Thursday, May 1, 2014

UST GS: The World of the Mysterious Bagworm

Dr Abe V Rotor
Living with Nature - School on Blog
Paaralang Bayan sa Himpapawid (People's School-on-Air) with Ms Melly C Tenorio
738 DZRB AM, 8-9 evening class Monday to Friday

Shot holes caused by bagworm (Cryotothelea heckmeyeri) besiege a talisay tree (Terminalia catappa), Cebu City 

                            Pagoda bagworm, Crypothelea hekmeyeri Heyl., in pseudo colony 
                            on duhat leaf; below, enlarged side view of the pagoda-shaped insect.

Sheepishly she peeps from under a pagoda she built;
Like the turtle she hides, creeps ‘til finally ceases to eat.
A Venus de Milo she emerges, sans wings she must wait,
Love scent in the air she urges a winged groom her mate.

She lays her eggs in the tent, broods them ‘til they hatch,
With heart’s content; leaves and dies after the dispatch.
To the Great Maker, life’s full of sacrifice and obligation;
Mother keeps young and home, the species’ bastion.

- AV Rotor, Bagworm
Light in the Woods, 1995

My pastime reading under a spreading duhat tree standing at the backyard of our old house was disturbed one summer. This favorite shady spot almost disappeared as the tree my father planted before I was born completely shed its leaves. Our yard turned into a litter of leaves. Our tree appeared lifeless.

Summer is when this tree is a deep green canopy, loaded with flowers and luscious, sweet fruits, and laughing children, their tongues and hands bearing the stain of its black berries.

The culprit cannot be the drought spurred by El Nino, I thought. Duhat is highly tolerant to prolonged dry spell, because its tap roots can reach deep seated ground water.

Even before I discovered the culprit - a shy insect protected by a pagoda-like bag - my children had already set up a field laboratory in their a tent, complete with basic research tools, and books of Karganilla, Doyle and Attenborough. For days our backyard became a workshop with the touch of Scotland Yard, Mt Makiling and Jules Verne.

My children called the insect living pagoda because of the semblance of its house with the Chinese temple, and because of its turtle-like habit of retreating into its bag. Leo, our youngest fondly called it Ipi, contracted from “insect na parang pagong at pagodang intsik”.

Ipi belongs to the least known family of insects, Psychidae, which in French means mysterious. Yet its relatives, the moth and the butterfly, are perhaps the most popular and expressive members of the insect world.

Curious about the unique bag and how it was built by such a lowly insect, Matt and Chris Ann worked as research partners. They entered their data in a field notebook as follows:

1. Base diameter - 2 cm
2. Height of bag - 2 cm
3. No. of shingles on the bag - 20
4. Size ratio of shingles from base to tip – 10:1
5. Basic design – Overlap-spiral

We examined the specimen in detail with a hand lens and found that the bag has several outstanding features. My children continued their data entry, as follows:

1. Water-resistant (shingle roof principle)
2. Stress-resistant (pyramid principle)
3. Good ventilation (radiator principle)
4. Light yet strong (fibrous structure)
5. Camouflage efficient (mimicry and color blending)
6. Structural foundation - None

The pagoda bag has no structural foundation, I explained. It is carried from place to place by a sturdy insect which is a caterpillar, larva of a moth. Beneath its pagoda tent, it gnaws the leaf on the fleshy portion, prying off the epidermal layer to become circular shingles. Using its saliva, it cements the new shingles to enlarge its bag, then moves to a fresh leaf and repeats the whole operation. As
the larva grows, the shingle it cuts gets bigger,

This is a very rare case where construction starts at the tip and culminates at the base, noted my wife. Remember that the structure is supposed to be upside down because Ipi feeds from the underside of the leaf, I said. “An upside pagoda,” our children chorused.

As Ipi grows, the shingles progressively increase in size and number, thus the bag assumes the shape of a storied pagoda. Thus there are small
Pagodas and larger ones, and varied intermediate sizes, depending on the age of the caterpillar which continuously feeds for almost the whole summer during which it molts five times.

