648 CLASS IV. INSECTA. 



somites (Fig. 407). They later shift into the lumen of the sac, 

 the dorsal wall of which forms the terminal filament of the 

 reproductive gland. The cells from neighbouring sacs ultimately 

 become contiguous and the investment of the gonad is derived 

 from adjacent mesodermic cells. 



Metamorphosis. The egg of an Insect gives rise to a creature 

 which always differs more or less from the adult, and in 

 many cases the difference is so great, that, unless we 

 knew their life-history, we should hardly class the larvae 

 amongst the Insecta. The typical number of segments for the 

 species is however always found in the larva, so the difference 

 between young and old is not so marked as between such a larva 

 as a nauplius and the form it gives rise to. In many orders, 

 which in certain features of the development most closely re- 

 semble the Myriapods, the change from larva to adult is com- 

 paratively gradual. The animal undergoes a series of ecdyses 

 and after each casting of the skin it approximates a little more 

 closely to the final form. Such a series of small steps is 

 termed incomplete metamorphosis. In the remaining orders, 

 the larva differs more or less markedly in structure and in 

 habit from the adult. 



It has been thought that in some groups such as the Thysanura 

 no metamorphosis whatever occurs, and that the young are 

 simply miniature copies of the adults, and merely increase in size 

 during development. These insects were called Ametabola. 

 But Heymons * has recently shown that there is a slight 

 metamorphosis even in some of the most primitive Thysanura. 

 He concludes that apart from a few exceptional cases no such 

 thing as true ametaboly exists in the Insecta, but that all un- 

 dergo some metamorphosis. He proposes the division of the class 

 into two categories. f 



(i) Epimorpha, in which the young differ from the adults 



* Heymons, t)ber die ersten Jugendformen von Machilis alternata Silv., 

 S. B. der Ges. Naturforsch. Freunde, Berlin, 1906 (Dec.), pp. 253-256. 

 Heymons' views are elaborated in an important paper in Spengel's Er- 

 gebnisse und Fortschritte der Zoologie, Jena, i, 1907, pp. 137-188. 



f Insects are also often divided into forms with incomplete metamor- 

 phosis or Homomorpha, and those with complete metamorphosis or 

 Heteromorpha. The correspondence of these groups with those adopted 

 by Heymons is shown thus : 



' Epimorpha 



Homomorpha . -j Hemimetabola | 

 Heteromorpha = Holometabola /Metamorpha. 



