The larvae usually live in Molluscs, the mature worm in vertebrates, and the immature but metamorphosed Trematode in either host and also in pelagic and littoral marine and fresh-water invertebrates.
But the larvae of the Ecaudata are mainly herbivorous and the digestive tract is accordingly extremely elongate and coiled up like the spring of a watch.
The larvae are furnished with large heads, powerful mandibles and well-developed legs, but the body-segments are feebly chitinized, and the tail-end is swollen.
The larvae known as caddis-worms are aquatic. The mature females lay their eggs in the water, and the newly-hatched larvae provide themselves with cases made of various particles such as grains of sand, pieces of wood or leaves stuck together with silk secreted from the salivary glands of the insect.
In the Limnephilidae the maxillary palp is three-segmented in the male, the larvae are variable in habit, many forming cases of snail-shells.
Another instance of mimicry affecting the larval form is supplied by the moth Endromis versicolor, the caterpillars of which resemble the inedible larvae of saw-flies.
Beetles and larvae are frequently carnivorous in habit, hunting for small insects under stones, or pursuing the soft-skinned grubs of beetles and flies that bore in woody stems or succulent roots.
Beneath the cuticle of these regions are situated the luminous organs, consisting of layers of cells which may be regarded as a specialized portion of the fat-body) Both the male and female fireflies emit light, as well as their larvae and eggs, the egg being luminous even while still in the ovary.
Thus, wasps catch flies; worker ants make raids and carry off weak insects of many kinds; bees gather nectar from flowers and transform it into honey within their stomachs - largely for the sake of feeding the larvae in the nest.
The larvae are stout and soft-skinned, with short legs in correlation with their burrowing habit.