METAMORPHOSIS OF INSECTS. 



353 



dl... 



Most caterpillars moult four or five times ; at each 

 moult the outer layer of the skin is cast off, the new 

 skin arising from the hypodermis, or inner layer of the in- 

 tegument. The skin opens on the back behind the head, 

 the caterpillar drawing itself out of the rent. In the 

 change from the caterpillar to the chrysalis, there are re- 

 markable transformations in the muscles, the nervous, 

 digestive, and circulatory system, inducing a change of 

 form, external and internal, characterizing the different 

 stages in the metamorphosis. 



While the changes in form are 

 comparatively sudden in flies and 

 butterflies, the steps that lead to 

 them are gradual. How gradual 

 they are may be seen by a study of 

 the metamorphosis of a bee. In 

 the nest of the humble or honey 

 bee, the young may be found in all 

 stages, from the egg to the pupa 

 gayly colored and ready to emerge 

 from its cell. It is diflicult to 

 indicate where the chrysalis stage 

 begins and the larva stage ends, 

 yet the metamorphosis is more 

 complete — namely, the adult bee 

 is more unlike the larva, than in 

 any other insect. 



Besides the normal mode of de- 

 velopment, certain insects, as the 

 plant-louse (Aphis), the bark-louse 

 (Coccus), the honey-bee, the Po- 

 listes wasp, the currant saw-fly (Nematus), the gall-flies, 

 and a few others, produce young from unfertilized eggs. 

 Certain moths, as the silk-worm moth (Bomlyx mori) and 

 others, have been known to lay unfertilized eggs from which 

 caterpillars have hatched. This anomalous mode of repro- 

 duction is called parthenogenesis, and fundamentally is only 

 a modification of the mode of producing young by budding 

 which is universal in plants, and is not unusual, as we have 



Fig. 303. —Embryo of the Louse. 

 am, serous membrane; rf6, amnion; 

 as, antennte ; vk forehead. — After 

 Melniliow. 



