A Students' Guide to Butterflies

The function of the larvae is very simple: to eat and grow. Coupled with a giant stomach and the ability to shed its skin, the larva is an eating machine designed to gather the energy necessary to transform into the adult. The duration of time in this stage averages about three to four weeks. Larvae, being insects, have six legs, and although less pronounced than the adult, three body segments- head, thorax and abdomen.

The larva have mandibles for chewing their food. The mandibles must be strong to break though the leaf and crush it. Here we have one of the major differences between the larvae and adult stage: Larvae eat only solid plant material from their host plant, whereas adults eat only liquids. It is also important to bear in mind tha the larval host plant has nothing to do with the adult's diet.

The eyes of a larva serve for little more than distinguishing day from night. Because caterpillars need to be as inconspicuous as possible from predators, most are nocturnal feeders. During daylight hours they remain motionless in discreet corners of their host-plant.

Underneath the head are glands which exude a liquid which when exposed to the air forms silk strands. The silk strands can be useful for three reasons. First, they secure the larva to the surface they are walking on. Second, some species use the silk as an escape route. For example, when an ant attacks a small larva, the larva may drop off of the leaf and dangle from the thread in midair. Third, the silk will be used to form a button of silk from which the pre-pupa will attach itself to the underside of the leaf where it will hang.

The thorax is a short segment of the larva. It is distinguished by being immediately behind the head and possessing the three pair of true legs on the underside. When later transformed to the adult butterfly, the wings will extend from the area directly above the true legs.

The thorax of the larva from above is very inconspicuous and hard to distinguish from its other parts. The best way to recognize this segment of the larvae is to turn it over where the six true legs can be seen underneath. Where the last pair of legs end is where the thorax changes to abdomen.

The abdomen, consisting mostly of the digestive system, represents the largest segment of the larvae. As the larva spends most of its active hours feeding on its host plant, the abdomen is where the giant stomach is found. In some cases, the abdomen can represent as much as 80% of the entire body size.

On the underside of the abdomen are a number of prolegs or sucker-feet. This is a misnomer as the prolegs do not have suction cups at all. Rather, they are equipped with thousands of tiny hooks (very much like Velcro) which assist with walking. As the larva lays down a sticky silk pad, the hooks on the prolegs will firmly grasp the silk. In this way larvae may attach themselves securely to whatever surface they may walk on no matter how sheer.

Along the sides of the abdomen one may find a series of spots. These are the spiracles -valves extending the length of the larva's body which allow the larva to breath.

Larvae, rather than having internal skeletons have exoskeletons. As the larvae grow they must shed their skins just as snakes and shell fish do. This process of shedding the exoskeleton or skin is called molting, and each successive molting produces a new instar. The word instar refers to the stage of development of the larva. When a larva emerges from its egg it is a first instar larva, and molting four times the larvae will pass through five instars. At the end of the larva's fifth instar it will have reached its full size and will form the prepupa. Inside the larva are all the parts of the butterfly. There is no complete breakdown of the larva and reformation to a butterfly. There is merely a specialization of body parts. The larva as a basic butterfly with additional parts to allow it to behave like a caterpillar. These additional parts would be suction feet, strong mandibles, a long body, and an enormous stomach.

The larval stage is the only stage of butterfly development where the organism grows. Take special note of how different the larval stage is from the adult butterfly stage (not only in appearence, but function as well). The job of the larva is simply to eat, grow, and gather the energy needed to transform into an adult while inside the pupa. The larva will grow from the size of a tiny egg to the proper size for the adult. The growth rate is phenomenal. If a normal 7 pound human baby were to grow at the same rate as a Monarch larva, in just one short month that baby would be the size of a London double-decker bus and weigh many, many tons.

As the larva reaches its full grown size in the final instar, it begins a stage just before the pupa stage known as the prepupa. Here the larva stops eating its host plant, empties its stomach, and begins wandering to find an ideal spot in which to become a pupa. The larava may wander long distances (with respect to its size), sometimes down and off their host plant and up another. When an ideal spot is found, the larva will once again use its silk glands to hang itself upside-down. This is a very delicate manuver in which the larva will attach its hind prolegs to the surface where it is hanging. The larva will create a small ball of silk to make this attachment, which is called a cremaster. Once attached, now hanging upside-down, it will slowly begin to condense its body. While condensing, the larva is also forming the outer layer of the pupa beneath its exoskeleton. These two processes happen simeotaneously so that when the larva sheds its skin one last time, the pupa is formed beneath.

This information is provided by Evan Fitzgerald, who was an intern at The Butterfly Farm in 1999. Evan graduated from the University of New Hampshire with a Bachelor's Degree in Environmental Science in May 1998. Evan can be reached at: evan@butterflyfarm.co.cr

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A Students Guide to Butterflies

Module 3: Larvae Stage