i8o COMMON BRITISH INSECTS. 



which can be protruded to the length of half an inch 

 or so. Many naturalists suppose that this horn is in- 

 tended for the purpose of driving away the ichneumon 

 flies when they attack the larva. I can, however, 

 scarcely accept this theory, because the ichneumon 

 flies are terribly injurious to many other caterpillars, 

 which yet are supplied with no apparatus for driving 

 them away. This organ, whatever purpose it may 

 subserve, gives forth a very strong odour, much re- 

 sembling that of fennel, and so powerful that even in 

 the open air it can be perceived at some distance. 



The colour of this splendid Butterfly is almost 

 entirely yellow and black. On the lower wings, how- 

 ever, there is a row of six cloudy blue spots, sprinkled 

 with yellow dots, and at the anal angle of each lower 

 wing is a large red spot with a slight blue crescent on 

 the upper part. This Butterfly was once spread over 

 a considerable part of England, but now seems to be 

 restricted to the marshy parts of Cambridge, Hun- 

 tingdon, and Norfolk. It has been taken in many 

 other places, but I believe that in all those instances 

 it was not native to the place, but had been artificially 

 introduced. In 1845 or 1846 I once saw a specimen 

 in a field by the Cherwell, close to Oxford, and chased 

 it for some time, but unsuccessfully. Whether or not 

 this was an introduced specimen, I have no means of 

 ascertaining. 



The egg of this insect is light green in colour and 

 oval in shape. It may seem rather superfluous to 

 say that an egg is oval in shape, but we shall presently 

 see that many eggs of Butterflies are anything but 

 oval in shape. Just before the egg is hatched, its 



