Metamorphosis is one of the keys to the success of the insects. Of all the insect orders it is the ones that go through a larval and pupal stage that are the most diverse. This way of life seems to be beset with all manner of pitfalls. Larvae are generally vulnerable to dessication and the pupa is a sitting duck for all sorts of pathogens and predators.
As seemingly risky as this way of life it has some huge benefits. Crucially, it allows two lives for the price of one and a division of labour. The larval stage is an eating machine and so all it needs are the structures to do this and do it well, i.e. well developed mouthparts and a metabolism tuned to efficiently turn this food into body mass. The adult on the other hand can concentrate on the problem of finding a mate and continuing the life-cycle. The larvae has done all the growing, so apart from requiring fuel for scuttling around and flying the adult doesn’t really need to spend too much time with its face buried in food. With these two distinct stages the youngsters and the adults will never be competing with one another for places to live and things to eat, which means that any given habitat can support more of them.
As if developing through all these stages wasn’t enough, many insects have heaped another layer of complexity on top in a process known as hypermetamorphosis. The larvae that hatches from the egg of one of these insects isn’t a sluggish grub-like creature, but a slender-limbed, extremely active beast, known as a triungulin, which scampers about looking for its host. The host depends on the insect in question that employs this strategy. Meloidea beetles, such as oil beetles, blister beetles, etc. are the most well known insects that go through hypermetamorphosis and these parasitise various solitary bees. The triungulins clamber onto the bee when its lapping up nectar from a flower and are taken back to its nest. If they’re lucky the mother bee will visit one of the nectar-filled brood cells in to which she has already laid an egg. One of the passengers will clamber on to this egg and use it as a raft before eating it and metamorphosing into a more typical, grub-like larvae. These chunky creatures can writhe around in the nectar and gorge themselves on it. The fat beetle grubs take on various shapes during their time in the bee’s nest and they eventually pupate to take on their bizarre adult form.
The photo below shows an interesting European Meloid (Cerocoma schaefferi), loads of which were found on flowers in an old quarry in northern Spain. Like all of their kind they also go have a triungulin stage, but instead of solitary bees, solitary wasps are what they seek. The wasps in question (Tachytes sp.) are specialist predators of mantids and grasshoppers and their kin. The wasps hunt and provision their nests with these insects and it is these the Cerocoma larvae eat.