The Pill Millipede Misleads with Convergent Evolution

The Pill Millipede (Glomeris marginata)

One of the most striking examples of convergent evolution I have come across is that of the Pill Millipede (Glomeris marginata) and the Pill Woodlouse (Armadillidum vulagre). Convergent evolution describes the acquisition of the same biological trait in unrelated lineages (1), and in both these animals the architecture of their exoskeleton allows them to curl into a ball (or “pill”) when alarmed. Closer examination of both species shows they are obviously not related, the most distinguishing feature being that G. marginata has two pairs of legs per segment, and many more to boot than the seven pairs possessed by A. vulgare. G. marginata also has shorter antennae and its dorsal plates are edged with cream rims. It can be found in leaf litter in mature woodlands (2), whereas A. vulgare is far more common - individuals can often be happened upon in urban gardens.

Convergent Evolution: Pill Millipede (Glomeris marginata) on left and Pill Woodlouse (Armadillidum vulgare) on right.

However, G. marginata may have further features, that show its apparent relatedness to insects, that are a result of convergence. Traditionally, morphological features such as loss of second antennae, formation of Malpighian tubules, postantennal organs and trachea have indicated that the myriapods are close relative to the insects (3). Yet these features have been shown to be prone to convergence (4), and molecular evidence also points to a far more distant relationship than previously thought. Of the four extant arthropod subphyla, the hexapoda and crustaceans are now seen as sister groups and mitochondrial genome analysis has revealed the same to be true for the myriapods and the chelicerates (5). This is backed up further by 18S/28S rRNA sequences and Hox gene analysis of the myriapods (3)

Overturned Pill Millipede (Glomeris marginata)

Analysis of neurogenesis in G. marginata has provided morphological evidence to support these claims (3). In the development of neuroblasts in G. marginata, groups of cells invaginate to form the the ventral neuroectoderm. This is similar to neurogenesis in spiders, but quite different than that in insects where single cells give rise to the neuroectoderm.

References:

1. Miller et al., 2010. Convergent Evolution
2. Sterry, p. 172
3. Dove and Stollewerk, 2003. Development 130 pp. 2161-2171
4. Dohle, 2001. Annales de la Societe Entomologique de France 37 pp. 85-103
5. Hwang et al., 2001. Nature 413 pp. 154-157