THE METAMORPHOSES OF THE RESPIRATORY SYSTEM. S3 



known ; but the indisposition of their alHes, the grasshoppers, to 

 fly for any distance, is readily to be noticed by any one walking 

 in the fields. They will jump and fly for a few yards, but it is 

 very unusual to see them doing anything more. How, then, is 

 it that these heavy and iMt over-active insects can occasionally 

 maintain a prolonged and elevated flight ? Their respiratory 

 system explains the apparent anomaly. When they are quiet 

 their trachcje are as flat as ribbons, and contain but little air. 

 On departing for their long voyage, however, the locusts, like the 

 cockchafers, blow themselves full of air, and their bodies become 

 very light in relation to the bulk of air they replace. More- 

 over, the muscular efforts of prolonged flight produce a certain 

 amount of animal heat, and this rarefies the air contained in 

 the vesicles and tracheae ; so that the principles of Montgolfier's 

 balloon were foreshadowed in the insect long before they were 

 elaborated by man. 



The vesicles arc evidently superadded to the tracheal of the 

 larva and caterpillar, and their growth during metamorphosis is 

 accompanied by a prolongation of the s)'mpathetic nerve over 

 them. 



There are two kinds of transformations noticed in the insects 

 or the Articulata, the complete and the incomplete, and, besides, 

 there are hyper and retrograde metamorphoses. The stages or 

 phases of development are usually three in number, but there are 

 four in some instances, and a great many more when skin-shedding 

 is accompanied by contemporaneous internal structural changes. 

 .Some Articulata do not undergo any metamorphosis. 



The vast majority of the articulate animals bring forth eggs 

 which contain the embryo ; this becomes, after hatching, the larva, 

 caterpillar, grub, or maggot; and either the larva differs consider- 

 ably from the embryo, or it is so much like it as to merit the 

 term of hatched embryo. But a few of the Articulata produce 

 living larvae, the process of hatching going on within the body of 

 the mother, which is then said to be ovoviparous ; or the young 

 are developed within the parent more as offshoots of parts of her 

 tissues than as the contents of eggs. The union of the males and 

 females before every production of eggs is found to occur in tnc 

 great majority of instances, but there arc some insects which 



