Order Polydesmida: Development

Stadia

The drawings below are a kind of growth chart for a female polydesmidan. Like all arthropods, millipedes can only grow by shedding their hard exoskeleton and replacing it with a new, larger one. Polydesmida do this in a highly regular way. At each moult they add legs at the hind end, and you can age a polydesmidan just by counting legs.

The drawings show the eight mobile life-stages, called stadia (singular stadium), in the development of a typical head-plus-20-segments, or H+20 polydesmidan (more on that 'H+20' below). These drawings, of course, are not to scale. In reality, stadium I might be only one-tenth the body diameter of stadium VIII, and one-tenth the length. Note also that only the legs on one side of the body are drawn.

Look first at stadium I. At the left end of the animal is the head (coloured green) with antennae. There are no eyes: all Polydesmida are blind. Immediately behind the head is the collar-like collum (blue) which partly covers the first pair of legs. There are three segments with one pair of legs each, then two legless (apodous) segments. At the rear end is the complex structure called the telson (yellow), which is always legless. A stadium I polydesmidan thus has only six legs. As the polydesmidan grows and moults, it adds segments and legs as shown in the drawings (see also the summary table below). In H+20 species, stadium VIII is the final life-stage, the one in which the millipede mates and dies, and there are 31 leg-pairs. Male H+20 Polydesmida in stadia IV, V, VI and VII are missing their eighth pair of legs. This is the first pair on segment 7 counting back from the head. Instead of legs, immature males have a pair of low, rounded bumps at this position (see sketch for male stadium VII).

When H+20 males become mature in stadium VIII, they suddenly produce a pair of gonopods in place of the missing legs (see sketch for male VIII). Gonopods are used for sperm transfer in mating and have complex, almost indescribable shapes which are characteristic from species to species.

H+20 vs. H+19

You now know how to age and sex a H+20 polydesmidan. 'H+20' Polydesmida are called that because they have 20 segments behind the head in stadium VIII, i.e. the collum, 17 leg-bearing segments, one apodous segment and a telson. There are also H+19 Polydesmida. These grow in just the same way as H+20 species, but they mature in stadium VII and die before reaching stadium VIII. In H+19 males, the gonopods appear in stadium VII after lurking unformed behind low bumps in stadia IV, V and VI.

H+19 Polydesmida tend to be a little smaller than H+20 species, but not always. In the Tasmanian fauna, the widespread H+19 species Tasmaniosoma armatum is considerably bigger in stadium VII than some of the smaller H+20 Polydesmida in stadium VIII.

Table of stadia

The table below summarises polydesmidan development in numbers. It applies to all the Polydesmida occurring in Tasmania, but in a few other places there are variations on these patterns. For a comprehensive account of millipede growth and a full discussion of variations see Enghoff et al. (1993).

In the table 'ring' refers to a segment other than the head, collum and telson. There is still some uncertainty among millipede specialists about segmentation just behind the head in Polydesmida. For the purposes of this website, the collum is assumed to represent a separate segment just anterior to the first leg-bearing segment.

The full table applies to H+20 Polydesmida. Ignore the stadium VIII column and you have the table for H+19 Polydesmida.