CHAPTER V 



DEVELOPMENT 



EMBKYOLOGY EGGS MICROPYLES FORMATION OF EMBRYO VEN- 

 TRAL PLATE ECTODERM AND ENDODERM SEGMENTATION- 

 LATER STAGES DIRECT OBSERVATION OF EMBRYO METAMOR- 

 PHOSIS COMPLETE AND INCOMPLETE INSTAR HYPERMETA- 



MORPHOSIS METAMORPHOSIS OF INTERNAL ORGANS INTEGU- 

 MENT METAMORPHOSIS OF BLOWFLY HISTOLYSIS IMAGINAL 



DISCS PHYSIOLOGY OF METAMORPHOSIS ECDYSIS. 



THE processes for the maintenance of the life of the individual 

 are in Insects of less proportional importance in comparison with 

 those for the maintenance of the species than they are in Verte- 

 brates. The generations of Insects are numerous, and the in- 

 dividuals produced in each generation are still more profuse. 

 The individuals have as a rule only a short life ; several successive 

 generations may indeed make their appearances and disappear 

 in the course of a single year. 



Although eggs are laid by the great majority of Insects, a 

 few species nevertheless increase their numbers by the production 

 of living young, in a shape more or less closely similar to that 

 of the parent. This is well known to take place in the Aphi- 

 didae or green-fly Insects, whose rapid increase in numbers is 

 such a plague to the farmer and gardener. These and some other 

 cases are, however, exceptional, and only emphasise the fact that 

 Insects are pre-eminently oviparous. Leydig, indeed, has found 

 in the same Aphis, and even in the same ovary, an egg-tube 

 producing eggs while a neighbouring tube is producing vivi- 

 parous individuals. 1 In the Diptera pupipara the young are 

 1 Ada. Ac. German, xxxiii. 1867, Xo. 2, p. 81. 



