Biology

Watch a short video of newly emerged Paralecanium expansum metallicum crawlers, filmed down a microscope. They have hatched from eggs underneath the adult female, whose lower surface is visible.

Most soft scales, and members of several other scale insect families, have a strange life cycle that involves spending almost their entire lives in a sessile (immobile) state.

Paralecanium expansum metallicum life cycle

Phase-contrast photomicrograph of a crawler

Phase-contrast photomicrograph of a P. expansum metallicum crawler. The specimen has been stained pink to make it easier to locate on the slide.

First-stage nymphs, termed crawlers:
  • hatch from eggs that are protected underneath the adult female’s body
  • can walk short distances
  • act as the main means of dispersal from one plant to another, or from one area to another

P. expansum metallicum crawlers provide a very effective means of dispersal as they are:

  • only 0.5mm long
  • extremely light
  • readily carried by air currents

Once the crawler has selected a feeding site it undergoes its first moult.

From the second stage onwards 

P. expansum metallicum females:

  • do not have functioning legs. Adult females don't even have the vestiges of legs
  • simply feed on the host plant’s phloem sap: if the plant dies, so do they

Some other soft scales do have short-but-functional legs but mobility is still extremely limited.

In complete contrast to females, male scale insects have:

  • long legs
  • a single pair of wings in most cases
  • no functioning mouthparts

So they have very limited time in which to seek out females in order to mate.