Aristotle’s Silkworm (Saturnia pyri) in Endemic Liquidambar Forest

Filmed in Günlüklü forest, Fethiye, Turkey on May 27th 2015. A Turkish version of this article was published in the Atlas Magazine.

Saturnia pyri is the largest moth in Europe. It lives in Southern Europe, parts of Asia and North Africa but it occasionally makes a northward move. It was recorded in England only once in 1984. Its caterpillars are polyphagous and feed on many different tree species such as blackthorn, ash, walnut, poplar, plum, apple, pear and cherry (please see a list of host trees at the bottom of this post). However, in this rare encounter it is documented for the first time to feed on the endemic oriental gum tree (Liquidambar orientalis). These endangered relic tree populations form monostand forest patches in Southeastern Turkey.

Here in this observation we see a caterpillar in it’s final (L5) instar stage. It is ready to pupate and is in search mode moving around the trunk of a downed oriental gum tree to find a suitable location. Once it finds the spot it will start spinning a cocoon and will form a chrysalis inside. You can see this activity in a separate post paired with this observation.

Both the caterpillar and butterfly forms of this moth are quite striking and have been observed by notable personalities in history including Aristotle, the father of scientific natural history tradition who has made impressive personal observations in the Northern Aegean seacoast. Aristotle describes a large silkworm in book V section 19 of his Historia Animalium:

“From one particular large grub, which has as it were horns, and in other
respects differs from grubs in general, there comes, by a metamorphosis of the grub,
first a caterpillar, then the cocoon, then the necydalus; and the creature passes
through all these transformations within six months. A class of women unwind and
reel off the cocoons of these creatures, and afterwards weave a fabric with the
threads thus unwound; a Coan woman of the name of Pamphila, daughter of Plateus,
being credited with the first invention of the fabric.”

Entomologist William T.M. Forbes of Cornell University provides a detailed analysis of the Aristotle’s Silkworm where he suggests the caterpillar described in Aristotle’s account must have been Saturnia pyri since domesticated Chinese silkworm (Bombyx mori) does not posses “horns”.

The late-Renaissance painter/illustrator Jacobo Ligozzi painted a butterfly form of this moth as a part of a Euphorbia illustration and also made a detailed illustration of the caterpillar in 16th century.

The famed artist painter Vincent van Gogh is also among the notable figures. In May 1889 van Gogh encountered the Great Peacock moth in the garden of the clinic at Saint-Rémy-de-Provence. He was so impressed with its beauty that he first charcoal sketched and then later made a painting based on the sketch.

Caterpillars are under constant threat from parasitoids. Parasitoids are parasites that kill their hosts. Caterpillars of Saturnia pyri have many defensive adaptations including visual deception, poisonous chemicals and even deterrence using sound (stridulation).

As we can observe in this recording the larvae of Aristotle’s Silkworm are well-defended chemically. A gas chromatography analysis showed that most of the defensive chemicals are aromatic and didn’t change even the caterpillars fed on different tree species. The caterpillars have poisonous sky-blue protrusions called “scoli” (tubercles in the form of spinose projections of the body wall). There are six scoli on each body segment (except the head and tail segments which have four) and from each scolus a number of black bristles come out. On mechanical disturbance, the larvae are able to discharge a clear secretion out of the tips of these hollow bristles. Production of protective poisonous chemicals are costly requiring expensive metabolic activity. Therefore caterpillars of some moths and butterflies eat their own discarded skins to regain protective chemicals each time they molt. Curiously, according to filmmaker Dr. Grochowalski of the Kraków University of Technology who has filmed the entire life cycle of the Great Peacock Moth none of the instars showed this kind of behavior.

Additionally Saturnia caterpillars use a visual deception adaptation to deter parasitoids. The blue bumps on their bodies mimic eggs of parasitoid wasps while the white spots resemble the eggs of tachinid flies. These structures are also present in the caterpillar of another Saturniid moth native to North America (Hyelophora cecropia). Parasitoid eggs develop fast and if a female parasitoid lays eggs slightly later than a previous one her offspring will be at a great disadvantage. Her eggs will not have enough time to develop. In order to increase survival chance of her offspring a female parasitoid must be very careful not to oviposit on a host that has already been parasitized.

The caterpillars of the Great Peacock moth have another defense in their evolutionary arsenal. When attacked by a predator in addition to the chemical secretions the caterpillars respond by creating stridulations. A study carried out by researchers in Canada has recorded the vibrations generated by caterpillars after simulated predatory attacks. The following video shows responses of a caterpillar to different types of predatory attacks. First scene shows a mock attack near the head capsule region and the second one a more sneaky hind attack from the posterior using blunt forceps which simulates an avian predator. The final third scene shows close-up movements of the mandibles during two chirp trains. During the first chirp train, each of the three chirps is associated with the right mandible sliding against the inner surface of the left mandible. During the second chirp train, the animal switches sides so that the left mandible slides against the inner surface of the right:

A list of host trees for Saturnia pyri:

Acer (maples)
Aesculus hippocastanum (horse chestnut)
Alnus (alders)
Betula (birches)
Corylus avellana (hazel)
Cydonia (quince)
Fagus (beeches)
Fraxinus excelsior (ash)
Juglans regia (walnut)
Liquidambar orientalis (Oriental gum)
Malus domestica (apple)
Malus sylvestris (crab-apple tree)
Olea europaea subsp. europaea (olive)
Platanus orientalis (plane)
Populus nigra (black poplar)
Prunus armeniaca (apricot)
Prunus avium (sweet cherry)
Prunus domestica (plum)
Prunus dulcis (almond)
Prunus persica (peach)
Prunus salicina (Japanese plum)
Prunus spinosa (blackthorn)
Pyrus communis (European pear)
Rubus (blackberry, raspberry)
Salix (willows)
Syringa vulgaris (lilac)
Tilia (limes)
Ulmus (elms)