THE LIFE CYCLE OF LEPIDOPTERA
© REG FRY 2007
The life cycle of Butterflies and Moths is:- The Egg, The Caterpillar or Larva, The Chrysalis or Pupa and finally the Adult Butterfly or Moth.
BUTTERFLY AND MOTH EGGS
Eggs come in a wide variety of shapes, sizes and markings. Those that are going to hatch out within a week or two are often laid on a leaf, some on the upper side of the leaf and some on the underside and they may be laid singly, in pairs or in a large batch depending on the species. A few species drop their eggs into grasses when in flight. Many species that overwinter as eggs lay them on the trunk, branch or twig of a tree and often close to a bud.
A few examples of egg shapes and designs are shown below. The first row contains examples of three Butterfly eggs and the second and third rows examples of Moth eggs.
THE CATERPILLAR OR LARVA
Most caterpillars have 3 pairs of true legs and with a few exceptions up to 5 pairs of prolegs or claspers as illustrated in the picture below. The true legs are segmented with joints and become the walking legs in the adult butterfly or moth. In most cases the number of prolegs varies from 2 to 5 but some leaf-mining caterpillars and Limacodidae species have none and some Zygaenoidea have more than 5 pairs. The number of prolegs and their size is often helpful in determining which family or families the caterpillar is likely to be part of and hence can help in identifying which species it is, examples from a few families can be found below. As a caterpillar grows in size it becomes too large for its skin which it sheds, typically 4 times, before it changes into a chrysalis. In some cases the number of prolegs which are visible increases as the caterpillar grows and changes its skin.
Family: Noctuidae
This is a large and varied family with around 350 species either resident or regular migrants to the UK. Most Noctuid caterpillars have 5 pairs of prolegs but 3 families have some species with one or two pairs missing or under-developed. In the subfamily Plusiinae, 14 out of 16 species have only 3 pairs - they are missing those on abdominal segments A3 and A4 as in shown by the example of a Golden Twin-spot caterpillar (Chrysodeixis chalcites) below. In the subfamily Hypeninae out of 11 species the proleg is rudimentary or absent on segments A3 and A4 in 4 species and is only rudimentary on segment A3 in 3 species - one example of the latter, The Snout (Hypena proboscidalis) is shown below although the partial proleg on A3 is not visible in this photograph. In the subfamily Catocalinae out of 7 species 1 only has a rudimentary proleg on A3 and 3 species have prolegs rudimentary or absent on A3 and A4.
Family: Geometridae
This is another large family with around 300 species either resident or regular migrants to the UK. Virtually all these species have only two pairs of prolegs, being without those on abdominal segments A3, A4 and A5. The absence of these three pairs of prolegs results in them moving by a looping process of arching and straightening the body - hence they are often called 'loopers'. The photograph of a Pale Brindled Beauty (Apocheima pilosaria) caterpillar is shown below. I am aware of a few species which are classified as Geometers which 'break the rule'. The Orange Underwing (Archiearis parthenias) (picture below) and Light Orange Underwing (Archiearis notha) both have varying size vestigial prolegs on segments A3 to A5. The March Moth (Alsophila aescularia) (picture below) and the Light Emerald (Campaea margaritata) both have vestigial prolegs on A5. The Scalloped Hazel (Odontopera bidentata) pictured below has two tiny pairs of prolegs on A4 and A5.
Family: Nolidae
This is a small family with only 4 species resident in the UK and another an occasional migrant. The caterpillars of these 5 species are all missing a pair of prolegs on abdominal segment A3 and thus only have 4 pairs of prolegs. The picture below shows an example of the Least Black Arches (Nola confusalis).
Family: Limacodidae
This is another small family with only 2 species resident in the UK. The picture below shows an underside view of The Festoon (Apoda limacodes) which has sucker-like structures in place of any prolegs.
SAWFLY LARVAE
Sawfly larvae are often mistaken for Lepidoptera caterpillars so I have included details and pictures here which illustrate the main differences so you should be able to identify any species you find from either group. In most cases sawfly larvae have 6 or more pairs of abdominal prolegs so if the total number of pairs of true legs and prolegs is 9 or more then they are sawfly larvae. The pictures below illustrate examples of the sawfly larvae Craesus septentrionalis (left) and Craesus alniastri.
