44 



to mine much in the same manner, but being larger it makes a wider 

 mine — o'5 mm. — and, as the excrement does not occupy the whole 

 breadth of the gallery, the mine has a greener appearance. 



In three and a half days the caterpillar will have eaten out the 

 parenchyma to a length of some 25 mm. It is now in its second 

 instar, and if we examine it we shall find it has made some advance 

 in structure, for it now has very small prolegs, but they are without 

 crochets. The colour is nearly the same as in the first instar, but 

 the orange band is not so wide in proportion, and has a tendency to 

 break up into spots. After changing its skin a second time the 

 larva soon resumes its work in the leaf. It continues as before to 

 form serpentine windings, going apparently haphazard anywhere in 

 the leaf. Whether these windings are due to chance alone, or 

 whether the larva takes the path of least resistance, has not, I think, 

 been yet determined. This part of the gallery is about i mm. 

 in width and 36 mm. in length. The excrement is here placed less 

 evenly over the surface and tends to run into lines and blotches, 

 which gives the mine a more variegated and paler appearance. 

 Some five days are occupied in making this portion of the mine. 

 The larva is now 3 mm. long, and has quite serviceable legs and 

 prolegs ; it is pale yellow in colour, and when newly changed still has 

 the orange band beneath. After a day or two this band begins to 

 vanish from the tail upwards, and when the larva is nearly full grown 

 in this instar there remains only a spot or two of orange on the 

 thoracic segments, and this colour finally disappears altogether. 

 Besides this, the larva somewhat changes its shape at the end of 

 this stadium, becoming less cylindrical, more flattened dorsally and 

 ventrally, so that it assumes somewhat the appearance of a dipterous 

 maggot. It now makes a large chamber at the end of its mine and 

 then turns round and hides the head and thorax under the excre- 

 ment which is deposited on the upper cuticle of the leaf. In this 

 condition the pale yellow larva is very inconspicuous and can only 

 be made out with care. The mine looks as though the larva had 

 deserted it, and when the larva is seen it looks as if it were dead. 



It would appear that this special period of the larva's existence 

 must be a very critical one, as it takes so much care to conceal itself 

 while undergoing tliis third change of skin. It remains in the 

 position described for three or four days before casting its old skin. 

 It may be particularly prone to ichneumon attack during this period. 

 In preparing for either of the two previous changes of skin the larva 

 simply widens the mine slightly (which enables it later to thrust the 

 old head aside) and ceases to feed, but does not change its position 

 at all. After the change the larva, now in its last stadium, appears 

 in the mine as a pale, yellowish-green caterpillar with a broad, dark- 

 green central line. Its mine is now 3 mm. in width, and the larva 

 eats out nearly all the parenchyma, leaving only a very slight layer 

 on the upper cuticle, and a slightly thicker layer of cells on the 

 lower cuticle. The mine therefore always retains a greenish appear- 



