458 



be detected. Tliere is no evidence, however, that any rupture 

 of the cell walls ever takes place. Between these fat cells 

 lie the larval tracheoles, and the chemical disintegration 

 rather than phagocytosis of these, in places which are usually 

 quite inaccessible to the phagocytes, is readily understood. 



The fat cells anterior to and above the brain are in the 

 form of a single layer of cells; during pupal life they are 

 often to be observed showing rhythmic movements, due, un- 

 doubtedly, to the contractions of the heart. The fat cells in 

 the postero-ventral part of the head cavity are much more 

 numerous; as in the thorax (alitrunk) they are loosely dis- 

 posed and cell walls are always clearly visible. 



From the late larva till the time of death of the insect 

 the fat-body undergoes a gradual degeneration and absorption, 

 and, although it is quite probable that the fat- body stores 

 up reserve products as the imago feeds, yet at no time is 

 there to be observed in Nasonia a formation of new fat cells; 

 the same individual fat cells which have persisted in the 

 senescent imago occurred already in the first larval instar. 

 It is this gradual degeneration that I shall here describe. 



In the larva at about the time of defaecation many of 

 the larger grains of storage material within the fat-body begin 

 to develop very minute crystals within them, and sometimes 

 quite large n.umbers of these may be present, all within a 

 single grain (fig. 92). In the small, more centrally situated, 

 and eosinophilous grains these crystals are not to be seen, but 

 frequently contain small chromatic granules, probably the 

 pseudonuclei of Berlese. But these crystals do not, as a 

 rule, persist long within the grain; already in larvae several 

 hours after defaecation they are no longer to be seen. Even 

 the chromatic granulations of the small grains seem to dis- 

 appear in the early pupa. Sometimes, however, crystals are 

 visible as late as several hours after pupation. 



The nuclei of the fat cells assume curious appearances 

 towards the end of larval life. The heavily granulated 

 structure and general compactness of the nucleus is lost, and 

 it may become finely granular and slightly branched, while 

 at other times it elongates greatly and stretches as a great 

 dumb-bell-shaped band almost from one side to the other of 

 the fat cell (fig. 92). At no time have I observed nuclear 

 division. 



However complex may be the changes going on within 

 these cells as they gradually liberate their storage substances 

 and then die, the processes which are to be observed micro- 

 scopically are easily described. The fat globules and storage 

 grains begin to decrease in number, at first slowly, then 

 rapidly. In the head of the three-day pupa the fat cells 



