16 BRITISH BUTTERFLIES. 



etc. — or in both, as will best protect it from the enemies which would 

 otherwise prey on it. Similar, therefore, as the newly-hatched larvae 

 may be, they present, usually, at their first moult, a marked change in 

 their appearance, and this frequently becomes more pronounced at 

 each successive moult until the larva is fullgrown. The difference 

 between the newly-hatched larvae of the Vanessids, Argynnids, etc., 

 and their adult forms is very great, e.g., compare the newly-hatched 

 and adult larvae of Dry as pap Ida, Aglais articae, etc. If we study the 

 habits of these larvae we shall find that these changes culminate in 

 producing just that form which is best protected by the particular 

 environment which surrounds it. The preservation of the more 

 suitable, and the weeding out of the less suitable, individuals by 

 natural causes, is known as "natural selection." Butterfly cater- 

 pillars have, therefore, been brought to a high state of fitness to their 

 surroundings by natural selection. Their independent mode of life 

 makes the larval specialisations run in entirely different directions 

 from those which are most effective in the preservation of the pupa, 

 or imago, where the conditions of its environment are so entirely 

 different. 



All newly-emerged larvae, however, do not conform to the generalised 

 type just noted, but hatch from the eggs already in a highly 

 specialised condition, e.g., the larva of Papilio machaon is well provided 

 with spinous processes when it leaves the egg, and others have 

 undergone even more development before hatching. We assume that, 

 as these larvae hatch in a more than usually specialised condition, 

 they go through a generalised stage of development in the egg before 

 reaching this more specialised one in which they hatch, and that these 

 earlier stages have at some distant time taken place outside the egg, and 

 that the necessities of a changed environment have forced these later 

 stages into the egg, so that the larva is more specialised and more able 

 to respond to its present environment when hatching takes place. We 

 have, however, no real evidence that this is the case. 



The larva undergoes a certain number of moults or changes of skin 

 before it becomes adult. The period between one moult and another 

 is called a stage or stadium. The appearance of the larva at any 

 particular moult is known as its instar. Thus a larva that moults 

 four times has five stadia, and five different appearances of its plumage 

 or instars corresponding with the stadia. It may be noted that the 

 head, being chitinous, is of fixed size throughout a given stadium, and 

 that this is of great importance in determining the stage in which a 

 larva is, apart from the size of the body. It is also to be noted that, 

 at a larval moult, not only does the larva cast off its old skin, but the 

 linings of the mouth, gullet, and even of the large air-passages are shed 

 as well, and, if the cast-off skin be examined, you may observe the latter 

 as fine thread-like processes curled up, and starting from the spiracles 

 down the sides of the caterpillar's body. Previous to moulting, the 

 larva spins a silken pad on which it rests for two or three days, inserts 

 into this the hooks of its prolegs, and here it remains motionless whilst 

 the new skin is being matured beneath the old one. During this 

 time a surface fluid collects between the two skins, the old skin splits, 

 and the larva in its new skin frees itself from the old one, which 

 remains attached to the pad by means of the prolog hooks, which cling 

 tightly to it. 



