The following diagram shows the four stages in the moths life-cycle. We successfully reared over 20 caterpillars to monitor their growth on different foodplants. Our observations are set out below.

The Emperor Moth is one of Britain's largest moths and flies in April and May. The males fly during the day looking for the females that fly at night. We were shown a female in April and it had laid eggs in the moth trap. These hatched in May and Class 1 were given 25 caterpillars to look after and rear to the pupa stage.

The Experiment

We split the class into five groups and each group was assigned a different foodplant to give to their five caterpillars. So we had groups for blackthorn, bramble, hawthorn, hazel and willow. We were pleased that nettle wasn't one of the recognised foodplants!

Every day we had to clean out the rearing pots, removing the old plants and frass (caterpillar droppings), and add new fresh leaves. We were careful not to let the rearing pots get too hot and not to leave the lids off in case the caterpillars escaped!

The caterpillars grew very quickly. Their skins don't grow so they shed their old skins to reveal new ones from time to time. This is called "molting". Each new skin is called an "instar" so after the first molt the caterpillar is referred to as a "second instar".

We noted down each date when they changed their skins and we measured the length of the caterpillars to see if there was any difference in their rate of development on the different foodplants.

When we first saw the caterpillars they were 10mm long and by the time they changed into a pupa they were 65mm long.

Observations

The caterpillars changed their skins or molted approximately every 10 days during which time they were almost doubling in size and each time their new skin was greener than before.

Key dates:

5 May - hatch 2mm
17 May - instar 8mm
21 May - into school
26 May - instar 20mm started getting greener
8 June - instar 40mm (willow) - bramble were only 25mm and not changing skins
20 June - changed to pupa (willow) at 65mm

The caterpillars were so large, by the time they had grown to 40mm we had to give them more room by splitting them into more rearing pots. All caterpillars seem to do is eat!

Summary

We were very surprised by the wide variation in the growth rate between caterpillars. Those fed on willow developed more quickly than any of the others. Hazel and hawthorn were not far behind but those fed on bramble and blackthorn developed much more slowly. Overall we managed to rear 80% of the caterpillars to the pupa stage.

Hopefully we will see some of them emerge in Spring 2010 although we know that sometimes they can stay as a pupa for 2 to 3 years! We will have to wait and see.

Update!!

Go to our Emperor Moth Update page to find out what did happen next!