THE LARGE CABBAGE WHITE BUTTERFLY 189 



right and left. If, on the other hand, it be a female, a similar 

 structure is visible on the 8th as well as on the gth segment. 

 In this connection it is useful to remember that it is the eighth 

 segment which has the rudimentary, functionless respiratory 

 stigmata. It is difficult to see any explanation of this pheno- 

 menon other than that at one time the pupa was sexually 

 mature, i.e. was the end of the life-history. 



Many larvae, especially of the autumn brood, after making 

 all preparations for pupation, fail to accomplish the change ; 

 for just when this is due, or rather overdue, to take place there 

 emerges instead through the skin of the unfortunate larva a 

 number of small grubs, which at once proceed each to spin its 

 own yellow cocoon on or close to the now dead caterpillar. These 

 are the grubs of a small ichneumon fly (Microgaster glomeratus) 

 which have lived as parasites within the caterpillar, and have 

 at length killed it and devoured all that lay within the 

 skin. The ichneumon fly itself is a small four-winged fly 

 provided with a piercing ovipositor, with which the eggs are 

 inserted into the skin of the caterpillar's back. This little 

 parasite is one of the chief agents in preventing the excessive 

 multiplication of caterpillars, and is one of the gardener's best 

 friends. 



The colour of the pupa is liable to wide variation, and within 

 a fairly large range of tints is more or less in conformity with 

 the colour of the surface to which the larva has attached itself. 

 For example, pupae on a tarred shed are very dark, while those 

 on a white window- frame are very pale. Interesting experi- 

 ments in this direction can be made by lining the sides and roof 

 of breeding-cages with differently coloured stripes of paper and 

 noting the colour effect produced upon the pupae, and also if 

 the larvae exhibit a preference for or prejudice against any 

 particular colours. It has been recorded that pupae attached 

 to a red brick wall were always upon the mortar between the 

 courses and never upon the bricks themselves ; but this might 

 have been the result of a preference for a material and not neces- 

 sarily for a colour. It appears that it is the whole skin of the 

 larva that is sensitive to the colour of the surroundings, and 

 an adaptive development of pigment results by some mechanism 



