O. A. Merritt Hawkbs 147 



projection of the body wall, from a minute papilla to a conspicuous 

 prominence," but he considers that the word tubercle should be used, 

 p. 16, "as a general term to indicate the location of a seta or a group of 

 setae or a process of the body wall bearing such a group." Fracker has 

 given the name scoli to the particular tubercles which occur in Satur- 

 nian and Nymphalidian larvae, scoli being defined as thorny processes 

 bearing spine-like setae. 



The three upper rows of scoli of both cynthia and ricini are, to use 

 Fracker's words, " well developed, conspicuous, cylindrical, higher than 

 wide," but the infra-spiracular series are short and the thoracic are 

 dumpy. 



The larvae with reduced tubercles appeared after the last ecdysis and 

 seemed, within their individual lives, to reverse the process by means of 

 which the typical Saturnian .scolus has been developed within the 

 species. Fracker(3), p. 44, states that the scolus has been evolved from 

 papillae, i.e. " setae surrounded at the base by a small chitinized ring." 

 The larvae which had the most perfectly reduced tubercles showed in 

 some segments, on careful examination, traces of the chitinized ring, 

 but the setae had disappeared, just as they have disajjpeared on the short 

 infra-spiracular papillae of the normal larvae. 



The number of abnormal larvae in F., was small, only 47 occurring 

 out of a total number of approximately 900 adult larvae. Only the 

 adult larvae can be counted as this condition appeared in the last instar. 

 These 47 larvae were S, RS, and P forms, of both sexes and gave rise 

 to both dark and light moths ; this particular phenomenon was therefore 

 not correlated with any of the other characters studied in this series of 

 papers. 



In order to know the anatomical value of the scoli, Dr A. D. Imms, 

 of Manchester University, very kindly ciit sections for me, as I did not 

 have the adequate apparatus at my disposal. The scoli (Fig. 2) consist 

 of two parts, a basal portion with a greatly thickened cuticle, pre- 

 sumably for support, and a distal portion over which the cuticle is 

 thinner and quite smooth. This cuticle is evidently different from that 

 on the basal portion and the general body surface, as it stains much less 

 deeply. The hypodermis consists of columnar cells, among which are 

 the large trichogenous cells with their large flattened nuclei — this layer 

 is at places invaginated to form what are apparently glands, although 

 they are not connected with any opening in the cuticle. 



The general body cavity extends into the distal portion of the 

 tubercles, through a somewhat narrow passage, which perhaps acts as a 



