2r6 INSECTS. PHYLUM ARTHROPODA 



changed from a hollow bag to a flat double membrane. There may 

 be one, two, or three generations in the year, according to the 

 species.' The' winter is passed in a quiescent state, sometimes as 

 an egg, sometimes as a larva, sometimes as a pupa and sometimes 

 as an adult, but in British species, with one exception, there is 

 only one method of wintering in each. The large white, Pieris 

 brassiccp, lavs its eggs in batches of six to a hundred on leaves of 

 plants of the family Cruciferae, such as cabbages and turnips. 

 The black and yellow larvae hatch in from four to seventeen 

 days, and are gregarious, feeding together on a mat of silk which 

 they' have spun over the leaf. There are four moults, and the 

 pupa is formed after about a month. It is fastened to a support 

 both by a girdle of silk round the body and by a terminal pad 

 in which the claspers are embedded. There are two or three 

 generations in a summer, and while the earlier caterpillars may 

 pupate on the food plant, the later ones usually carry out a 

 vertical migration and anchor themselves on the under side of a 

 ledge of a wall or fence. Metamorphosis takes about a fortnight, 

 but the late summer pupae have their development inhibited by 

 the cold weather, and may survive the winter. If they do, the 

 imago emerges in April. Butterflies of the first generation are 

 on the wing from April until June, and those of the second from 

 June until the end of August. In favourable weather there may 

 be a third generation which flies until October. A few imagines 

 survive the winter, but generally an individual does not live for 

 much over three weeks, and feeds on the nectar of several flowers, 

 especially beans, clover, and lucerne. These are Leguminosae, a 

 different family from that to which the food plants of the larvae 

 belong. Examples of butterflies which winter in other ways are 

 the brown and purple hairstreaks, Thecla hetulce and T. quercus 

 (eggs), the meadow brown, Maniola jurtina (larvae), and the 

 small tortoiseshell, Aglais urticcB, and brimstone, Gonepteryx 

 rhamni (imagines). It is natural that most of the earliest butterflies 

 of spring should be those which have survived the winter in the 

 adult state. 



Order 21. Diptera. These are ' flies ' in the narrow sense, two- 

 winged insects with the hind pair replaced by knobbed balancers. 

 The mouth-parts are sucking but may also be piercing, either 

 primarily by means of mandibles, as in the gnat described on 

 p. 243, or secondarily by stiffening of the labella, as in the tsetse 

 fly Glossina and the stable fly Stomoxys. The three thoracic 



