54 CLASSIFICATION OF INSECTS. 



from X. to XVI., which we shall also speak of at times 

 as the second series of orders. These divisions are 

 convenient and avoid the use of the terms *' lower " 

 and "higher" orders, which have seemed to us ob- 

 jectionable for reasons that will appear in the text. 



The common ancestor of the Thysanura and all 

 other insects was, therefore, probably distinct from 

 anything now living, but, nevertheless, possessed cer- 

 tain common characters with the adults of Lepisma and 

 Campodea, and with the larvae of other orders, all of 

 which still continue to inherit to a greater or less 

 extent some of its peculiarities. If all the evidence 

 from these sources be brought together, the known 

 laws of heredity justify the naturahst in asserting that 

 this ancestor must have been devoid of scales, a 

 naked, wingless, six-footed, active insect, with -only 

 slight differentiation between the three regions of the 

 body, and having three simple thoracic rings of nearly 

 the same size, which were not subdivided by second- 

 ary sutures. It was this generalized common form, 

 which has been inherited by the Thysanura, and by 

 the larvae of other insects, and not the exact form 

 and pecuHarities of Lepisma or Campodea, a fact 

 clearly pointed out by Packard in Our Common In- 

 sects. These two genera are simply the closest copies 

 of the ancestral form now in existence ; and we have, 

 therefore, following after Packard, employed the term 

 " Thysanuriform," ^ rather than " Campodeaform " or 

 " Lepismaform," to designate the larvae which repeat 

 ancestral characteristics. 



1 Third Rep. U. S. Ent. Com., 1883, " Genealogy of Insects," 

 p. 297, note. 



