312 Mr. Poulton's notes in 1886 



certainly seemed to be influenced by dark or light 

 surroundings. While the great majority of my larvae 

 possessed the uniformly developed well-marked black 

 bands described by Dr. Dixey, in a few individuals these 

 markings were present to a very slight degree. 



11. Eelation between phytophagous larv.e and 

 various species of food-plant. — One of the most in- 

 teresting things about insects is their extraordinary 

 specialisation in relation to plants, and the complete 

 interdependence between these widely separated groups of 

 organisms. Not the least interesting feature of this rela- 

 tionship is the fact that so many species of phytophagous 

 larvae are rigidly limited to a few or often to only a single 

 species of food-plant. If such larvse are offered other 

 food-plants it is well known that they will generally starve 

 without touching them. And yet there can be no doubt 

 that the food-plants must have been often extended or 

 changed as the range of a species altered, and, although 

 such change may be frequently in the direction of allied 

 or representative species of plants, this is by no means 

 always the case. Thus, taking a single instance, the 

 common food-plants of S. ocellatus are sallow and apple, 

 the occasional food-plants laurel and poplar. But if a 

 larva becomes so specialised in relation to a food-plant 

 that it will starve without touching another species 

 there seems to be a great barrier in the way of any 

 alteration, and the continued abundance of the animal 

 would seem to be endangered by a double series of risks, 

 i. e., those which beset the animal itself, and those 

 which beset the plant upon which it depends. Many 

 observations, however, tend to prove that the rigid 

 specialisation largely grows up in the life of each 

 individual, and is therefore not inherent in the species. 

 This seems to be shown by the following observations: — 

 The larvae of S. populi feed commonly upon poplar, 

 rarely upon sallow and laurel : during the past season I 

 offered laurel to some half-grown larvse which had been 

 previously fed upon Papains nigra, and, although left 

 without other food for some days, the laurel was 

 untouched. The larvae similarly refused Populus alba. 

 So also S. ligustri commonly feeds upon privet, lilac, 

 and ash, but it has been occasionally found upon holly ; 

 and 1 have unsuccessfully offered the latter food -plant to 



