322 



Intkoduction. 



The insect transformation presents one of the most in- 

 teresting of the many phenomena of living things about us. 

 To the popular imagination it is a manifestation of the super- 

 natural. To the biologist it offers unrivalled material for 

 the study of several fundamental tissue reactions : extensive 

 tissue degenerations followed by correspondingly great tissue 

 regenerations; delayed cell differentiation and cell regenera- 

 tion, and sometimes even, it seems, cellular dedifferentiation ; 

 while the cases of phagocytosis at times met with are extra- 

 ordinary. Nevertheless, its study has been very neglected. 



Numbers of the great early anatomists — Malpighi, 

 Swammerdam, Lyonet, Diirckheim — turned their attention 

 to the structure of insects, and though they were able to show 

 that the larvae of insects had already the same general 

 anatomy as had the adult insects, yet the difficulties of the 

 dissection of the soft semi-fluid contents of the pupal shell 

 proved so great, that the process of transformation was not 

 elucidated. 



Reaumur, it is true, had been able to show that the limbs 

 of the adult insect were to be found invaginated beneath the 

 surface of the body of the nymph. Newport (1832) had 

 observed the concentration of the ganglia of the ventral 

 nerve cord as it changed from the larval to the imaginal 

 condition, but beyond these facts nothing was known; and 

 Oken, who wrote his voluminous "Allgemeine Natur- 

 geschichte" at about this time (1836), summarized his know- 

 ledge of the process thus (vol. 5, p. 714): — ''At the last 

 moult the insects become covered by a homy shell, which is 

 devoid of feet and oral appendages. Consequently in this 

 stage they lie quiet for several weeks, often throughout the 

 whole winter, without feeding or moving, and in this con- 

 dition are spoken of as pupae or nymphs. Under this shell 

 is gradually formed the perfect insect, the fly with its three 

 body parts, with its new feeding organs, feet and wings; 

 finally the skin splits dorsally, the insect creeps out, waits a 

 few minutes till it has hardened, and then crawls or flies away, 

 to seek other food or to reproduce. This gradual step-like 

 development is spoken of as a transformation or meta- 

 morphosis." 



It was not till the publication in 1864 of Weismann's 

 great memoir on the metamorphosis of the blow-fly that any 

 light was thrown on the process. Weismann, without any 

 modern technique available to him, and using only the old 

 method of hand dissections, studied the process with remark- 

 able accuracy. His observations were made more on broad, 

 general, anatomical lines. He was able to show that the 



