Larva a.k.a. caterpillar (An introduction and external morphology)

   As we have discussed earlier in a post mentioning the life cycle of butterflies, we know that the second stage of the cycle is larval stage. The animal at this stage is technically called larva (plural larvae) or in common tongue the caterpillar. This is the stage preceded by egg and which successes into the pupal stage. Main purposes of this stage are to eat, grow and accumulate nutrients.

   Larvae of almost all the species of butterflies eat their way out of the egg and firstly they consume the shell of their own egg. After that they start exploiting their host plants or in some cases their prey (Apefly Spalgis epeus larvae eat mealybugs). Plant eaters have a wide diversity in the choice of which part of the plant they eat. Some eat only the young leaves, some the flower buds, some young fruit, some even eat roots and some eat everything. 😅

  Lepidopteran larvae has a worm-like appearance (some sluggish looking too) and have a tagmatized body consisting of 3 tagmata namely head, thorax and abdomen.

   Head is equipped with finely crafted instruments which are used to cut and slash plant material. In a large larva we can identify some of these external structures.
    Head is covered with a hard sclerotized epicranium which is divided into two halves (right and left). Triangle shaped frons, clypeus below it and labrum below the clypeus can be seen from the anterior side of a larva. Behind labrum is the pair of mandibles, maxilla and labium. There is a pair of galea originating from the maxilla (maxillary galea) and this pair becomes the proboscis in the process of metamorphosis. Labium has the tiny protruding spinneret onto which the duct of the silk glands opens. The tiny pair of antennae can be seen beside the mandibles. There are stemmata (usually 6 stemma on each side) on the ventrolateral area of the epicranial lobes. (There are many more important structures associated with the head, but I'm not planning to cover all that and internal anatomy up, for this info is more than necessary for conservation purposes.)
   

Anterior view of the head of a 5th instar Papilio polymnestor (Blue Mormon) larva

   Then comes the thorax. This consists of three segments and each bear a pair of true legs (thoracic legs) which again (each leg) consists of five segments, orderly from proximal to distal coxa, trochanter, femur, tibia and tarsi. This means the larva has 6 true legs. These are powered by muscles.
   Thorax is connected to head by the smooth cervix (neck). First thoracic segment or the prothoracic segment, has a pair of spiracles each side and in the case of Papilionidae, also has the evertible osmeterium (which looks like a forked tongue) dorsally.

   Spiracles are the openings to the outer environment of the internal network of air tubes which acts as the respiratory system of all insects (called the tracheal system).

   Last part of the body is the abdomen. It is made up of 10 segments. 3,4,5,6 and 10th segments have one pair of prolegs each. Which means a larva has 5 pairs of prolegs (10 prolegs in total). These prolegs are not like the thoracic legs. They have hook-like structures called crochets and work from the force that blood pressure creates inside the larva. First four pairs of prolegs are called abdominal prolegs and the last pair is called the anal prolegs. 1,2,3,4,5,6,7 and 8th segments have a pair of spiracles each on either sides. At the posterior end is the anal opening.
  There are some more special structures in the abdomen and some are only present in some families only...  (Dorsal Nectar Organ- DNO, Pore Cupola Organ- PCO, Tentacular Organ- TO, honey glands and wax or powder glands etc.) We will discuss these special features in their family accounts.

Lateral view of a 5th instar Papilio polymnestor (Blue Mormon) larva 

    Larval stage is the most harshest. Predation, diseases, parasites, shortage of food and many other factors test these delicate creatures everyday to their limits. Birds, reptiles, amphibians, arthropods and even some mammals prey on these guys. Parasitoid flies and wasps use caterpillars as food source and the incubator for their young. Bacteria, viruses and fungi also attack them. Cordyceps spp.  are well known fungal pathogens to caterpillars.

    But caterpillars are not defenceless in this game. They bear many peculiar extensions such as horns, fleshy filaments, setae, spines, branching spines etc. and are found in every colour of the visible (and invisible) spectrum, millions of colour patterns and shapes and many peculiar behaviors, all are custom tailored by evolution to overcome the above mentioned enemies and maximize their survival.

    Caterpillars molt their skins 4 times, growing bigger and bigger after each molting. The resulting five stages of the larvae are referred to as instars (just hatched-1st instar, after the first molting-2nd instar etc.). After growing to the full size (5th instar) they stop eating and often wanders to find a good secure place, lay a silk girdle or make a good cover and after everything is ok, they molt their skins for one last time and become pupae. Some fail in this process and die, unable to form the pupa.

Larvae of some common butterflies of Sri Lanka