I recommend: Grimaldi, David. 2005. Evolution of the Insects. Cambridge University Press.

Reduction of leg number is probably related to a Hox gen mutation.

http://labs.biology.ucsd.edu/mcginnis/Hoxmacroevoweb.pdf

http://www.ncbi.nlm.nih.gov/pubmed/11859370

http://ucsdnews.ucsd.edu/archive/newsrel/science/mchox.htm

Further:

http://phys.org/news/2011-07-mysterious-fossils-clues-insect-evolution.html

http://www.biomedcentral.com/1471-2148/8/120

http://phys.org/news/2011-05-evolution-reverse-insects-recover-lost.html

https://www.researchgate.net/publication/51766046_Pancrustacean_phylogeny_in_the_light_of_new_phylogenomic_data_support_for_Remipedia_as_the_possible_sister_group_of_Hexapoda

Regards,

Joachim

Article Pancrustacean Phylogeny in the Light of New Phylogenomic Dat...

Thank you. I have Grimaldi's book but it doesn't discuss about all the theories available.