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A proleg is a small, fleshy, stub structure found on the ventral surface of the abdomen of most forms of of the order Lepidoptera, though they can also be found on larvae of insects such as symphyta. In all the orders in which they appear, mainly Hymenoptera and Lepidoptera, prolegs of any form evolved independently of each other by convergent evolution. They appear to have evolved from ancient inactive genes that have since been activated again. Lepidopteran prolegs are novel traits, not leg homologs
Prolegs of lepidopteran larvae have a small circle of gripping hooks, called " crochets". The arrangement of the crochets can be helpful in identification to family level. (1977). 9780412613906, Springer. ISBN 9780412613906 Although the point has been debated, prolegs are not widely regarded as true arthropod leg, derived from the primitive uniramous limbs. Certainly in their morphology they are not jointed, and so lack the five segments (coxa, trochanter, femur, tibia, tarsus) of thoracic insect legs. Prolegs do have limited musculature, but much of their movement is Hydraulics powered.
| Number of prolegs on insect larvae at abdominal segments A1–A9/10=S4-12/13 (T1–3: thorax segments with 6 legs)Stefan von Kéler: Entomologisches Wörterbuch. Akademie-Verlag Berlin 1963. p. 80. |
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Additional (stubby) prolegs on segment 8 have been reported on the Geometridae species Campaea perlata and Alsophila pometaria.
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