THE YOUNG INSECT 2:j 



that just before the eicada nymph is ready to assume the 

 winged state, it ascends to the surface of the ground, and 

 there often constructs a pillar-like cell of mud in which 

 it passively awaits its final transformation. This appears 

 to be a purely voluntary act, for the faculties of the 

 insect remain quite unimpaired. Indeed, many individuals 

 never build earth-cells at all, but continue their under- 

 ground life until the last moment, and climb up the trunk 

 of a tree just before the skin splits down the back, allow- 

 ing the adult insect to issue forth. 



The case of the cicada is especially interesting, because 

 we seem to see in it the dawning of the pupa habit, 

 though no pupal form is assumed. Another step in the 

 evolution of metamorphosis is exemplified by the tiny 

 black insects known as thrips, which are often so numerous 

 in flowers. In its last stage the thrips nymph, though it 

 moves, is sluggish, while its limbs are enveloped in a 

 membrane and its wings enclosed in sheaths. Finally, 

 among the scale insects, we come upon a kind of connect- 

 ing link between incomplete and complete metamorphosis. 

 These insects get their name from the fact that many 

 of the species exude a waxy secretion from their bodies 

 which hardens into a protective scale, or shell. They have 

 sucking mouth-parts, and many of them are very injuri- 

 ous to vegetation. Perhaps the most familiar species in 

 England is the so-called mussel scale, or bark louse, which 

 may often be found in myriads upon the stems and 

 branches of apple trees in neglected orchards. While the 

 female insects, which are wingless and strangely degraded 

 in form, undergo a typically incomplete metamorphosis, 

 the winged and active males pass the last stage of their 

 development in a passive, pupa-like condition, enclosed 

 in a waxy cocoon. 



Because the life-cycle of any one insect is really an 



