48 [February 1902. 



Observation of the conditions under which the larvae of Bi/hlia 

 exist in this locality tend to throw some light on the probable cause 

 of their marked dimorphism. In the short herbage of the open veldt 

 round Estcourt the nettle upon which these insects feed stools out 

 into a low clump-like growth, amidst which the green larva is well 

 concealed, but the dark fortn is decidedly conspicuous To any one 

 who has only observed the latter under these circumstances the 

 explanation of its distinctive colouring must present a decided 

 difficulty. But widely distributed species such as these must en- 

 counter varying conditions throughout their range, and to these they 

 must be able to adapt themselves. Such is doubtless the case in the 

 present instance. For in the rank growth of this country this 

 particular nettle assumes a somewhat creeping habit with more widely 

 distant leaves, and is especially fond of twining itself among the close 

 woody stems of several small shrubs. When a dark Byhlio larva is 

 seen upon the plant under these conditions the protective value of 

 its dark and light stripes at once becomes obvious, and this probably 

 accounts for the much greater prevalence here of the dark form in 

 both the larva and the pupa. It would be of much interest to 

 ascertain the larval colours assumed by these two species in various 

 parts of their extended range, and especially in relation to the habits 

 of their food plant. The differentiation of the larvae into two forms 

 is probably of considerable antiquity, as may be judged partly from 

 the marked character of the dimorphism, all tendency to intermediate 

 variation having been eliminated ; but principally from the fact that 

 the two types may be readily distinguished from the first moult. The 

 larva of acheloia cannot be distinguished for certain from that of 

 ilithyia until its fourth stage. And since in its third stage it exhibits 

 distinct traces of the dark central line characteristic of the latter 

 insect, this fact, in conjunction with its smaller range, points to the 

 conclusion that it is a later development from the typical species. It 

 is probable that India was the original home of the genus, and it 

 possibly took part in that great influx of animal life from Asia into 

 Africa, which, according to the suggestion originally made by Huxley, 

 took place in later Miocene times. The new conditions under which 

 it then found itself would thus lead to the preservation of suitable 

 varieties, resulting in the establishment of B. Gotzius and its various 

 local forms. Following out this supposition the Madagascar sub- 

 species B. Gutzius-anvatara would have arisen subsequent to the 

 isolation of that island, owing its origin to the occasional immigration 

 of examples of Gotzius from the mainland And this view is supported 



