark chitinous spine is obsolete; the horn-like 

 tubercle is half as high as the segment is thick." In the next stage it is neai"ly as long as the 

 eighth segment is thick vertically. "The horn is slightly retractile in this stage, and the base 

 is movable, being capable of withdrawal and extension and is distinctly nutant, the apex some- 

 times hanging over backward." In the fourth stage the horn becomes larger, higher, and more 

 acute than before; "it is freely elevated or allowed to fall over backward, is soft and flexible, 

 but very slightly retractile, and bears a few scattered, tine bristles." In the final stage the horn 

 is high, stiff, not granulated, but somewhat annulated. The horn is more like that of a sphingid 

 than a ceratocampid, in not forming a solid spine, though this is not invariably the case in the 

 Ceratocampidtv. 



In the allied genus Dasylophia the larva is hatched with the two tubercles / on the eighth 

 abdominal segment still separate, and thus represents an earlier stage in the ontogen}- than in 

 Pheosia. In stage I this pair of tubercles each end in a bristle; in stage II and later stadia the 

 tubercles are fused to form a low, flattened tubercle, the setaj being lost. 



In Tlypitrpax aurora and the species of Schizura, whose segments are nearh- all dorsally 

 humped, each side ending in a seta, the fleshy tubercles on the eighth abdominal segment are as 

 large or nearly as large as those on the first abdominal segment. In .S'. leptlnoidcn the fusion of 

 what corresponds to the caudal horn is seen in PI. XXVI, fig. 1, tuljcrcles // having apparently 

 become atrophied. The cause of the fusion of these tubercles, whether on the first or eighth 

 abdominal segment, is obscure, but probablj' it is a mechanical one. 



In some respects the most specialized genus is Heterocampa, and in two of the species whose 

 ontogeny is known, i. e., II. ohVupui and //. (/uttiritta, certain segments are armed with a pair 

 of dorsal chitinized solid spines or horns. Those on the eighth abdominal segment are well 

 developed, being long, erect, and bearing a few secondary spines. (See Pis. XXX and XXXI of 

 my Monograph of the Notodontida?, also fig. 85.) Their bases are, however, wide apart and 

 there are no signs of a tendency to fusion. They disappear at the first molt, and are probably 

 either defensive structures inherited from some earlier form, or have arisen with comjrarative 

 rapidity, a sport or mutation due to stimuli from without. The ninth abdominal segment in 

 in these two species is well developed, but smaller. 



These caudal hoi-ns do not appear in the first stage of 77. hiu/ulata, II. nianteo., II. ynicolor, or 

 H. astarte, which are thus proved to be more recently evolved. . In the tailed forms, such as 

 Hetfrocampa unicolor, Macrurocampa and Cerura, it is possible that the growth energy 

 expended in the production of the armature has been transferred to a process of hypertrophy of 

 the anal legs. 



Reference should also be made to the species of Notodonta, Synunerista, etc., which bear a 

 hump or broad, low tubercle on the eighth abdominal segment, apparently resulting from the 

 fusion of the primitive setiferous tubercle. 



The caudal horn of the Boinbycidie. — In the majority of the species of this interesting group, 

 represented by Bomh)/.i' mori. there is a well developed caudal horn. 



In stage I the larva is similar to a young Malacosoma (Clisiocampa), having the fine hairs 

 arising in tufts from small warts, but already the two warts on the eighth abdominal segment 

 have united into one. 



On examining a series of larva- in the fourth stage the caudal horn is seen to be soft, fleshy, 

 thick at the base, and rather densely clothed with comparatively coarse setiv: it rather suddenlv 

 contracts toward the end. which is somewhat acute, pointed, the i\\) dark chitinized. and bearing- 

 no signs of its originally double origin: the slender end of the horn licars no hairs and is very 

 finely granulated. 



