680 TEXT-BOOK OF ENTOMOLOGY 



hypodermis is accomplished under the influence of the leucocytes 

 (Fig. 632, fr), which attack the larval hypodermis-cells and absorb 

 their contents piece by piece, and so till themselves with bits of the 

 hypodermis-cells and their nuclei ; since these fragments have the 

 shape of roundish granules, they were called by Weismann granule- 

 balls. These granule-balls, which fill the body-cavity of the later 

 pupal stage, are nothing else than the leucocytes (blood corpuscles) 

 which have absorbed the fragments of tissue of the larval body. 



It should here be said that the destruction of the larval tissues is 

 not to be attributed to the previous death of the cells, but is the 

 result of the action of the leucocytes on tissues which, though 

 weakened in their vital power, are still living. While the com- 

 pletely healthy, active tissues, i.e. those of the imaginal buds, with- 

 stand the attacks of the leucocytes, the less healthy larval tissues 

 are by the attacks of the leucocytes divided into fragments and eaten 

 and digested by them. This process is most marked in the his- 

 tolysis of the larval muscles. The destruction of most of the larval 

 organs depends, therefore, on the capacity of the amoeboid blood-cor- 

 puscles for taking food and on intracellular digestion, as was first 

 shown by Metschuikoff, who has given to these leucocytes the name 

 of " phagocytes." 



This process of histolysis goes on in the same way in the head and 

 abdomen as in the thorax. In the abdomen, as Ganin first proved, 

 there are in each of the eight segments of which it consists in the 

 larva four small cellular islets or imaginal buds (Figs. 631, hi, 

 632, f), from which originate the new hypodermis. 



Van Rees has lately found in the abdominal segments another 

 pair of smaller imaginal buds. The four imaginal buds occurring in 

 the last segment are situated close to each other, encircling the anal 

 opening (Fig. 633, ims), and take part in the formation of the hind- 

 intestine, the rectal poaches and rectal papillae. To this segment 

 also belong the two pairs of imaginal genital buds (rudiments of 

 the external sexual organs) which were first found by Kiinckel 

 d'Herculais in Volucella. 



The newly formed hypodermis spreads rapidly over the outer sur- 

 face of the body, so that hypodermal areas corresponding to the 

 separate imaginal buds soon unite. Simultaneously with this com- 

 pletion of the definite epithelial layer the larval hypodermis becomes 

 completely destroyed by the phagocytes. 



The muscles. A similar process of destruction by phagocytes 

 affects the greater number of the larval muscles, except the three 

 pairs of thoracic muscles employed in respiration, and which pass 



