Key to orders

1A All three pairs of abdominal appendages two-segmented, with a terminal vesicle and five setae ...... Eosentomata

1B Abdominal appendages I two-segmented with a terminal vesicle and three to four setae; those on abdominal segment II and III two-segmented, with a terminal vesicle and three to four setae, or uni-segmented, without vesicle and with one to three setae ......................................................................................................................... 2

2A Median setae present on meso- and metanotum................................................................................ Acerentomata

2B Median setae absent on meso- and metanotum .................................................................................... Sinentomata