THE METAMORPHOSIS OF INSECTS 181 



stars of insects with a complete metamorphosis; and in this sense 

 only is it used in this book. 



The adaptive characteristics of larvae. The larvae of insects with 

 complete metamorphosis, like the naiads of those with incomplete 

 metamorphosis, exhibit an acquired form of body adapting them to 

 special modes of life; and in this case the cenogenetic or "sidewise 

 development" is much more marked than it is in insects with an 

 incomplete metamorphosis. Here the form of the body bears but little 

 n-lation to the form to be assumed by the adult, the nature of the 

 larval life being the controlling factor. 



The differences in form between larvae and adults are augmented 

 by the fact that not only have larvae been modified for special modes 

 of life, but in most cases the adults have been highly specialized for a 

 different mode of life; and so great are these differences that a 

 quiescent pupa stage, during which certain parts of the body can be 

 made over, is necessary. 



Here, as in the case of insects with an incomplete metamorphosis, we have an 

 illustration of the fact that natural selection can act on any stage in the develop- 

 ment of animal to better adapt that particular stage to the conditions under which 

 it exists. Darwin pointed out in his "Origin of Species" that at whatever age 

 a variation first appears in the parent it tends to reappear at a corresponding age 

 in the offspring. This tendency is termed homochronous heredity*. 



It is obvious that the greater the adaptive characteristics of the immature 

 forms, the less does the ontogeny of a species represent the phylogeny of the 

 race to which it belongs. This fact led Fritz Muller, in his "Facts for Darwin", 

 to make the aphorism "There were perfect insects before larvas and pupae." The 

 overlooking of this principle frequently results in the drawing of unwarranted con- 

 clusions, by those writers on insects who cite adaptive larval characteristics as 

 being more generalized than the corresponding features of the adult. 



The more obvious of the adaptive characteristics of larvae are the 

 following : 



The form of the body. As indicated above the form of the body of a 

 larva bears but little relation to the form to be assumed by the adult, 

 the nature of the larval life being the controlling factor in determining 

 the form of the body. As different larvae live under widely differing 

 situations, various types of larvae have been developed; the more 

 important of these types are described later. 



The greater or less reduction of the thoracic legs. In the evolution 

 of most larvae there has taken place a greater or less reduction of the 

 thoracic legs; but the extent of this reduction varies greatly. The 

 larvae of certain Neuroptera, as Corydalus for example, have as perfect 



*Hom6chronous : homos (6/xo'v), one and the same; chronos (XP^OS), time. 



