Many species spend more time in their larval stage than their adult stage. So the "sack of fat with a mouth" is sometimes the true insect star and the beautiful, ornate, or winged version may not even have the ability to eat...only mate.
You are correct in thinking that the larval stage may seem ridiculous, but most insects have thousands or hundreds of thousands of eggs since attrition (natural selection) rates can be high among the seemingly unprotected grubs.
Larva can have many benefits, however. They are often adapted to different environments (aquatic, subterranean, nocturnal) to their adult counterparts which can offer different opportunities for camouflage, feeding or pupating. This also reduces competition between the larva and the adult specimens for sustenance etc.
Flight for example, uses a great amount of energy and in species which do not eat as adults, it is vital that the fat grubs gorge on carbohydrates to sustain the adult for the few hours or days that it might have to find a shag.
Genetic variation. Some cicada grubs exist underground for up to 17 years, only emerging during certain climatic or seasonal events. This staggering emergence helps to ensure enough genetic diversity and to also avoid predators or a freak snowstorm wiping them all out.
Pupae can often survive winters or climactic conditions which neither the larva, nor the adult can survive. So that little crispy shell can be a good way to ensure survival. In order to survive the cold or wet winter, the larva must eat, eat, and eat some more.
Scientists believed metamorphosis began to occur 280 million years ago (fossil records) where the first embryos hatched as tiny worms and not like their adult counterparts as seen before. One theory suggests that this type of life cycle first arose because the 'worms' were forced to hatch prematurely by some kind of nutrient deficiency or dramatic climactic shift.
There is no known common ancestor and there are also several forms of metamorphosis in insects and as seen in non insects, such as amphibians (frogs, toads, salamanders etc) so it is likely an example of convergent evolution.
· 4 months ago