NOVITATES ZOOLOGICAE XXVI. 1919. 



309 



citron, " limdo galego," " tamanqueira," Piper aduncuin, and at least two other 

 species, but only once on Piper beletnense. Also a Rutaceous herb in gardens 

 known as " ariida." 



The egg of thoas is cadmium yellow and of moderate size, and is laid generally 

 u^Jon the upper surface of the freshest leaves, where it is easily detected. 



As I have it on Dr. Ruber's authority that the entire Citrus genus is an 

 importation, and that some 400 years ago there was not a single orange or lemon 

 of any species growing in South America, we are forced to the conclusion that 

 the many different larvae which now feed on the leaves of Citrus, apparently 

 by preference, were formerly restricted in their choice of diet afforded by nature 

 to something less palatable. That a number of Papilios of the country, and 

 other species like Rothschildia betis frequently taken in Para on orange and 

 lemon, should show an almost exclusive attachment to a department of plant-life 

 which is not indigenous, is surely a fact strange enough to require an explanation. 

 I once made the discovery of thoas feeding in nature on Fagara rhoijolia (taman- 

 queira), a thorny tree with leaves like the mountain ash, and smelling like lemon ; 

 and it was significant to learn that this natural alternative pabulum belonged 

 to the same botanic Order — Rutaceae. Moreover, I had already taken the larvae 

 of both Rothschildia betis and ericina feeding on the leaves of this tree, and the 

 combination of circumstances not unnaturally suggests the theory that Fagara 

 rhoijolia and its allies, together with various species of the Piperaceae Order, 

 are the original food-plants of the present-day orange feeders. 



While on the subject of food-plants, it is worth a passing mention that the 

 larvae of almost all the Papilios yet found exhibit a certain predilection which 

 they share in common. I refer to their partiality for odoriferous and even 

 pungent-scented leaves, such as characterise Aristolochia, Citrus, Fagara, Piper, 

 Umbelliferous plants like carrots and parsnips, and Anonaceae, all of which are 

 pre-eminently endowed with essential oils of powerful odour. This particular 

 adaptation of an extensive lepidopterous family to widely dift'erent representatives 

 of the vegetable kingdom may, of course, be accidental, and have no real bearing 

 upon their undoubted association as members of a great genus, but it is at least 

 noteworthy and interesting. 



Returning to thoas, the young white and yellow-brown larva clearly fore- 

 shadows the adult, except that it has more yellow in its composition, and up 

 to the final instar is very oily looking. Both then, and even after, it bears a 

 striking resemblance to a piece of freshly deposited bird's dung. When fidl- 

 grown it is sometimes to be found on the branch, but is more usually to be 

 seen resting fully exposed upon the upper surface of a leaf of its food-plant. 

 Viewed at a certain angle from the front, it bears a distinctly snake-like and 

 forbidding appearance, the thoracic segments being humped up and exhibiting 

 a dark eye-like mark on either side. 



In the North American Papilio cresphontes this snake-mimicry is carried 

 to as great a pitch of perfection as in the Sphingid genus Xylophanes ; and 

 though I am unable to conceive how this can be attributed to " natural selection," 

 such parallel instances, together with many others almost equally remote from one 

 another, yet all obviously designed to imitate a small serpent, surely preclude 

 the possibility of mere coincidence. Whatever the actuating cause, the effect 

 without doubt is protection, the disguise being employed as a preventive 

 measure against birds and lizards. 



