24 



the larvae are said to be coarctate. Chapman has pointed out that 

 although there is apparently no near relationship between the Coleop- 

 tera and Hymenoptera, they are very much alike in shape and general 

 appearance, whilst both are helpless from their quiescence, and have 

 resorted to the formation of a cell or cocoon for their protection. 

 The general similarity between the " free " pupae of certain Lepidop- 

 tera and the Trichoptera has been one of the factors in leading us to 

 assume a close affinity between these orders. Poulton has demonstrated 

 that the appendages of the lepidopterous pupa are not merely cases for 

 the homologous structures of the imago, and he has further shown that 

 there is often in the pupal structure no corresponding imaginal organ 

 until shortly before the emergence of the imago. In some pupae the 

 imaginal organs are formed long before the emergence of the imago ; 

 in others their formation is delayed until a very short time before 

 emergence, development then proceeding with great rapidity. 



We have already stated that the early entomologists believed that 

 the " encasement " theory of Swammerdam was the true explanation 

 of the metamorphosis of insects, and it was not until Herold, in 1815, 

 objected to some of the views that the theory involved, that any real 

 doubt as to its correctness was expressed. Malpighi, as early as 

 1667, noticed in Bombyx mori that just before pupation the antennae 

 could be seen concealed in the head of the larva, and that the legs of 

 the imago grew in those of the larva. Swammerdam, by throwing a 

 full-fed caterpillar, just on the point of pupation, into boiling water, was 

 able to show, by stripping off the skin, the immature form of the 

 imago. 



Reaumur in 1734, and Lyonet in 1760, both detected the organs in 

 the process of formation ; yet they did not appear to suppose that the 

 organs were being formed, they only looked upon them as being 

 liberated. Herold, however, in 181 5, showed that the wings did not 

 become visible in the lepidopterous larva until the very end of larval 

 life ; and that, as the larval organs disappeared, they were replaced by 

 entirely new organs. In Herold's view the phenomena of metamor- 

 phosis were to be explained by the assumptions that the accumulation 

 of fatty material went on in the larval state until the imago had 

 attained its full dimensions or volume ; that it then became a pupa, and 

 in this stage the organs were developed and took on their definite 

 form. Kirby and Spence, in 1828, however, maintained the old views, 

 and state that " a caterpillar is not a simple animal, but a compound 

 one," and further, that " a caterpillar, at first scarcely as large as a bit 

 of thread, contains its own teguments threefold, and even eightfold, in 

 number, besides the case of a chrysalis and a complete butterfly, all 

 lying one beside the other." Of course there is some truth, mixed up 

 with much error, in the views propounded by Kirby and Spence and 

 those asserted by Herold. 



In 1864 Weismann discovered the germs of the imago (imaginal 

 buds) when he was at work on certain Muscidae, and from these obser- 

 vations deduced his theory of "histolysis," i.e. the complete destruction 



