^ Fig. 78.— Pupa- 

 rium of blow-fly 



THE STRUCTURE OF THE BLOW-FLY 139 



finally almost black. Within this hard and dry skin the larva 

 changes to a pupa, and the pupa to a fly. If the pupating 

 insect is examined when the pupa-case is red-coloured, it will 

 be found to contain a creamy fluid, with little 

 visible structure. By the help of carefully pre- 

 pared microscopic sections, it has been discovered 

 that in this stage the larval organs break up, 

 and new (imaginal) organs take their place. The 

 destruction of the larval structures is effected 

 by the activity of peculiar blood corpuscles 

 (phagocytes) which eat up the muscles, epi- 

 thelial cells, etc., and ultimately transfer their 

 own substance to the growing tissues. The 

 new organs of the fly are developed from special 

 cellular patches, forming part either of the epi- 

 dermis, or of the epithelium of the internal 

 cavities. These centres of development arise 

 by infolding of the epidermis or epithelium. e"nd'osi'ngt^h^"true 

 Such pouches or infoldings can be seen in a ^"jP^'g ^"^f '=°|,"j 

 simpler form in late larvse of such insects as 'a/vai skin, mag- 

 Chironomus, Lepidoptera, etc., 

 where the larval legs are wanting, or too 

 short to receive the much longer legs of 

 the pupa and imago, the bases of which, 

 as they grow, become telescoped into 

 the body, while the tip may be still en- 

 closed within the larval legs. Should the 

 legs, wings, mouth-parts, and other similar 

 organs be almost or altogether undeveloped 

 in the larva, but large and complex in 

 the fly, the infoldings,' become deeper still, 

 and seem to lose all connection with the 

 epidermis or epithelium to which they 

 belong. This is the case with the blow- 

 fly. The larva has, at most, insignificant 

 rudiments of eyes, feelers, mouth-parts, 

 wings, and legs, while all are of elaborate 

 structure in the fly. Moreover, the 

 alimentary canal and other internal organs require to be 

 almost completely regenerated during the pupal stage of the 

 blow-flyr The new imaginal structures develop from centres 



Fig. 79. — Pupa of 

 blow - fly, removed 

 from pupariuin, mag- 

 nified . 



