534 The Outline of Science 



maggots, and various grubs, and these may be more sedentary 

 in habit. In the course of the life-history of many insects a 

 marked change of form takes place metamorphosis. According 

 to the degree of metamorphosis, insects are divided into three 

 groups : ( 1 ) When no metamorphosis occurs and the young are 

 hatched as miniatures of the adults, e.g. the most lowly insects, 

 the Springtails and Bristle-tails. (2) An intermediate group 

 comprises those insects which show partial metamorphosis. In 

 this type the insect is able to move and feed practically through- 

 out its development; the change is a gradual one. Through a 

 series of moults, made necessary by the inexpansible armour of 

 chitin, the insect reaches the adult condition. 



For instance, the young Locust, as it emerges from the egg, 

 has a pale, soft body swathed in transparent skin. It sheds its 

 mantle, and, gaining strength in the sunlight, becomes firm and 

 black, only differing from its parents in size, colour-markings, 

 and the absence of wings. It feeds hungrily on vegetable sub- 

 stances, and grows and moults, each moult leaving it larger, 

 brighter, and hungrier than before, until after the third moult its 

 wings begin to show. The moulting process lasts only about half- 

 an-hour, and the locust only stops feeding for a few hours. No 

 phase of torpor or quiescence occurs in this "half -metamorphosis" 

 type, and after the fifth skin-casting the locust is a perfect, 

 winged insect, soft and helpless and very vulnerable for a time, 

 but rapidly regaining firmness and vigour. 



(3) When complete metamorphosis occurs, a quiescent 

 pupal or chrysalis stage comes between the larval and adult 

 stages. Growth occurs during the larval stage, a period of 

 voracious feeding, rapid growth, and numerous moults. The 

 larva eats far more than is necessary to maintain its life, and lays 

 up a reserve store which provides for the resting pupal stage 

 which follows. The pupal stage is a time of little or no external 

 activity but great internal changes. The larval tissues are broken 

 down and their substance is reconstructed into the very different 



