12 STRUCTURE OF THE LEPIDOPTERA 



of insects are, and to what extent these metamorphoses render them 

 fit for the work they have to do. 



It is certain that the chief work of insects, taken as a whole, is 

 to remove from the earth the excess of animal and vegetable matter. 

 If they are to do this work effectually, it is clear that they must 

 be very voracious feeders, and also be capable of multiplying their 

 species prodigiously. Now each of these powers requires the special 

 development of a certain set of organs, and an abnormal develop- 

 ment of one set must necessarily be produced at the expense of the 

 other. Hence we find insects existing in two distinct stages, with 

 or without an intermediate quiescent state, during the first of which 

 the digestive apparatus is enormously developed, while the repro- 

 ductive organs occupy but very little space ; then, during the other 

 stage, the digestive apparatus is of the simplest possible de- 

 scription, and the organs of reproduction are in a perfect state of 

 development. 



Allowing, then, that the chief work of the insect is the removal 

 of surplus organic matter, we can see that a large share of its life 

 should be spent in the larval or grub stage, and that the perfect 

 state need not occupy any more time than is necessary for the 

 fertilisation of the eggs that almost completely fill the body of the 

 female at the time of her emergence from the chrysalis shell. 



Many insects undergo their metamorphoses by slow degrees, 

 but the Lepidoptera, after existing for some considerable period 

 without any important visible change in structure, pass by a rapid 

 transition into the next state. Thus, a caterpillar, that has not 

 altered in general form for several weeks, changes into a chrysalis 

 within the course of a few days ; and again, after a period of quies- 

 cence that may extend throughout the whole of the colder months, 

 becomes a perfect butterfly or moth within twenty minutes of the 

 moment of its emergence. 



But this suddenness is more apparent than real, as may easily be 

 proved by internal examinations of the insect at various stages of 

 growth ; showing that we are led astray by the rapidity of external 

 changes the mere moulting s or castings of the skin while the 

 gradual transformations proceeding within are not so readily 

 observed. 



We have already said that the life of the perfect butterfly or 

 moth is short. A few days after emergence from the chrysalis case, 

 the female deposits her eggs on the leaves or stems of the plant 

 that is to sustain the larvae. Her work is now accomplished, and 



