500 



COMPARATIVE ANATOMY 



CHAP. 



play an important part in the breaking up and disappearance of the larval organs ; 

 since they, as phagocytes (leucocytes) seize upon the elements of the larval organs, 

 and like amtebfe take them into their protoplasm. The phagocytes thus laden with 

 the wrecks of the larval organs and floating in the body cavity are themselves later 

 turned into food for the developing imaginal portions, especially for the epithelia, 

 into which they immigrate and break up. 



At the end of larval life there are found in the thorax imaginal discs similar to 

 those in the Corcthra larra. They here lie, however (Fig. 355), much deeper in the body, 

 and are connected with the hypodermis by means of long cell strands which are hollow 

 only in the neighbourhood of the imaginal discs. The thoracic limbs and wings begin 



Fig. 356.—^, B, C, D, Diagrammatie representation of the development of thewings, legs, and 

 the imaginal hypodermis of the Muscidas from the imaginal discs of the larva during meta- 

 morphosis, diagrammatic transverse sections. Ih, Chitinous integument of the larva, from which the 

 subjacent hypodermis (i/ti/) has withdrawn ; iid, imaginal discs of the wings ; iiv, of the thoracic legs ; 

 is, the strands connecting them with the hypodermis ; fi, wing rudiments ; &, leg rudiments ; ihy, 

 imaginal hypodermis, spreading out in D from the imaginal discs. The imaginal rudiments of the 

 hypodermis are indicated by thick black outlines, the larval hypodermis by two thin parallel 

 lines. 



to form in just the same way as in Coretlira as outgrowths within the imaginal sacs. 

 At a later stage the processes of the imaginal discs which are connected with the 

 hypodermis shorten and become hollow. The larval hypodermis then opens over the 

 imaginal discs, which have moved outwards, and the feet and wings come freely 

 to the surface. A new hypodermis layer spreads out from the imaginal discs over 

 the thorax : this is the rudiment of the imaginal hypodermis, while in proportion as 

 the imaginal hypodermis spreads, the larval falls to pieces and disappears, in such a 

 way that the larval and imaginal hypodermis taken together at all stages of the 

 metamorphosis form a continuous cover round the body. While in the thorax the 

 formation of the imaginal hypodermis proceeds from the imaginal discs, in the 

 abdomen it proceeds (later than in the thorax) from the formative centres, the so- 

 called islands, in the hypodermis. In each abdominal segment there are four larger 

 and two smaller islands (Fig. 356). 



