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Myriapoda (comprising, among others, millipedes and centipedes) can have hundreds of legs (Illacme plenipes having up to 750 legs). Interestingly, the number of legs (or leg pairs) appears to differ not only between myriapod species, but even between individuals of the same species:

Body light cream-colored, thread-like, extremely narrow and long (max. width: ♂ 0.55, ♀ 0.64; max. length: ♂ 28.16, ♀ 40.40). Adult individuals with 84 – 192 segments, and with 318 – 750 legs (VMNH paratype ♀ with 192 segments and 750 legs, more than any other organism known on Earth).

Source: "A redescription of the leggiest animal, the millipede Illacme plenipes, with notes on its natural history and biogeography"

This raises the question: Is the number of legs in myriapoda determined entirely by the genome, or do environmental factors play a role in precisely how many legs eventually develop?


Note: Myriapoda generelly hatch with very few legs, and more legs develop during successive moults. Some examples can be found here.

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Your question is "How much of the variance in number of legs is explained by genetic variance versus environmental variance?". If needed, you will find here and here some basic information in order to understand the concepts of phenotypic, environmental, genetic variances and the different concepts of heritability. –  Remi.b Jun 1 at 15:35
    
It seems that this would be fairly easy to test(we have guesses for what they eat) with a small colony of I. plenipes kept under observation. Unfortunately, considering how rare it is, that experiment seems not to have been conducted. It's also possible that I. plenipes simply continues adding legs indefinitely. –  Jeremy Kemball Jul 8 at 15:48

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Three factors that influence the number of legs are:

1) Sex : In some species of myriapoda, the females have been found to have more leg segments than males (reference) eg: Himantarium gabrielis

2) age

Growth is by adding segments and legs with successive molts (anamorphic), and myriapods continue to add additional segments and legs after they have reached sexual maturity (reference).

3) Working of a "segmentation clock" (set of genes) that periodically adds segments (reference).

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