58 THE BOOK OF BRITISH BUTTERFLIES. 



extremely difficult mattej to distinguish the insect from the 

 flower-head on which it rests, for the latter consists of a 

 large number of tiny white flowers, between which show 

 little bits of green, producing a chequering very similar 

 indeed to that on the insect. This is by no means an 

 isolated example: cases of "resemblance" are common 

 not only among the butterflies, but amongst all classes of 

 insects, not to mention divisions of the Animal Kingdom 

 outside the insect group. How many caterpillars owe 

 their safety from numerous enemies to their similarity 

 in colouring to the leaves on which they feed ! 



The eggs are laid singly about the end of April or 

 beginning of May, on the flower-stalks of some of the 

 following cruciferous plants, and possibly others : Garlic 

 Mustard (Sisymbrium - alliarid), London Rocket (Sisym- 

 brium Irio\ Hedge Mustard (Sisymbrium offidnale), Flix- 

 weed (Sisymbrium Sophia\ Cuckoo-flower (Cardamine 

 pratensis\ Tower Mustard (Turritis glabra\ Treacle 

 Mustard (Erysimum cheiranthoides\ Charlock (Sinapis 

 arvensis\ Winter Cress (Barbarea vulgaris\ Penny Cress 

 (Thlaspi arvense), as well as Horseradish and other 

 garden plants. The eggs hatch in nine days, or even 

 less, and the young larvae feed on the pods and seeds 

 (seldom on the leaves) of the plants enumerated above. 

 They are full-fed on the average about the beginning of 

 July, and after changing into the chrysalis form remain 

 in that stage on the stems of the food-plant till the 

 following April, when the butterfly appears on the wing 

 and remains out till nearly midsummer. 



The larva (Fig. 42), which is about ijin. in length, 

 and tapers somewhat toward the anal extremity, is 

 rather dark green dorsally, but yellower on the ventral 

 surface. These two colours are separated by a clear white 

 line, sharply defined below, but gradually fading into the 



