Lepidoptera larval stages are called caterpillars. They hatch from the egg layed by the adult and develope into a pupa, that undergoes metamorphosis. The word larvae, not neanidae or others, is used because we refer to holometabolous insects: those insects undergo a complete metamorphosis: the juvenile stages is really different from the adult (ex. caterpillar and moth), presenting different structures and different overall morphology. In hemimetabola, as Phasmatidae, the juvenile stage is similar to an immature adult.
In this website, and elsewhere, the larval stages will be called L1 (larval stage N°1), L2, L3... ecc. according to their progression. When the caterpillar hatchs from the egg it is called L1.
The larval period is part of holometabolous insect post-embryonal development. In this phase the animal life is commited to eat! Caterpillars need to grow a lot, they actually increase their size of various orders of magnitude during their larval lifes, reaching the critical weight that allows them to pupate (reviewed in BA Edgar, 2006). To sustain this impressive size increase, caterpillars pass through several larval instars, usually 5.
Larvae store a lot of energy allowing metamorphosis to take place, the size of the adult is determined by the size of the caterpillar at the end of its larval growth. Although several Lepidoptera species are able to eat during their adult life, this is not increasing their size.
Caterpillar breeding represents the funniest part in Lepidoptera breeding. A general rule in Lepidoptera breeding is: the less you interfere with them, the better they will stay. Observation is the best way to interact with them.
When eggs inside the Petri-dish or the plastic box (as shown in eggs/pupae care chapter) start to hatch, caterpillars will spread inside the box; for this reason is better to avoid big containers. This is Time To Put the first fresh host plant leaves inside; caterpillars will sense the presence of the host plant and converge on it. In general, after hatching, caterpillars tend to rest for a while and then spread almost randomly for several hours; sometimes up to few days depending on the species.
Many larvae eat their own eggshell after hatching to reintegrate salts. Thus, better leave the eggshells inside the container for a while after catepillars hatch.
It is possible to breed caterpillars inside a Petri-dish for the first instar, in some cases till the second. This method has the advantage of reducing the space in which they can walk around and create a micro-enviroment in which leaves are drying slower. On the other hand the breeder should clean them more often and be very carefull in case of toxic hostplants or species sensitive to overcrowding or ventilation.
The number of L1 caterpillars inside a standard 80mm Petri-dish could range from 5-10 for delicate species (ex. A. dubernardi) to 50 for easy ones as S. ricini. This also depends on the number of times your are able to clean them daily. Usually, we let them hatch inside Petri-dish and then transfer them to small plastic boxes at the beginning of L2.
When caterpillars are very small (e.g. L1,L2,L3) they will consume a very small amount of food and the leaves could dry before they have finished to eat them.
To avoid this, place inside the container a small amount of leaves. When it is Time To Change the food, or clean the box, remove all the old leaves to avoid caterpillars to waste time on them.
If you can, avoid touching the caterpillars, especially when they are in the first instars, they are very sensitive. If you need to move them, try to handle them with a leaf.
If caterpillars are moulting from one instar to another, they will ancorate to the ground/leaf/box or to wherever they are. They will stay without moving for a couple of days before breaking the old instar's cuticole and start moving again. In this phase they need to be left attached to the substrate. Don't worry if caterpillars sometimes seem too static, they are probably moulting!
Starting from L3/L4, caterpillars will consume a larger amount of food and they will be tranferred to bigger boxes to avoid overcrowding and allow more space for the host plant. Since caterpillar's growth is exponential, the major part of the food will be consumed in the last larval instar (usually the 5°). During every instar, larvae will be less active in the first part of the instar. When they get closer to the end of the instar they will be more active and will dedicate much more time to eat.
At the end of the last larval instar, caterpillars will cease to eat and expulse liquid stools. This is time for them to look for the right place to pupate: the "wandering" stage. Depending on the species they will look for a different place to pupate. Some of them will just find a place between the leaves while others will go to the bottom of the container and try to dig (underground pupators).