20 HOUSE, GARDEN, AND FIELD 



pressure within the bladder, that the end of the hard 

 outer shell is torn open, a specially prepared cap being 

 often detached (Fig. 4), and through the orifice the fly 

 creeps out. During the act of extrication the bladder can 

 often be seen alternately swelling and collapsing. Blue- 

 bottles, house-flies, cheese-flies (see p. 114), and many 

 more escape in the same way from the barrel-like case 

 within which they were developed. 



The holly-fly (Fig. 5) belongs to the enormous family of 

 the Muscidse, and is not very unlike a diminutive house-fly, 

 being only 2j mm. (one-tenth of an inch) 

 long. It is nearly black, but the proboscis 

 and halteres (balancers) are white. The 

 female fly lays an egg upon a young holly- 

 leaf, and thus starts the new generation. 



The little maggot feeds all autumn, 

 winter and spring, devouring the soft green 

 cells of the evergreen leaf. Its body is 

 sufficiently transparent to show a green line 

 traversing it from one end to the other ; 

 this is the alimentary canal, filled with 

 skin after ~~emer- f resh green food. By the help of the micro- 

 gence of holly-fly. 3CO p e we can see that the head, as in other 



The place of escape 



is seen near the maggots, is reduced to a vestige ; even 

 upper end. ^e paired jaws have disappeared, and the 



biting apparatus is a simple arrangement of hooks or 

 teeth, which serve to loosen and bruise the soft leaf-cells. 

 Two or three galleries are often run through the same leaf, 

 and as many pupae are of course developed. They seem 

 at first sight to share the gallery between them, but if 

 the leaf is minutely examined, it will generally be found 

 that partitions are left, and that each pupa inhabits a 

 separate cavity. The larva of the holly-fly is preyed upon 

 by two parasitic Hymenoptera, which subsist upon the 

 tissues of their living host. Shortly before the winged 

 fly is due, an insect of a quite different kind may emerge 

 from the blister made by the holly-fly larva. 



