ENTOMOLOGY 19 



within soon becomes too small, and the larva has to throw off this 

 old shell and replace it by a new and more commodious one. This 

 action of throwing off the old shell is called moulting, and the proc- 

 ess has to be repeated a number of times before the larva reaches 

 its full size. During the larval existence of an insect there is stored 

 up all the material required to produce wings and organs of repro- 

 duction, as well as to transform the other organs, as eyes, legs, etc., 

 into their final shape. When fully grown, the larva is transformed 

 into the third stage, or pupa (chrysalis, nymph). In this stage the 

 insect is usually quiescent, at least apparently so, though in reality 

 it is a very active stage, as the most wonderful changes have to take 

 place inside the stiff and rigid pupal shell, and frequently within a 

 very short period. After a certain time the skin of the pupa breaks 

 open, and the fourth and final stage or imago appears, ready to per- 

 form all the functions of a winged, sexual insect. Although these 

 transformations seem to be very sudden, they are really nothing but 

 continuous growth, arrested at intervals by the inflexibility of the 

 outer skeleton. The metamorphoses of insects vary very much, and 

 serve as the basis for separating all insects into two groups, those with 

 a complete metamorphosis, as described above, and those with an in- 

 complete one. A complete metamorphosis is one of the most won- 

 derful transformations known to natural history. From an egg 

 hatches a worm-like creature, always hungry, growing rapidly until 

 its full size is attained, when it suddenly stops feeding, and changing 

 to an apparently lifeless object, becomes a pupa. Remaining almost 

 motionless in this condition it breaks open and gives forth a much 

 larger being, possessing many organs not found before in the earlier 

 stages, and able to fly about to mate and deposit again eggs. In a 

 complete metamorphosis the different stages such as egg, larva, 

 pupa and imago do not resemble each other at all. In an incom- 

 plete metamorphosis we have no such notable changes of form. The 

 egg hatches into a being that looks very much like the parent, being 

 of course quite small, and lacking all traces of wings or sexual 

 organs. This larva feeds just as ravenously, and has in conse- 

 quence of its rapid growth also to moult a number of times, and 

 during these slight changes in size it acquires gradually rudimentary 

 wings, which increase in size until the adult stage has been reached. 

 But during this whole period of growth no quiescent state like that 

 of a true pupa appears, and the young insect resembles its parent 

 throughout the period of growth. Butterflies are a good illustration 

 of a complete metamorphosis, and locusts of an incomplete. 



The mouth-parts of insects give us also an excellent means for 

 classifying them into three groups. One group possesses a biting 

 and sucking mouth ; the second one contains insects which chew their 

 food by means of a pair of horny jaws acting in a horizontal direc- 

 tion; the third group possesses apparently no jaws, and the species 

 belonging here are sucking insects. They obtain their food by pierc- 

 ing and sucking by means of four bristles enclosed in a jointed beak, 

 or fluid food by means of a long and flexible tongue. 