Apart from the difference in the number of pairs of prolegs the construction of caterpillar prolegs is significantly different from those of sawfly larvae. I think this is fairly obvious when comparing the examples from the two groups shown above. However another significant difference is that caterpillar prolegs have tiny hooks known as crochets on the underside of their claspers. The picture below illustrates the hooks which are visible on the last pair of prolegs of a Broad-bordered Bee Hawkmoth (Hemaris fuciformis) caterpillar.
THE CHRYSALIS OR PUPA AND EXAMPLES OF COCOONS
When a caterpillar is fully grown it usually changes colour when about to change into a chrysalis or pupa. This stage marks one of the most dramatic changes in the development of the insect. The pupal case is developed under the caterpillar's skin and the skin softens and splits to reveal the pupa underneath. The picture below shows a typical Noctuid moth pupa and the areas where significant parts of the adult insect will form up within the pupa casing are clearly marked. The 3 pairs of legs (the true legs) are attached to the the thorax of the adult insect and from the front to the back of the head are named as follows:- The Prothoracic, the Mesothoracic and the Metathoracic legs.
When they are ready to pupate caterpillars look for a suitable place to complete this part of their life cycle which may be on a plant or tree, under leaf and other litter on the ground or in many cases under the earth.
Considering butterflies first, some butterfly caterpillars spin a silk pad on a leaf, plant stem or even a fence post. They then hold onto the silk pad with their hind (anal) claspers and hang, head down, waiting to change into a chrysalis. During the pupation process the skin of the caterpillar splits at the head end and the old skin is gradually worked up the body of the newly formed chrysalis until it is gathered at the top by the silk pad. At this stage hooks which have formed in the cremaster work their way outside the old skin and get a grip onto the silk pad. The chrysalis is now able to wriggle around until it has discarded the old skin and it remains hanging by the cremaster hooks whilst the chrysalis hardens and it remains suspended in this way until the butterfly eventually emerges.
Other butterfly species (for example, the Whites or Pierid species) suspend themselves the other way up, i.e. with their hind feet at the bottom
with the head facing upwards. In these cases they need additional support so as well as a silk pad they spin an additional silk girdle around their 'middle' to keep themselves
in the correct position whilst changing into a chrysalis. Examples of both suspension methods are shown in the picture below.
Some butterfly caterpillars which spend a relatively short time as chrysalis attach themselves to leaves and others that spend the winter as chrysalis
pupate on the ground under plant litter etc. and may spin a loose cocoon around themselves.
The majority of moth caterpillars burrow underground and spin differing density cocoons around themselves before
pupating. Some of those which overwinter as chrysalis spin cocoons on twigs, branches, leaves or even in plants such as heather. Two types of cocoon
are shown in the picture below. The left hand picture illustrates the type spun by the Puss Moth and 'kitten' caterpillars. These are very hard
cocoons made up of silk and chewed bark - when the adult moth emerges it has to eject a liquid acid to soften an area of the cocoon before it can escape.
The right hand cocoon is the shape of a cylindrical cigar tube which is constructed by the Oak Eggar and some other Eggar caterpillars (note that the fibers that normally
attach the cocoon to the surrounding herbage have been removed). It is not a particularly hard cocoon and the adult moth is able to force its way out of one end when it emerges.
The picture below illustrates another type of cocoon which is in the shape of an upturned rowing boat. This cocoon was spun
by a Green Silver-lines caterpillar
and there are quite a few other caterpillar species that spin similar cocoons either on leaves or on twigs etc. In most cases there is an opening vertical
split in the rear end of the boat which closes when the adult moth has escaped.
Finally, one of the most ingenious cocoons is the pear-shaped cocoon which is spun by the Emperor Moth caterpillar. The picture below shows
an Emperor Moth cocoon which has been cut in half so that you can see the way it is constructed. The caterpillar spins a neck at the top of the cocoon and within this
a circle of fibres pointing upwards but not joined in the centre so that the moth can easily force its way through this one-way trap-door when it emerges.
Perhaps the best 'engineer' of them all?
THE ADULT BUTTERFLY OR MOTH
The drawing below illustrates the main elements of an adult butterfly. Most of the elements are obvious but where present in the adult butterfly or moth the palps are used for tasting and the proboscis for feeding on nectar and other liquids.
