222 INTRODUCTION TO ZOOLOGY CHAP. 



these two membranes are more widely separated to allow of 

 the passage of the veins or nervures shown in Fig. 151. The 



structure of the nervures does 

 not seem to have been very 

 fully investigated, but they 

 always contain a chitinous sup- 

 porting rod, and, in some cases, 

 a trachea and blood-cells have 

 been seen in them. 1 



In the abdomen 



Thp 



Abdomen. nme segments seem 

 to be represented 

 in the female, and ten in the 

 male, but in each case the last 

 two are difficult to distinguish 

 FIG. 151. Anterior and posterior in the imago, as they are more 

 wing of Pieris brassicae with or legg w j t hdrawn into the 



scales rubbed off to expose . , ml . , 



"nervures." body. They are more clearly 



seen in the chrysalis. 



There are six very distinct pairs of abdominal spiracles, 

 and a seventh pair is said to exist. There is also a pair of 

 spiracles on the first thoracic segment. No appendages at all 

 are borne on this part of the body. 



The head is shown in Fig. 143, and it and the 

 a ' characteristic proboscis are described on p. 213. 



The butterflies live an active aerial life, flitting 

 with a zigzag motion from flower to flower, sucking 

 nectar from them for food. They are the com- 

 monest butterflies seen on a sunny day in flower and vegetable 

 gardens. The butterfly, if a female, should be carefully 

 watched as she hovers over the flowers, for she may be about 

 to lay her egg-clusters on the cabbage or nasturtium leaves. 



The change from the crawling voracious caterpillar to the 

 quiescent pupa, and then again to the active winged butterfly, 

 is very striking, but it is much more gradual than appears 

 externally. By dissection it has been shown that, in the 

 caterpillar, even some time before pupation, the rudiments of 

 the organs of the perfect insect have already been formed, 

 but they are at that stage compressed within the body. At 

 pupation they are suddenly pushed out, and so cause the 

 1 Cambridge Natural History r , vol. vi. p. 330. 



