BUTTERFLIES AND MOTHS 319 



flies (which see). The leaf of the cabbage is selected inasmuch as the 

 leaves of this, and of many different plants of the same family (besides 

 many other cruciferous species), form the food of 



B. The Larva. 



The latter escapes from the egg after about a fortnight, feeds inces- 

 santly, grows rapidly, and after casting its skin several times (why? 

 see p. 310) develops into the familiar " caterpillar." From these two 

 facts viz., that the larva lives on the leaves of several plants, and 

 especially of such as are cultivated in large quantities we may under- 

 stand the extraordinary abundance of the cabbage butterfly. (Follow 

 the development of the insect from the egg to the butterfly.) 



1. Cabbage leaves, like all green vegetable substances, do not form a 

 very nourishing food (see Part L, p. 101). Hence the larva must consume 

 a large quantity of it, firstly for the purpose of satisfying its needs, 

 secondly in order to grow rapidly, so as to be mature and ready to enter 

 the pupa stage before the first night frost (which would prove fatal to it) 

 sets in, and lastly for the purpose of laying up within its body a store 

 of reserve material on which the life of the pupa may be sustained 

 during the period of quiescence (compare Part L, p. 66, Section C, 2). 

 These reserve materials are found in the larva in the form of fat. For 

 elaborating large quantities of food the larva requires a wide and long 

 intestine. 



2. The intestine, not being coiled, renders an elongated, tubular, and 

 vermiform body a necessary condition. (This " vermiform " character of 

 the body is more or less pronounced in all insects which pass through a 

 pupa stage.) 



3. The larvae of the cabbage butterfly (like those of all Lepidoptera) 

 are not obliged to go in pursuit of their food, and only rarely have to 

 undertake long migrations. They are accordingly slow and lazy creatures 

 with very short legs. (Contrast with those larvae which lead a predatory 

 life. Why are caterpillars unable to dispense entirely with legs, like 

 maggots? How is their sluggishness of importance to them in the 

 storing up of reserve materials ?) 



4. The three pairs of thoracic legs would not, however, be sufficient 

 to support the elongated body. (Whilst creeping over a horizontal 

 surface the insect would have to drag the hinder portion of its body 

 after it ; in creeping vertically downwards, this part of the body would 

 " tip over " ; and in creeping along the under side of leaves or similar 

 objects, or along vertical surfaces on a horizontal or oblique direction, 

 it would hang downwards.) Hence, in addition to the thoracic legs, 



