47 



holometabolous orders of insects. The word literally means a 

 " mask," and was applied to the caterpillars of butterflies and moths 

 by the old philosophers, because they considered that the larva 

 masked the later stages ; that, in fact, the larval skins enveloped the 

 pupa, and this the imago, and that having shed its normal number 

 of skins, the imago, always present within the larva and pupa, was 

 liberated. To a certain extent this idea of the old naturalists was 

 accepted until comparatively recently, but was completely overthrown 

 by Weismann's discovery of the imaginal discs in the larva and the 

 mode of their development. This discovery has, of course, com- 

 pletely changed our ideas of the nature of metamorphosis, and revo- 

 lutionised our knowledge of the fundamental processes concerned in 

 the change from larva to pupa, and from pupa to imago. 



We have already referred to the periodical moults or sheddings of 

 the skin that larvae undergo. Previous to exuviation taking place the 

 larva indulges in a prolonged rest ; later the old skin splits in the 

 region of the head, and gradually, and usually without any apparent 

 effort on the part of the larva, the skin slips back over segment after 

 segment until it is completely cast. With the exception of a series 

 of almost imperceptible vermiform movements of the segments the 

 larva appears to be taking no active part in the process, the skin 

 slipping off as the larva advances much like a glove is slipped off the 

 finger. There is a fluid circulating between the two skins just 

 previous to the moult, and this acts as a lubricant during the process, 

 and there is no doubt that the development of the hairs and other 

 cutaneous structures beneath the old skin facilitates the change. 

 When the brva has just completed a moult its skin is exceedingly 

 soft and delicate, but the latter soon hardens. Not only is the in- 

 tegument shed with its hairs and setae, but linings of all internal 

 organs that have had an ectodermal origin (e. g. the trachea?, mouth 

 lining, and part of the alimentary canal) are cast with it. In the 

 apodous larvae of certain Hymenoptera, bees, &c, the delicate skin 

 is not shed whole, but breaks away in shreds or fragments during the 

 process of moulting. The old notion that the larva shed its skin 

 because it was no longer large enough for the growing animal is not 

 now accepted. There can be little doubt that moulting is really an 

 excretory process, by means of which the accumulation of waste 

 matters is periodically got rid of. 



It is necessary now to return to the subject which we recently left, 

 viz. that relating to the actual development of the imago of the 

 Holometabolic insects. Herold was the first author who objected to 

 the erroneous theory, held by Swammerdam and others, that the 

 newly-hatched larva had in it at the time of birth all the parls of the 

 larva, pupa, and imago, each of which became visible in turn at 

 every subsequent moult. As we have before mentioned, however, 

 it was not until the real nature of the internal changes was dis- 

 covered and explained by Weismann that any radical difference of 

 opinion as to the nature of metamorphosis was accepted by natu- 



