ORIGIN OF THE TRACHEA. 



171 



flies, the Psocus, the Chrj'sopa, the lice or parasitic Hemiptera, 

 together with Thrips, thus forming the establishment of lines 

 of development leading up to those Neuroptera with a complete 

 metamorphosis, and finally to the grasshoppers and other forms 

 of Orthoptera, together with the Hemiptera. 



We have thus advanced from wingless to winged forms, i. e., 



199. Chrysopa. 200. Panorpa. 



from insects without a metamorphosis to those with a partial 

 metamorphosis like the Perlas ; to the May flies and Dragon 

 flies, in which the adult is still more unlike the larva; to the 

 Chrysopa (Fig. 199) and Forceps Tails (Panorpa, Fig. 200) and 

 Caddis flies, in which, especially the latter, the 

 metamorphosis is complete, the pupa being 

 inactive and enclosed in a cocoon. 



Having assumed the creation of our Leptus 

 by evolutional laws, we must now account for 

 the appearance of tracheae and those organs so 

 dependent on them, the wings, which, by their 

 presence and consequent changes in the struc- 

 ture of the crust of the body, afi"ord such dis- 

 tinctive characters to the flying insects, and 

 raise them so far above the creeping spiders, 

 and centipedes. Our Leptus at first undoubtedly 

 breathed through the skin, as do most of the 

 Poduras, since we have been unable to find 

 tracheae in them, nor even in the prolarva of a 

 genus of minute ichneumon egg parasites, nor 

 in the Linguatulae and Tardigrades, and some 

 mites, such as the Itch insect and the Demodex, 

 and other Acari. In the Myriopod, Pauropus, 

 Lubbock was unable to find any traces of tra- 

 cheae. If we examine the embryo of an insect shortly before 

 birth, as in the young Dragon fly (figure 201, the dotted line 

 t crosses the rudimentary tracheae), we find it to consist of 



201. Embryo of 

 Diplax. 



