494 



purely larval cells occurs during the feeding period, and there 

 is a great growth in size. At metamorphosis some of the more 

 specialized tissues (muscles) are phagocytised (evidently after 

 dying); the cells of the less specialized tissues (hypoderm, 

 oesophagus, rectum) cast out parts of their substance, and 

 having evidently rejuvenated, remain as the adult tissues. In 

 the case of the other organs, those of the imago are formed 

 from arep-s of cells which till now have lain dormant — the 

 imaginal discs. 



(5) Le'pidoptera. — Larvae emerge with typical insect head 

 and mouth appendages. Legs present though very reduced. 

 The larval cells divide just before the various moults during 

 the larval period. The larval tissues seem to disappear by 

 phagocytosis and redevelop from imaginal discs. Very little 

 is known, however, about the Lepidopteran metamorphosis. 



(6) Muscidae (Weismann, Kowalevsky, Van Rees, Perez, 

 and others). — The larva emerges as a legless maggot, with 

 reduced spiracles, and with poorly developed mouth append- 

 ages. Except for a tracheal proliferation cell division does 

 not occur during larval life. At the end of the feeding period 

 the larval cells die and are phagocytised; the imago develops 

 from imaginal discs of scattered imaginal cells. 



(7) Chalcid Wasps (type: Nasonia). — The larva hatches 

 in a very primitive condition; the head is still in a biseg- 

 mented state; the only mouth appendages present are rudi- 

 mentary mandibles ; malpighian tubes have not yet developed ; 

 the proctodaeal invagination has not yet even opened into 

 the archenteron. There is no cell division during larval life, 

 except possibly in the tracheal system (most of the apparent 

 proliferation here is, however, due to growth in cell size). 

 After three days all the larval tissues — even the almost in- 

 significant peritoneal membrane — ^having grown in cell size, 

 die. Some are removed by phagocytes, others dissolve in the 

 blood. The imago develops from imaginal discs or scattered 

 imaginal cells. 



It seems to follow from the above account that a meta-^ 

 morphosis^ — ^a period of more or less violent transformation in 

 contrast with "normal" progressive development — occurs in 

 its simplest form in the dragon-flies. No definite pupa is 

 formed here, and the insect, though restless and refusing to 

 feed, is never helpless. But in all the others the internal 

 changes are of a much more violent nature, amounting at 

 times to an absolute death — ^in the fullest sense of the word — 

 of the larva. If imaginal tissues were not present the 

 "corpse" would be left to decompose; the imaginal cells, 