If there are no longer new shingles added to the bag it is presumed that the insect had stopped growing. It then prepares to pupate and permanently attaches its bag on a branch or twig, and there inside it goes into slumber. The attached bag appears like thorn as if it were a part of the tree, and indeed a clever camouflage on the part of the insect. Here suddenly is a parasite becoming a symbiont, arming the host tree with false thorns!

My children's curiosity seemed endless. I explained that like all living things, bagworms have self-preserving mechanisms. They must move away from the food leaf before it falls off. They must secure themselves properly as they tide up with their pupal stage. After a week later they metamorphose into adults. Here on the twigs and branches they escape potential predators. Here too, the next generation of newly hatched larvae will wait for new shoots on which they feed.

Matt picked one bag after another to find out what stage the insect is undergoing. I recalled my research on Cryptothelea fuscescens Heyl, a relative of C. heckmeyeri, the pagoda species. Chris Ann took down notes

1. Specimen 1 - Bag is less than 1 cm in diameter, caterpillar in third instar (molting), voracious feeder.

2. Specimen 2 - Bag large, construction complete, insect in fifth or sixth instar, morphological parts highly distinct, head and thorax thick, three pairs of powerful legs.

3. Specimen 3 - Insect in pupal stage, expected to emerge in one week, chrysalis (skin) full, dark and shiny. Feeding had completely stopped.

4. Specimen 4 - flag empty, opening clear, chrysalis empty.

5. Specimen 5 - Bag contains eggs laid on cottony mass, chrysalis empty.

The last specimen is intriguing. Where is the insect? Why did it abandon its lifelong home? A puzzle was painted on the face of our young Leo. So I explained.

Let us trace the life history of Ipi and its kind. Both male and female bagworms mature into moths. The winged male upon emerging from his bag is soon attracted by love scent emitted by a waiting female moth still ensconced in her bag. The scent is an attractant scientists call pheromone. Then in the stillness of summer night, her Romeo comes knocking. Without leaving her bag she receives him at an opening at the tip of the pagoda bag. A long honeymoon follows, but signaling an ephemeral life of the couple.

The fertilized female lay her eggs inside the bag, seals it with her saliva, then wiggles out to the outside world but only to fall to the ground - and die, because Nature did not provide her wings!

“Poor little thing,” muttered Cecille apparently in defense of the female species. “Nature did it for a reason,” I countered, “otherwise we would not have bagworms today.” The wingless condition of the female bagworm is the key to the survival of the species.

The sun had set, the litter of leaves had been cleaned up. And the silhouette of our leafless duhat tree against the reddening sky painted gloom on our subject. As dusk set in, I noticed nocturnal insects circling the veranda lamp. A moth paused, then passed over our heads and disappeared into a tree. “Bye, bye,", cried Leo Carlo.

Summer was short, the rains came early and our duhat tree developed robust foliage. Cicadas chirped at the upper branches and an early May beetle hang peacefully gnawing on young a leaf. I was reading Rachel Carson’s Silent Spring when a gust of wind brought down a dozen tiny bagworms hanging on their own invisible spinnerets. My children were aroused from their reading of The Living Planet.

We had unveiled the mystery of the pagoda bagworm, but above anything else, we found love and appreciation on the wonders of Nature and the unity of life itself. ~~
Another species of bagworm (Crypthothela fuscescens Heylerts),  Family Psychidae. Photos taken at Angels' Hills, Tagaytay. The larva builds a bag of dried twig of the same diameter and length and attaches on the host plant until it reaches maturity.  The spent bag simply remains hanging. Lower photo shows an exposed larva purposely for study.

Bagworm (Crypthothela fuscescens Heylerts), Family Psychidae
Photos taken at Angels' Hills, Tagaytay.

Bagworms make bags out of pieces of stems and leaves attached to the host plant. The male insect emerges leaving behind its molt at the opening of the bag. The female is wingless and does not leave the bag. When ready for mating she exude sex pheromone to attract a winged male through the posterior opening to fertilize. After laying her eggs inside the bag, she pushes her way out and drops to the ground and dies. In a week's time the hatchlings emerge from the nursery bag and soon find food and start building their own bags. Lowest photos: full size bagworms. The caterpillar molts five or six times before becoming into pupa, and consequently adult. Exposed caterpillars in their fifth and final instars.

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