Why does a Manduca larva undergo five molts?
Insects have their skeleton on the outside, this is called an exoskeleton. Exoskeletons make growing bigger difficult. Manduca weighs about 1 milligram when is hatched and about 10 grams just before pupation. To grow that much, the insect is doing some pretty amazing things with it’s exoskeleton.
There is a limit to how much the insect can grow. Because of the growth limits Manduca must shed it’s exoskeleton often. The process of shedding the old skin while building a new, bigger one underneath is called, molting. Some insect molt only 3 times while others molt more than 5 times during the larval stage.
During molting, the insect usually is usually quiescent (doesn’t eat) for many hours to allow a new exoskeleton to develop. After molting, the mandibles are hardened and the insect eats the old shed exoskeleton except the very hardened hood capsule.
The abdominal horn is black and large in comparison to its body. The stripes are not yet visible at this stage. The first period takes place between emergence and the first molt. A larva at this stage is very aggressive and if crowded will rear back on the prolegs (the legs found on the abdomen) and swing its body around in a violent fashion. This behavior tends to lessen as the larva gets older. The color remains rather pale for about 40 hours after emergence. The insect eats a huge amount during this period, increasing its weight nearly 6-fold and begins to look blue-green. 48 hours after hatching, Manduca stops feeding and begins to weave a silk mat. Glands in the head and thorax produce a silk thread that is laid down on the food. The insect then crawls onto the mat, digs in its crochets, a row of velcro-like claws, into the silk and begins to molt again.
In the second stadium, the characteristic diagonal stripes begin to appear. The stadium is divided into a feeding period of about 48 hours followed by a quiescent molting period of 24-36 hours. The second instar does not make a silk pad.The second instar grows from about 8 mg to about 30 mg
The third instar shows the same growth pattern as the second. A third instar weighs about 30 mg at the beginning and by the time it stops feeding 36 hours later, it has grown to over 150 mg. At about 40 hours it stops eating and begins the molting process.
The fourth instar grows from 150 mg to about 1.1 grams but the time it takes may vary. Some insects can complete this period in as few as 80 hours while others take up to 120 hours. The feeding period is different for each, but the quiescent period remains about 30 hours.
The fifth instar represents the last larval period in which it feeds voraciously. The insect grows from about 1 g to over 11 g if fed properly. Normally, fifth instars feed about 5 days. The circular structures on the sides of the larva are called spiracles. These are the external openings for breathing. During this feeding period,take a look at the last two thoracic segments. The segments don’t have spiracles. In place of the spiracles, faint white tissues form called imaginal discs. These tissues will develop into the front and hind wings. At pupation, the imaginal discs expand quickly to form the wing case and the adult wings. Other adult tissues such as legs, proboscis, and eyes also form in a similar way. The silvery white string-like structures under the exoskeleton are trachea, the oxygen carrying tubes. Until the fifth perios, the gender of the insect can not be determined. By placing the insect on its back and looking at the last abdominal segment with a magnifying glass, you can determine gender. If it is a male, there is a small dark spot in the center of the sex sternite. If it is a female, the sternite will lack the small spot.
The wandering stage of the fifth Instar
In the wandering stage, the insect appears restless and starts to move around the cup smashing the accumulated frass (insect feces) pellets. In nature, the insect would move off the food plant and look for a pupation site in the soil. The wandering stage is the best time to view the dorsal aorta and heart. Coloration in the dorsum (the back) is retracted, allowing the organ to be seen. The area often has an orange coloring.
The dorsal vessel is a long dark tube that runs down the center between the white stripes. There is a single chamber per segment and each chamber is connected to the next by a tube. Each chamber has flaps on either side that act as valves. When the chamber contracts, the valves close due to hydrostatic pressure. If you watch closely, you can see the chamber expand and contract.
Once you see the dorsal vessel appear, the larva begins burrowing behavior. Prepupation events are the most interesting behavioral changes that Manduca undergoes. Instead of climbing upwards, the nonfeeding larva now climbs down. Manduca will continue to search for good burrowing conditions for about 2 days. In nature, the insect pictured above would have already burrowed underground. They are typically found about 9 to 15 inches below the surface of the soil.