MUSTARD BEETLE. 65 



the thin fihii of the upper coat of the leaf, which had been left 

 remaining untouched when the egg was inserted from the other side 

 in the little cavity formed for it, and this upper film was now in some 

 instances beginning to split away. 



The operation of hatching, or emergence of the grub from the egg, 

 was very slowly performed. In the case of a larva which I observed in 

 the act of coming out of the egg, and which was then about three- 

 quarters emerged, it was precisely nineteen minutes before the operation 

 was completed, and the little grub stood beside the empty egg. During 

 hatching the grub was not of the smoky colour which it presently 

 acquires, but of a bright orange-yellow, with the four black spots on 

 the back of the second and third segments from the head showing very 

 noticeably on the yellow ground colour. The colour gradually became 

 duller, changing to dusky or smoky tints of orange or yellow, and 

 about forty minutes after the time of its first appearance from the egg, 

 the little black horns were observable, and the head and claw-feet were 

 turning grey, and the larva was now able to move about readily, making 

 use of its caudal sucker-foot to help it in progression. The other 

 larva3, which I was able to watch during or immediately after hatching, 

 were similarly of an orange colour, and changed gradually to the first 

 shades of the smoky grey colour, under which we know them only too 

 well on the Mustard leafage. 



The above-mentioned method of egg deposit, and then of covering 

 over of the eggs by the Mustard Beetle, is worth notice, as in the first 

 part we see how early in the attack (even before the maggots have 

 hatched out of the egg) the health of the leafage is damaged by the 

 preparations for their presence ; and next that the covering placed 

 over their eggs acts as a protection to them from the effects of the 

 remedial dressings which we might otherwise have hoped would help 

 to check the attack in embryo. 



The plentiful supply of grubs which hatched from the eggs enabled 

 me to experiment on the effect of " Paris-green " applied to them as a 

 dry dusting, with the result that nearly all so dusted were dead on the 

 following morning. 



On Jvine 9th, I chose a Mustard leaf infested by a large number of 

 the Mustard Beetle larvas, which had begun to hatch on it two or three 

 days before. This I dusted as lightly as I could with Paris-green by 

 holding the leaf upright, and throwing a little of the powder taken up 

 between my fingers and thumb at it, so that the under side of the leaf, 

 on which the maggots were dispersed, was for the most part just faintly 

 or hardly perceptibly tinted by it. In a few parts only there was a 

 little patch where the powder was noticeable. This was a little before 

 6 p.m., and on the following morning, on examination of what had 

 resulted at 9 a.m., I found that, with the exception of three of the 



