Lepidopteran larvae have both thoracic legs and abdominal prolegs, yet it is unclear whether these are serial homologs. A RNA-seq analysis with various appendages of
Bicyclus anynana
butterfly larvae indicated that the proleg transcriptome resembles the head-horn transcriptome, a novel trait in the lepidoptera, but not a thoracic leg. Under a partial segment
abdominal-A
(
abd-A
) knockout, both thoracic leg homologs (pleuropodia) and prolegs developed in the same segment, arguing that both traits are not serial homologs. Further, three of the four coxal marker genes,
Sp5
,
Sp6-9
, and
araucan
, were absent from prolegs, but two endite marker genes,
gooseberry
and
Distal-less
, were expressed in prolegs, suggesting that prolegs may be using a modular endite gene-regulatory network (GRN) for their development. We propose that larval prolegs are novel traits derived from the activation of a pre-existing modular endite GRN in the abdomen using
abd-A
, the same Hox gene that still represses legs in more lateral positions.